"Mike" wrote in message
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"John" wrote in message
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"Mike" wrote in message
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The recent very cold weather has thrown up a problem I hadn't noticed
until now.
Boiler is a Grant 70/90 Euroflame and it's 25 kW output should be more
than enough to heat the house through about 20kW rated radiators.
Grant were one of the early entries into the Condensing oil boiler market
but are not one of the frequently encountered manufacturers up North at
least. Is your model a condenser or a non condenser?
Non condensor
And indeed it did when the weather wasn't so cold outside. But now
it's
colder it can't keep up. Have traced the problem to the boiler output
temperature rising up to the cut-off limit (just under 90 degC) then
turning
off until the pump circulates the water enough for it to drop to the
turn-on
value (appears to be around 75 degC).
With the boiler in it's on or off state, the return temperature is
about
65 degC plus or minus a degree or two.
The hysteresis your stat exhibits is perhaps a bit excessive and a
suspect
stat may be involved here, however your return water temperature appears
to
show little variation between cut out and cut in on the boiler
temperature.
This does suggest the rate of flow through the radiator system to be too
low.
What should the stat limits be. The upper limit is programmable via a
knob
on front of boiler but manual doesn't say if the lower limit is fixed or
moves with it.
You will need to ask Grant for this info but some boiler stats have a small
differential adjustment screw concealed about the body somewhere. I'd try to
be certain that the flow rates through the boiler are adequate before
upsetting the factory adjustment of this though
Have tried the pump (Grundfos Alpha) in every position on it and all
give
the same effect, though some vary the time taken that the boiler spends
on
or off. But typically it is 2-3 mins on, 3-4 mins off. Obviously if I
could get it to be on a bit longer and off less the house would be warm
again.
I think you should get hold of a differential thermometer with sensetive
clip on probes or a non contact IR thermometer and check the differential
temperature across each radiator under steady-state conditions. If your
boiler is non-condensing the differential should be approximately
10/11degreesC.
Am using a Maplin non contact IR. As the system has three CH zones I
think
in better weather it got the system up to temp and then everything was ok.
But now under heavy load it can't get there.
If you study the FAQ on balancing a system it will help you to understand
the aims of balancing a system correctly and if your radiator load is as
you
mention the performance should be closer to the expectations.
That was what I used. Most helpful. I thought I had got the system to
around 80 deg in, 65 out for most radiators but as it turns out this was
only when the room thermostats were being overridden one by one.
I'd try turning all your stats full up so that all rads are operating
together before balancing the system and then go for a lower differential
across them say about 10/11 degrees C. As zones close down on the stats the
pump head will increase marginally (circulators are centrifugal) subject to
the response of the Alpha. Increased head will give a slightly faster flow
which in turn will reduce the differential tamp across the rads but not by
much. The reduced Dt will give a higher mean rad temp which will be to your
advantage
Does your assumed load include the hot water demand also or is that on
top
of your
figures?
There is provision for a hot water feed but it is currently not activated
as
I am going to be using a small hydroelectric generator to drive the
immersion heater so hopefully the boiler won't be called upon for this.
Sounds good - do you live near a fast flowing stream? I have a hankering to
move somewhere hilly with a rushing torrent when I retire and do something
similar. If the water is pure I'll install a sandfilter and UV unit to
become more or less self sufficient energy and water wise at least
Thanks
Mike
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