I have a back boiler with a thermostat on the top of it, scaled from 1-6. I
also have a room thermostat which just controls when the heating works as
far as I can see.
My question is what should I have the boiler thermostat set on? It is now on
'4' but is it more efficient to have it set on 5 or 6?
The hot water temperature seems ok, slightly too hot to hold your hand under
for long but I assume thats ok.
I am new to all this, as this is my first house so go slowly :)
The boiler thermostat controls the water temperature within the central
heating pipework. It basically determines how hot the radiators get.
The room thermostat turns the entire system on or off depending on how hot
the rooms are (not the radiators).
In terms of thermodynamic efficiency, any boiler (not just condennsing
types) will be more efficient at lower temperatures, so low is best.
However, there are two caveats:
1. This will promote condensing. Whilst modern condensing boilers have down
firing burners and drainage systems to take the acidic condensate away, and
ancient back boiler will have no such features. If water starts coming down
the flue, or forming on the heat exchanger, it will corrode. You'll probably
have to turn it well low to cause this, however.
2. You can't turn it so low that the radiators can't heat the room, or the
hot water cylinder never gets to temperature. The hot water cylinder stat
must be set at least five degrees below the water flow temperature, or it
may never turn off.
'4' sounds fine, provided the house gets hot enough and your hot water stat
is set reasonably low (i.e. 55C or thereabouts).
It should be a little box near the bottom of the hot water cylinder. It may
have a knob on it, calibrated in celcius. It will have a flex coming out
wired into a nearby junction box. It may be held on with a plastic strap
that goes all round the cylinder. You may confuse it with an immersion
heater, if you don't know what you are looking for.
You may not have one, in which cause, you should fit one and use it to
control the zoning arrangements. Otherwise, you are throwing money away.
hmm... no little box at the bottom...
shall i just chuck a wad of cash in the middle of my room and burn that
so how easy are these things to fit then? <grin>
just one more thing... 'aren't newsgroups brilliant!'
It could be anything from extremely simple to very complicated, depending on
what you have installed.
The following information should help in determining what your current
system is. We already know it is a back boiler.
1. How many pipes come out of the boiler? How large are they?
2. Does your programmer allow you select the heating, but have the water
turned off, or are you forced to have the water on? (Or you can turn the hot
water off, but it has no effect, the hot water cylinder is heated anyway).
3. Can you hear/feel the pump operating when the heating is off, but the hot
water is turned on?
4. Are there any zone valves? These are large valve, that look like a
stopcock, except the handle has been replaced by a plastic box about 8cm x
8cm x 6cm which a flex trailing out. We need to know how many such valves
you can find, where they are installed, and how much pipes come out of them.
First off... thanks for helping me out and appreciating I know absolutely
nothing about my system. This coupled together with the nice lady who
offered to take her trolley back with hers at Asda today reminds me how many
genuinely decent people there are out in this sometimes weird world.
To answer your questions as best I can... here goes...
1) Can't actually see my boiler, its all hidden in the wall behind the fire
on my chimney breast. I think to get to see all of this would mean some
deconstruction of the fire on the front?
2) Whenever the heating is on, the hot water is on. There is no option on
the programmer to have heating on without it.
3) (hangs head in shame) I don't know where my pump is either, although I
assume there is one somewhere. Just when I moved in I had the heating on for
a while to test it and the pump screamed for a while, but it was in the
morning and it stopped after 5 mins before I woke up to find it.
4) I am not aware of any zone valves.
I basically think its the oldest, simplest form of CH/HW there is. Its
reliable though, keeps the house at whatever temperature I ask, and makes
lots of nice hot water for my new power shower, bless it.
Is this any use at all?
Thanks again for your continued interest, if you're anywhere near Rotherham
I owe you a pint.
You probably have gravity circulation on your hot water cylinder. Adding the
thermostat to it may actually be quite difficult. This is because it will
probably need an additional zone valve and wiring between the zone valve and
the programmer. This is assuming the feed/vent arrangements are suitable for
the insertion of the valve. You may want to wait until the back boiler
expires, unless you are a "have a go" type. Let me know if you are!
I think I am an "if it ain't broke don't fix it" type. I think for the
upheaval that doing that would involve, I will pay more per month until my
boiler realises its in the new millenium and decides to retire.
Right, it's friday, enough CH talk... wheres the nearest drinking
Cheers again Christian, as a new home owner I am glad places like this
I suspect that you've got a gravity hot water and pumped heating system -
quite common in the 60's and 70's. If you could get to your boiler, you'd
find 4 water pipes connected to it. The 2 on the one side would be fat
ones - 1" or 28mm - and would go to the indirect coil of the hot water
cylinder - probably in the airing cupboard. Water circulates round this
circuit by gravity (convection) whenever the boiler is running, and the hot
water will eventually get very (probably too) hot, depending on the boiler
The other pair of pipes would be smaller (3/4" or 22mm) and would circulate
hot water to the radiators whenever the boiler AND pump are running. The
room stat would switch the pump on and off as required.
This sort of system is wasteful because:
a) there is no control of domestic hot water temperature, so it gets too hot
b) the boiler is on whenever the timer calls for hot water or heating - and
keeps cycling unnecessarily to keep itself warm even when both demands are
This can be improved quite dramatically by installing a zone valve in the
gravity circuit - controlled by a thermostat on the hot water cylinder - and
wired in such a way that the boiler only comes on when one or both demands
are unsatisfied. The parts required to do this can be bought for a few tens
of pounds - and the cost would be repaid very rapidly. Even if you pay a
plumber to do it, the payback should still be fairly quick.
Have a look at the C-plan on
http://content.honeywell.com/uk/homes/systems.htm and you'll get an idea of
waht is involved.
Thanks for that description, again in nice easy terms for me to understand.
It certainly sounds like the system I have, and is something worth
considering starting as I decorate through the house, to make sure the
additional wiring is all nicely tucked away. Will probably make it a gradual
job through summer to next winter when it will be most useful and save me
the most money.
Thanks again Roger,
Right... I am definately going to the pub now :)
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