Classic Hymer Mythbusters and common situations.

Here are some thoughts and musings on questions that get asked regularly ....

What follows is based on years of answering technical questions online and reading and analysing all the questions that get asked, and the situations that occur. 

Some of this is personal opinion, but based on what I see, but most of it is simply an accumulation of collective knowledge

DIESEL HEATERS

I have lost count of the times people say - I am going to fit a diesel heater - usually because their Truma heater is no longer working. It really isn't that simple, what is actually happening is that people see a diesel heater costing £100 online and think it's a cheap fix. It isn't, unless you are a solid DIYer with a good tool kit and knowledge - and if you are, then you also have the nouse to fix the Truma.

The Truma 3002/5002 is a brilliant heater, and is plumbed into the ducts that run around the van. It is there for a reason - Hymer put it there! Spares are available, and they are usually easy to fix.

Diesel heaters are noisy and expensive to run, they also need battery power. Fix the Truma if you can, especially if you use the van a lot or full time.

For a full and detailed article on diesel heaters in classic Hymers ....

https://hymers700.blogspot.com/2024/02/diesel-heaters-in-classic-hymers-in.html

COMPRESSOR FRIDGES

With new 3-way fridges now well over £1200, everybody thinks 12v compressor fridges are the answer. They are not, but they can be if you take everything into consideration.

A compressor fridge is a one trick pony - it needs 12v, and only 12v - if you have no 12v, you have no fridge. So the 12v it needs has to be supplied over and above your normal 12v usage. Average use for a 12v fridge in summer is about 30 amp hours per day - that's nearly half a standard 100ah leisure battery. So a 12v fridge only makes sense if you can replace that much power - every day. The easiest way to do this is by solar. If you have 200w or more of solar and a decent battery, then you can run a 12v fridge - but there is a catch - and that is that you can only do this in summer.

If you full time or use the van for long periods in winter, then it is a completely different kettle of fish. Solar is almost non existent in winter - so you need other sources of charge - regular plug ins or regular driving. It is almost impossible to run a 12v fridge completely off grid all winter - for winter full timing, gas is the answer.

Of course if you never off grid and always plug in, then you can of course run an electric fridge - in fact you can run an under the counter 230v domestic fridge that you can buy on Amazon for £80 - all you have to do is secure it in place and fit a door catch.

SOLAR IS NOT ABSOLUTE

A 200w solar panel does NOT supply 200w of power. It will only supply 200w of power with the sun directly overhead in clear sky and clear air - ie lab conditions. In the UK and Europe the sun is never overhead. So the most you will ever get is around 180w for an hour either side of midday at the June solstice - June 21st to be exact. From that day forward, the amount of power you get reduces steadily day by day until you get to the shortest day December 21st, the winter solstice. And to make matters worse, a panel only provides the max, if it is at 90 degrees to the sun. But a panel on a van roof is flat, which only makes it worse. At midday on December 21st if the sun is shining, that 200w panel will produce about 20w - and then only for a couple of hours.

The bottom line is that you simply can't get enough power from your solar setup in winter. You have power to burn in June, and next to nothing in winter. So if you use the van in winter you have to have a completely different routine to what you do in summer - you have to drive more, or plug in more. 

Full timing or long periods off grid in mid winter is very hard on solar alone.

BATTERIES in 2024

After years of messing around and researching, I can now say that lithium batteries have arrived! A cheap crappy 100ah old fashioned lead acid leisure battery is around £100 and will last about 3 years, less if abused. A decent one - Varta etc - is now about £150 and will last a bit longer - up to 5 years. But a half decent 100ah lithium battery with bluetooth will now cost about £350, will deliver 50% more power than a normal battery - which means that a 100ah lithium is as good as a 150ah lead acid battery, and it will last 10 years or more.

So the longevity alone is a good reason to buy - if you intend to keep the van that long. But the number one reason to buy lithium is not any of the above .... it is INFORMATION! Most (but not all) lithium batteries are "smart" - they have bluetooth and come with an app - and the app tells you everything about the battery. This info is priceless because it allows you to plan. It tells you exactly how much juice you have in the battery, how much you are using when you switch things on and off, and how much charge you are getting. This feature and this feature alone is worth upgrading to lithium, because the app is just so bloody useful! After a few weeks of use you soon know exactly what effect every activity has on your power situation. This allows you to make decisions based on facts, not guesswork. Wonder whether you can stay an extra day off grid? The app will tell you. Want to know how much solar is giving you? or an hours driving? The app will tell you. 

Reasons NOT to buy lithium? if you only use the van for summer weekends, or you mainly use campsites and plug in, or you drive every day for at least 2 hours - then you don't need lithium. And you have never had any complaints so far.

Reasons TO BUY lithium. You use the van a lot, you prefer off grid, or at least to have the choice. You use a lot of power ... or for the last few years you have been frustrated with your power system - these are all good reasons to upgrade.

And if you love your van life and dream about lots of power - then go large! Spend a grand and get 200, or 300 or even more amp hours of lithium - it is life changing .... it was for me.

PORTABLE POWER STATIONS - JACKERY BLUETTI ETC

In the context of a motorhome these have zero relevance at all. A PPS is nothing more than a battery and an inverter in a pretty box. There is only one reason to buy one of these, and that is to deal with situations where you need power and you are nowhere near any vehicle or building or power source - ie camping in a tent in the wild - in those situations they are good - if you can carry them.

But in a motorhome they have no use whatsoever because you already have a battery. So if you want more power in your motorhome you simply upgrade your system to satisfy your needs. And that upgrade will be well under the cost of any of these PPS devices.

And these devices are also misnamed - they are not "power stations" - they are batteries, and just like a battery, they have so much power, and when that power is gone, they need recharging - from plugging in or solar. Which is exactly the same as the electrical setup in your motorhome.

Technically, they are rated in watt hours, which confuses many people. Motorhome owners are generally used to thinking in amp hours at 12/13v (leisure battery voltage). To convert, you just divide the watt hours by 13. So a Jackblue portable power station advertised at 800wh, is the equivalent of a 60ah 12v battery. And will most likely be at least twice the price.

I am not saying the products are rubbish, just that they are not relevant at all in a motorhome - unless you also need to have power away from your van, in a field or on a mountain.

CHARGING THE ENGINE BATTERY 

A common question is how do I charge the engine battery? The short answer is, in a classic Hymer, you don't. The engine battery has only one purpose and that is to start the engine. When the engine is started, the alternator soon puts back the power that was used to start the engine, and you should always arrive at your destination with the engine battery completely full, and it will stay that way for months, ready to start the engine again. Self discharge of a starter battery is around 3% a month, so it will hold charge for many many months.

So a classic Hymer left the factory with no facility to charge the engine battery from anywhere except the alternator.

But then as mobile phones, satnav and reversing cameras arrived - many if not most of these were powered by the cigar socket in the cab - the nearest and most convenient source of 12v power - and the cab cigar socket is powered from the engine battery, not the leisure battery, because it is a Fiat or Mercedes factory feature, not Hymer. So then owners were arriving on site and leaving all that gear plugged in, or just having the radio on for a few days, and even if plugged in to EHU, or had solar fitted, found they had a flat starter battery.

This became such a "thing" in the 90s and 00s that dealers and manufacturers were getting sick of the support and warranty claims, which were not their fault. So products started to appear on the market to "fix" the "problem". The most famous of these is the BatteryMaster - this is a very simple box that connects between both batteries and allows some charge to bleed over from the leisure battery to the starter battery to compensate for this extra load and alleviate the problem.

Then solar controllers started to appear with dual outputs, and some later vans with Elektroblocks also had a trickle charger output to the engine battery.

Another factor involved was that back when the vans were new and expensive, many owners fitted expensive alarm systems, and alarm installers just connected these to the engine battery, as they did with cars.

So just like the horrible cigar lighter socket became the default 12v source in the cab, so engine battery charging from all sources also became a thing.

But it really should not be necessary in a classic - what is actually needed is to get back to original specification and not have anything plugged into the cab cigar socket, or if there is, to unplug it while parked up. Better still, it is better to never use the cigar socket at all - they are horrible things - and install proper power sockets in the cab, powered from the leisure battery. Modern life in the 2020s is almost completely powered by USB, so having USB sockets installed is the best answer.

So the general attitude has been that there is no point in wasting valuable leisure power by diverting it to a starter battery. But on the plus side, modern solar is now so cheap and efficient that you can afford to divert a bit to the engine battery, and that in any case, all that equipment is part of the van's general setup anyway.

So in terms of what the classic Hymer should do - the first thing is to be aware of the issues, and then take a decision based on your circumstances and lifestyle. Either you modify your van so that everything is powered from the leisure battery, leaving the starter battery full all the time, or you accept that some stuff is powered from the engine battery, and you modify things to replace that charge. It doesn't really matter what you do, as long as you prevent the classic situation of enjoying a nice stay somewhere, and then not being able to start the engine to drive home.

BATTERY VOLTAGE GOTCHAS - STATE OF CHARGE

One of the commonest mistakes that many owners make is seeing a (lead acid) battery voltage of 13v or more and assuming that the battery is full and all is OK. It's not! Any battery on charge, in any condition, flat full or empty, will display a voltage over 13v - IF ANY CHARGE SOURCE IS PRESENT! So if the engine is running, or you are plugged in, or solar panels are working in the day time, then the voltage will be over 13v regardless of the state of the battery.

The ONLY way to tell how much charge a battery has left, with minimum equipment, is to measure it's "resting" voltage. Resting voltage is a battery doing nothing - no charge sources present, and no major discharge going on - having a couple of lights on is OK. And it needs to be resting for at least 15 minutes.

Once the battery is at rest, then its voltage is an accurate indication of its state of charge - ie how full or empty it is. For a lead acid battery, fully charged is 12.8v, and just about empty is 12.0v. Google "12v battery resting voltage" and you will see lots of pretty tables you can download and print. And it's linear - so 12.4v is 50% full.  At 12v, decimal points of a volt really matter, so if you do this you need to use a multimeter - the Hymer panel voltmeter isn't accurate enough, although if you compare the needle position with your multimeter readings, you can get a reasonable idea from it - ie you know what position the needle is in at 12.8v and also at 12.0v.

But if solar is connected, even a bit of sun can up the voltage, so unless you can switch off your panels, the best time to do a resting voltage measurement is after dark. Same goes for EHU and engine - must be off.

Once you understand this simple principle, it is easy to measure and estimate lead acid battery performance.

You can also buy and fit "battery monitor" devices which do all this for you and give you a useful percentage display - but that's another subject.

OLD ALARMS

When the vans were new they cost big bucks and plenty were stolen. owners rightfully installed alarms, sometimes it was even an insurance requirement. But wind the clock on 30 years and these old alarms can literally ruin a holiday and strain a marriage! Most of the companies that made and installed them have either gone or been absorbed, and unless you have original documentation, and there is info still available online, it is really hard to unpick a problem with an old alarm. The ones with a fob that immobilises the engine are the worst - they are a time bomb just waiting to blow up in your face at the most inconvenient time. One day, they just stop working. if you are lucky, all you need is a new battery in the fob, and if you are that lucky, then buy a lotto ticket that weekend. But what usually happens is that after 30 years of solid service, eventually, after all those temperature changed and periods in storage, year after year, a bit of corrosion or vibration creeps in and one day a connection is broken or a circuit board gives up and you are bang in trouble. 

And it really can be a nightmare - these alarms were meant to be secure - so they can't just publish all the details on the internet! So troubleshooting is next to impossible. It is also quite difficult to disable a diesel engine. The Fiat engines have an electric fuel shutoff valve, which is usually disabled by the alarm, and as such is usually easy to fix. But the Mercedes (pre 95) have mechanical/pneumatic (vacuum) shutoff valves, so were much harder for alarm companies to disable, and as a result can be complex to untangle.

So what to do if you have an alarm like this? You either have to know about it in advance and how it works, so that you can un-immobilise your engine if it ever fails. Or you simply take it out - either yourself or pay a modern alarm installer to do it for you, or at least have them show you what to do and how to start the engine again if it happens to you.

Classic vans hardly ever get stolen - the thieves go for modern vans - so having no alarm is no big deal. And if you do want security, you can do a lot better with modern gear - mechanical immobilisers are easy and cheap to fit, GPS trackers are dirt cheap, and security is now at the next level with the internet of things - the same technology that gives us web cams, baby cams and doorbell cams now also offers us as much or as little security as you could possibly need for your van.

If you do decide to have security items permanently connected in your van, then make sure they are powered from your leisure battery, preferably with solar. Then solar should easily replace the power used by these devices, even in winter storage - and then no matter what happens, your van engine should always start after a period of storage. Another tip if you do suffer from flat engine battery after storage is to just remove the negative cable from the engine battery so it can't possibly discharge slowly. But this is a workaround, not a fix. You can also buy battery disconnect switches which do the same thing only more conveniently, and these can also double as useful security devices. You can even buy ones that are remotely switched with a fob.

I am not saying that security and alarms are unnecessary - just that any alarm installed 30 years ago is a problem waiting to ruin your day. 

LPG

I LOVE LPG! I really do ... the best money I ever spent (other than lithium!). But just as with solar and batteries, it is all down to how you use your van. If you are a summer user, and stay in the UK, and only buy bottled gas occasionally, then you don't need LPG. 

But if you use the van a lot, especially in winter, and travel to different countries, then LPG is a must, and it pays for itself in a few years. 

The cheapest setup is a single refillable 11kg bottle - but 2 are better. LPG is about 50% cheaper than Calor - Calor is a rip off! And no matter what you read, LPG is available everywhere except Morocco. Yes it is becoming less common on forecourts in the UK, but every big town has a supplier, all you have to do is use the LPG app and fill up when you can. LPG is NOT disappearing in the UK, it will ALWAYS be available - you just have to plan forward and fill up where you can, using the app. The only time LPG is a bad idea is if you are unlucky enough to live in the van in a remote area like Cornwall or Scotland, and you don't drive very much - ie you don't tour around - you use the van more like a static. In these cases you are better with bottled gas. But if you use the van for what it was designed for - holidays and touring, then LPG is available as you go - just not on every forecourt - and it has never been on every forecourt.

Underslung tanks are OK, but the only reason to fit a tank is if you want to use the gas locker for other things. A good example of this is so that you can use your now empty gas locker for bigger batteries.

USING CAMPSITE EHU POWER TO HEAT THE VAN - VS GAS AND WILDCAMPING - THE NUMBERS AT 2024 ENERGY PRICES. 

Power is now around 25 to 30p per unit, which is a kilowatt hour. The standard Truma 3002 is a 3kw heater, and is rarely used on full - average is about 2kw to keep a 5 series van warm in average cold weather. Same goes for any fuel. So you are looking at around 60p an hour to run a 2kw electric heater. So 24 hour use is as much as £14. And people do! That's why EHU pitches are now £25+. An oil filled rad at about 800w (0.8kw) i much cheaper, but the site owner doesn't know what you use. But many of them are now wise to the fact that owners run electric heaters, so they assume the max, or fit meters, or fit low amp breakers that will trip if you try and use heaters.

Only farmers who have big power bills anyway don't seem to bother - I can still find farm sites with EHU at around £15 but they are getting rarer - the going rate for a pitch with EHU is around £25, even more in posh places.

Now contrast that with gas. Calor is crazy expensive these days at around £50 for 13kg, but still comes in at around £2 a litre, or £4.00 a kg.

The Truma 3002 heater, fitted to most classics, consumes 30g/hour on low, and 280g/hr on max - and you rarely run it on max unless it is sub zero outside, so I suggest 150g/hr is a reasonable average. That gives you a running cost for 24 hours of around £14 - which is roughly the same as electricity at 30p a unit. So there is little cost advantage between the two, except for the difference between the power cost of the pitch and the actual cost of the pitch, which for a £25 pitch is £10. So wild camping on Calor gas is roughly £10 a day cheaper than a £25 pitch.

Now consider LPG, which is 80p a litre from a gas place, and around £1 on the forecourt. half the cost of Calor. Now you are paying £7 a day for the same heat output.

So refillable LPG is by far the cheapest way to heat a van off grid - so unless you can find an EHU pitch for a tenner or less, which is almost impossible, or any other place with "free" power, a £25 pitch with EHU is more expensive by far. Of course you have to take into consideration the installation cost of LPG, but for all year round full timers it soon pays for itself, especially if you always wild camp.

But even Calor is around £10 a day cheaper based on a £25 EHU pitch cost.

But if you always prefer to pay for a site pitch with EHU, at £25, because you don't like wild camping, then using a 2kw electric heater will save you around £10 in Calor gas, or £5 in LPG. And if you prefer no heating at all while in bed, and only use a 2kw heater for 12 hours a day, then those figures reduce by 50%.

Fridge on gas and cooking are of course another factor, but they take far less gas than heating. A fridge consumes 11g/hr on gas, which is around 0.25kg per day = £1 on Calor, 50p on LPG.

OK sounds complicated, but it isn't. Once you know the consumption of the appliances - my figures are from Truma's website - and the cost of the fuel, plus the cost per unit of electricity, then it's simple arithmetic.

ENGINES and TURBOS

I have lost count of the number of posts I see about owners asking about bigger engines or turbo conversions because their van is "too slow". Forget it. An engine change or a turbo conversion is a big job, and at commercial rates is simply unaffordable. The only time it is possible is if you either have the skills, or know somebody who has the skills, for free. It is the labour that costs money and with commercial labour at £50+ an hour it is simply uneconomical. Yes there are plenty of owners that have done it, but they have done it themselves. If you don't have the skills or facilities, then forget it. It is cheaper to sell your van and buy a faster one. 

And don't forget that you spend 90% of your time living in the van, and only 10% driving it. I have done 200k and 30 countries over 15 years, and a bigger or faster engine would have meant absolutely nothing.

SILVER SCREENS

Marmite! I hate them, why live in a padded cell with no view from those magnificent panoramic windows. Where do you store them? Do you fancy putting them up in the rain? Or packing them away sodding wet?

But it's horses for courses. If you are parked up in one place living in your van all winter, then screens will keep your van warmer and you will use less gas. Ditto if you are a winter sports fan and do regular ski holidays, and don't care about the view.

But for "normal" use .... well I can't tell you what to do, but do think about it. I had them on my first van 25 years ago and they were used twice and then sold. Or maybe I burnt them.

INTERNET AND WIFI

For solid wifi internet in the van you only need to spend £50 plus a PAYG data sim card. You don't need expensive gear or roof mounted antennas, and whatever you do don't buy a Mifi. 

If all you need is occasional internet - use the hotspot on your phone, not a mifi - they are technically identical.

Read this blog on the subject.

https://hymers700.blogspot.com/2022/11/no-need-to-spend-big-bucks-to-get-wifi.html

TV

Satellite TV in a van is now dead and buried, unless you already have it, there is no need to buy it. It is expensive.

For "normal" TV in the UK, all you need is a normal TV and either a roof mounted caravan TV antenna, or one of those little internal ones by a window. Terrestrial Freeview in the UK is available almost anywhere, and signals are far stronger than they used to be.

For all other TV - internet is the way to go. A 12v router costing £50, a PAYG sim card loaded with 200gb of data for £20, and either a smart TV, or a normal TV converted to smart by the addition of an Amazon Firestick plugged into its HDMI port, and you are good to go for at least a month. 4G is almost everywhere, and is almost always fast enough for rock solid TV - Iplayer, ITVX, Netflix, Amazon prime - plus if you are a bit of a nerd like me there is no end to the stuff you can get at.

As always, roaming abroad is more difficult because of UK roaming limits, but everything is doable with local sim cards.

RADIO

If you have a working FM/AM cab radio and you are happy with it, then great. But just like TV, radio is going internet. Whenever I want radio, I just use the phone and bluetooth to the car speakers. Drop outs occasionally happen, but I once drove from Manchester to Dover and it dropped out once.

DAB RADIO

Dab radio is a failed technology - forget it. Fine if you are not moving, but in a moving vehicle it is rubbish. Use internet radio instead.

STARLINK

And for those who must have fast internet 24/7 everywhere, Starlink is almost here. I say almost because at present it is a bit too cumbersome and power hungry for "normal" use. But soon I expect them to offer a simple dome on the roof that doesn't use too much power for about £50 a month. If comms and internet are your thing, then keep an eye on Starlink. In Jan 2024 they are already trialling Starlink direct to a normal mobile phone, no need for any extra equipment. In the next 2 or 3 years high speed internet and coverage everywhere on the globe will be an affordable reality. But right now (2024) it is cumbersome and consumes about 5 amps - too much for most owners and battery systems.

THETFORD TOILET AND SOG SYSTEM

There is a world of difference between occasional holiday use and full timing!

I have seen guys on camp sites empty their cassettes every morning and then rinse it out seventeen times, and then spray it with air freshener.

Mine on the other hand has been in constant use for 15 years - I have worn out 3 cassettes. I have had my hand inside it to retrieve stuff and I have emptied it in some horrendous disposal points. You just get used to it. If we have to, we can get a week out of a cassette - don't ask me how! But if you are in a gorgeous spot with no easy disposal then you can do it. I have also carried full cassettes on a scooter and a push bike, and even carried one through a crowded village - wrapped up in black bin liners - nobody even knows. It's the reality of full timing in out of the way places - you just do it.

My conclusion is that considering what it was designed for, the Thetford cassette system is worthy of great respect. Spare parts are available, and once you understand the basic design, it is easy to work on and fix. 

But I have no time for the SOG system, but only in the context of full time use year after year. The filters are expensive, the fans break, and I have spent more time fiddling with SOG than I have with the toilet itself. But for holiday use it is fine - it does what it says on the tin. But I wouldn't bother with it for full timing - it's just another thing to break.

In terms of fluids, I have tried everything, and I always end up back with Elsan Blue. Second choice is Thetford - but it's expensive. The green eco stuff is just that - eco stuff. But eco is the least of your worries when full timing. Bio washing powder or tablets, and cheap dishwater tablets also work, to an extent - I use them when no fluid is available, which is quite often when you tour places like Albania!

Disposal - after the first few years disposal is never a problem. Best thing I ever bought was a set of manhole keys and levers and the gradual knowledge of how sewer systems work, and what you can get away with. In 20 years and 30 countries I have never dumped irresponsibly - but I have dug some pretty deep holes.

If I was building a new van from scratch I would install one of those separator loos - they are really good. But I wouldn't go to the hassle of ripping out a C3 in a Hymer and installing one - the ergonomics don't work. One day somebody will design a separator the same dimensions as a Thetford C3 bench - and I would buy that.

PLATING, WEIGHTS AND DRIVING LICENCES 

All UK driving licences after 1st Jan 1997 allow the holder to drive a van up to 3500kg - and the DVLA tax class for a van under 3500kg is PLG - private light goods. And the tax amount for this is more expensive than for vans over 3500kg - perverse, but true.

Anybody who passed their test before 1997 is entitled to drive a vehicle up to 7500kg. This is nowadays called a C1 entitlement, also known as Grandad Rights.

Any van over 3500kg is classed as PHGV private heavy goods - ie a heavy goods vehicle used privately. The tax payable on this class is less than PLG under 3500kg. Don't ask why, it just is.

Generally speaking in classic Hymer terms, the 5 series vans are 3500kg or less, and the 6 and 7 series vans are plated at 4600kg, so need a C1 licence. 

Those are the basic facts. But then the whole thing can get very murky and confusing because most UK classics are imports, and many vans - many more than you would believe - are imported with incorrect V5s. Back in the 90s the DVLA were pretty lax with imports - they would basically stick anything you told them on the V5. So all sorts of weird situations have happened. I have seen S660s that are plated at 4600kg with V5s that state 3500kg. I have seen weight plates (VIN plates) either defaced or altered. I have also seen 5 series vans with plates that say 3100kg or 3200kg and hardly any payload - but in the UK these can be legally re-plated to 3500kg - there are specialist firms such as SV Tech in the UK that do this. 

So the moral is - the V5 on an imported classic is not gospel, and the DVLA do make mistakes. So if in doubt - check. Ask on the group.

SOLAR PANEL FIXING - SCREWS OR GLUE OR BOTH?

There has been a long running debate on fixing solar panels to Hymer roofs. The argument is what sort of bracket, and what sort of fixing. 

The problem for me and the group is that we are amateurs advising other amateurs, so although nobody is expecting to be sued, there is a moral responsibility to ensire sound advice. 

The bottom line is that adhesive only can work, and is used by professionals, often on newer vans. But pros work to a formula - a carefully designed formula, and they never deviate. Adhesive only fixings are all about surface preparation, choice of adhesive and final application, plus curing time. But for the amateur, there are too many variables, not the least of which is their own ability and experience.

The Facebook group has several professionals and ex professionals in the background, and they report that this is argued about even within the industry. But our professionals (and me!) are unanimous - when installing panels on a classic van, using both is the easiest and safest option, because it offers the best of both worlds.

Our professionals are adamant - all the stories of panels coming off are usually always those that were adhesive only, and the majority of these could be narrowed down to bad surface preparation - if you use glue on a dirty surface you are asking for trouble. Even using the wrong type of solvent cleaning fluid can be a factor.

Of course owners are free to do whatever they like, but if the question is asked on the group, screws and adhesive is the answer. My panels went on using this method in 2015 and every year I inspect and give a really strong pull. No change after 8 years. So at least I have some evidence to back it up. 

With regard to brackets - the big plastic ones do not have a good reputation - aluminium is more popular. 

And while we are on the subject of panels - flex panels are rubbish and a nightmare to get off when they break. Rigid panels are the way to go.