Frost free upright freezer problem. Long Post

I have a Bosch full height upright freezer that is (meant to be) frost free. It has the evaporator at the top and a fan blows air around the cabinet to freezer the contents. Normally as good as good and anytime you look at the evaporator it is either completely clear or the slightest film of frost visible.
How do these things work? I assume that there is an electrical heater on the evaporator that comes on after the compressor switches off to melt the frost and the water drains away. If there is cooling demand, it either abandons the defrost if unfinished or perhaps allows it to finish and then commences cooling?
Am I on track?
The problem is that I noticed that the frost had built up seriously a few weeks back and the temperature was rising inside. We *think* that the door had got left ajar but no-one will confess to this. I then spent some time playing a hot air gun over the evaporator melted all the visible frost and after switching back on normal temperature was achieved.
But slowly over a period of weeks, the frost has been building up but the temperature has been maintained ok.
I can imagine a conflict between the defrost process and the demand for cooling. Maybe the defrost part uses a 'just above zero" temp sensor to signify frost clear and that is conflicting with the cooling cycle?
I wonder if the heating cooling algorithm can't cope with the remains of ice that I did not melt away fully and it is slowly building up again or perhaps if the heating element has failed.
In normal use (without the door being left ajar!) the only source of frost should be a bit of warm air let in when the door opens and so it does not need a huge defrost capability.
I have had one suggestion of letting the whole thing defrost naturally over a couple of days, but the trouble is that it holds the vast proportion of our frozen food stock and we would need a dedicated feeding frenzy over several weeks to empty the thing.
Thanks if you have read this far! Any suggestions please folks?
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On Fri, 29 Sep 2017 17:30:52 +0100, Bob Minchin

Two thoughts.
1 - you have a leaking door seal (or a leak elsewhere) that slowly allows moist air from the room into the freezer, or the door isn't shutting properly (BTDT etc).
2 - to defrost it, get a _large_ cardboard box and line it with sheets of polystyrene, say 1" or more thick, bottom, sides, and a panel for the top, all cut to size and close fitting. Freeze several plastic bottles of water, or get a good collection of ice packs / freezer packs, and freeze them. Put them in the box, along with the contents of the freezer. Repeat until freezer is empty, then turn freezer off, leave the door open, perhaps direct a fan heater into it set on 'low', and put plenty of old towels in the bottom and on the floor in front of it to mop up the melt-water.
The cardboard box/boxes will keep the stuff cold for long enough to allow the freezer to defrost.
--

Chris

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You summarised it quite well - My daughter had one (not Bosch) and I had to replace the timer as a gear had stripped I recall.
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DerbyBorn wrote:

Thanks DB I'm not sure if mine has a timer module or if it is electronic. The badge on the front says "computer control" but that could be marketing garbage. I'll have to drag it out from the wall and see if there is a module round the back. I'll spend some time on YouTube thanks for the prompt. I should know that nearly everything appears there.
Bob
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They can get overwhelmed with ice due to an "incident" and then the defrost time doesn't quite clear it. A full manual fefrost is the way to go.
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http://www.ebay.co.uk/itm/Supco-Universal-Domestic-Fridge-Freezer-Defrost- Timer/282165782144?epid65909042&hash=item41b2642680%3Ag% 3AtMoAAOSwgmJX0DkL
http://tinyurl.com/y7egqbbq
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This and other videos will reveal the operation:

https://www.youtube.com/watch?v=-L80Fk5had4

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wrote:

Sounds unlikely given that it must be moving air from outside into the inside in normal use, not just moving the air around inside.

Ditto.

More likely the control system isnt doing what it should for some reason.
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On 29/09/2017 18:28, Chris Hogg wrote:

Rather than carboard boxes, I have used the fridge, which is much easier and quicker than the freezer to empty by eating everything - especially if done at the right time of the week.
If there is a removeable panel in the back of the freezer, a hairdryer or a steam wallpaper stripper will soon clear any ice behind it.
Make sure that the drain is completely clear. Any ice in there will cause a continual build-up when the water from the heating cycle of the frost-free function cannot drain to the evaporation tray.
SteveW
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Bob Minchin wrote:

If you can, take the cover off the evaporator plate and see if it is all frozen up inside. If it is, the timer could be the fault. Maybe. There should be/might be little white plastic plugs holding the cover in place.
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Mr Pounder Esquire wrote:

The evaporator seems to be just tubes and fins with air blown through. I have a hunch that my hot air defrosting is just melting a little ice which then drips down onto other ice and freezes again.
I'll try looking at the control board again. It has a couple of triacs on it and I suspect they switch the fan and the heater.
If I can be certain which is the defrost heater, I might be able to power that up manually, leave the freezer loaded but switched off and accelerate the defrost process? Worth a go I reckon.
Bob
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Bob Minchin wrote:

I called it a plate in error. Frost free do indeed have tubes and fins with a fan blower motor. My experience with refrigeration ended in 1999. So you did take the cover off then?

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Mr Pounder Esquire wrote:

Yes the front cover is off exposing about 5 individual plugs, one of which I'm pretty convinced is the defrost heater having looked at pictures of replacement one. One pair of connections measures 280 ohms or about 200 watts on 240v which seems about right for the job.
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Bob Minchin wrote:

Not all frozen up then?
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Mr Pounder Esquire wrote:

over a few weeks. I am becoming convinced that a good defrost will sort it out but will try direct connection to the defrost heater with the contents inside and a thermometer monitoring the contents temperature.
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On Friday, 29 September 2017 19:19:35 UTC+1, Bob Minchin wrote:

l
n
a classic formula for a fire with a frost-free.
A 2 day defrost really is the first move. If the problem recurs, you may ha ve a bad timer or defrosting element. The long term solution is to get a tr aditional freezer, that way you get to choose when to do the annual defrost instead of the freezer. And it lasts longer.
NT
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Only if you are actually stupid enough to want to waste your time manually defrosting for the rest of your life and have somewhere to put the contents of the freezer while you defrost it.

I have never had to manually defrost my frost free freezer or fridge either.

I don’t care.
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snipped-for-privacy@gmail.com wrote:

Well only if you think I'm daft enough to leave the defrost heater on without monitoring it carefully. I did keep an eye on it as well as a thermometer probe in with the food. I turned the heater off at -15C and it is much improved now. Overall I think you are correct in that a long term defrost will be the complete cure. So we are working toward condensing/reducing our frozen food stocks to take this one out of service for a couple of days.
In the meantime I shall devise a method of a much louder alarm than the pathetic squeak it now gives out which can't be heard outside of the utility room. I'll link it into the phone bell repeater system which covers the house and workshops.
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On 29/09/2017 19:10, Mr Pounder Esquire wrote:

That is what I have done to a frost free freezer a long while ago. I now avoid them.
If you can remove the rear panel inside the freezer then investigate and thaw all the ice you can see. The plastic doesn't take much heat so be careful.
Some posters have mentioned control and timer systems, but once the heat exchanger is clogged with ice, no heating (frost free) cycle seems able to clear it.
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Fredxxx wrote:

New home fully modernised 9 years ago. There was no way a frost free fridge freezer was going in there. My conventional fridge freezer is set on Number 2 and works fine. The compressor is very quiet and does not run for too long. I am aware that the thermostat is getting to the end of its life, but I can get one from the net for about 10 quid. Dead easy to fit.

Yip.
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