On 29/09/2017 18:28, Chris Hogg wrote:
On Fri, 29 Sep 2017 17:30:52 +0100, Bob Minchin
wrote:


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?

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.

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