thermostats

Premier_2

espresso wrote: »

Pity that you can't admit that you are wrong.

:rolleyes:

Automatic bypass valve




Safety temperature limiter




Also: Fitting thermosatic radiator valves (TRVs)

"I already have a thermostat on the wall. Won't they work against each other?"

Yes, but only if they're in the same room. Your wall-mounted thermostat is very commonly in the hall, so you shouldn't have TRVs there. If you do, the radiator's thermostat will shut the radiator off when the temperature is reached, only for the wall thermostat to fire up the boiler and compensate for the cool radiator. And so on, and so on. End result: your boiler takes a hammering and the room stays cold.

How do I do it?
...
. Put them on each radiator, but not in the living room or the room with the thermostat dial

http://www.bbc.co.uk/bloom/actions/radiatorvalves.shtml

espresso

Premier wrote: »

I was referring to the safety temperature limiter within the boiler itself

I don't see why you have muddied the waters by talking about the "safety temperature limiter" as it's irrelevant to this discussion.

Cardew

I see where Premier is confused.

He has got the safety cut out confused with the boiler water thermostat.

The boiler thermostat shuts the boiler off and on as it reaches the temperature set on the dial - in my case that is from 64C to 82C.

However should that thermostat fail, the boiler will carry on firing and the water in the system would boil. To prevent this there is a safety device - the safety temperature limiter in the diagram above. (in my system it is called the overheat protector and when it operates it(electrically)drops out the valve and that puts out the pilot light.

Anyway rather than scoring points, the OP(and anyone else) should not worry that he has a TRV on each radiator. I have, and so have millions.

Similarly if you don't have a room thermostat, or have it turned up, your system will work exactly as intended and the boiler will switch off as it is designed to do.

mech_2

Premier wrote: »

With respect to the adjustable thermstatic control knob on the boiler, this again is something entirely different to the safety temperature limiter.

Well I'm assuming the boiler thermostat is what Cardew was originally referring to. A safety device is irrelevant in normal usage. The thermostat would kick in first.

If your property take too long to warm up whether in winter or summer no matter how it is set up, again it suggests a fault in the system design (e.g. wrongly sized radiators. edit: or your boiler is oversized)

I don't think that's relevant. It's the relative amount of time the air in the room takes to reach the setpoint on the roomstat compared to how long the water takes to reach the boiler thermostat's setpoint. Heating a roomful of properly cold air will take more than a couple of minutes. Air only convects so fast and a cold brick building soaks up new heat like a sponge. In contrast, pumped water transports heat a lot quicker; that's why it's used. The water in the radiators will get up to temperature first, pretty much irrespective of what size they or the boiler are. The radiator sizing calculation will assume they're operating at a typical boiler thermostat setting.

Premier_2

mech wrote: »

Well I'm assuming the boiler thermostat is what Cardew was originally referring to. A safety device is irrelevant in normal usage. The thermostat would kick in first....

Maybe my mistake, but I'm sure he was referring to a bypass valve in post#14

mech wrote: »

I don't think that's relevant. It's the relative amount of time the air in the room takes to reach the setpoint on the roomstat compared to how long the water takes to reach the boiler thermostat's setpoint. Heating a roomful of properly cold air will take more than a couple of minutes. Air only convects so fast and a cold brick building soaks up new heat like a sponge. In contrast, pumped water transports heat a lot quicker; that's why it's used. The water in the radiators will get up to temperature first, pretty much irrespective of what size they or the boiler are. The radiator sizing calculation will assume they're operating at a typical boiler thermostat setting.

Simple heat transfer theory.

If the ambient temperature of the room is cold, the radiators will give out heat to them. Therefore the return temperature of the water will be lower than the flow temperature. The boiler will then reheat the water to flow around the system again.

Only when the radiators do not give out heat (e.g. because they are turned off, perhaps by TRVs), or in the unlikely event the room was as nearing the temperature of the water being pumped, will the return temperature be approximately that of the flow temperature.

Cardew

mech wrote: »

Well I'm assuming the boiler thermostat is what Cardew was originally referring to. A safety device is irrelevant in normal usage. The thermostat would kick in first.

.

Yes of course it is the boiler thermostat, the safety device cuts off the pilot light and hence gas and the pilot light would have to be relit.

To be honest I can't even believe we are having this conversation.

Plenty of people have posted on here that they don't even have a wall thermostat, even with modulation the water will still get too hot and the boiler shuts off.

I was only recently reading on a plumber's forum where some plumbers were saying how silly it is that wall thermostats are now mandatory; and stated they do exactly whay i have done for 20 years - whack it up to max and use the TRVs to control the temperature in each room.

Why would you want one location with a room thermostat to have to be up to temperature to control every room in the house.

My wall thermostat is in the hall, which most of the time I don't heat.

Putting it in a reception room is no good as I have other recepion rooms I might use; why should I heat a room that I am not using.

The same if someone wishes to go to bed early and wants the heating kept on - if you rely on a room thermostat you are unnecessarily heating a reception room/hall.

Some people have a gas or log fire in a reception room - If you rely on a wall thermostat you will have no central heating in the rest of the house.

Many people get up early and go off to work without using a reception room. They only want bedroom, bathroom and kitchen heated; not the reception room with the wall thermostat.

Cardew

Premier wrote: »

Maybe my mistake, but I'm sure he was referring to a bypass valve in post#14


Of course I was referring to an Automatic Bypass valve(ABV) in post #14 . This was to refute Premier's erroneous statement that you must have a radiator without a TRV.

I gave the quote from the Myson website on an ABV

It eliminates the need to fit an uncontrolled radiator to act as a system by-pass(i.e.you don't need to leave off a TRV) and it also increases system efficiency. The automatic by-pass valve also aids the operation of thermostatic radiator valves.

Loads of websites explain the same.

This was nothing to do with Premier's equally erroneous position on water temperature.


Simple heat transfer theory.

If the ambient temperature of the room is cold, the radiators will give out heat to them. Therefore the return temperature of the water will be lower than the flow temperature. The boiler will then reheat the water to flow around the system again.

Only when the radiators do not give out heat (e.g. because they are turned off, perhaps by TRVs), or in the unlikely event the room was as nearing the temperature of the water being pumped, will the return temperature be approximately that of the flow temperature.

I simply cannot believe that Premier has posted the above.

A CH system is designed so it will heat the whole house to the desired temperature with the ambient temperature at yC. This might mean having to heat xx radiators.

XX Radiators will dissipate a greatdeal of heat from the CH water. However much of the time it might only be heating a couple of radiators, - the extreme example I gave is only a heated towel rail.

I don't know how low an output even a modulating boiler can go - 8kW? and some boilers didn't modulate(they were on or off).

How on earth would a heated towel rail be expected to dissipate that amount of heat so the water would require reheating?

Premier seems to think that the boiler will be firing continuously regardless of the load.