Weather Compensation Combi Boiler

[Sarah29]

Hi - Am hoping someone can share a thought or experience using a weather compensation modulating boiler. 

 

I understand that we don't have any thermostats in the house if the weather comp system curves are setup correctly. All sounds great. But what happens if we have a separate log burner too. On a cold day when we have the log burner on, surely the boiler would keep putting heat into the room despite the room already being up to temperature?

 

Any thoughts would be greatly appreciated here, thanks

[SteamyTea]

My understanding is the 'curve' is the difference between the outside temperature and the heating flow, so if the room goes s hotter, for whatever reason, the boiler should compensate.

[Dave Jones]

with no room stat is measured the temperature of the flow and return and modulates based on that. Assuming you have rads (undefloor works same) the TRV valve in the room with burner would click off so the other rooms would heat up and click off as normal. The woodburner having the effect of speeding it up.

[JohnMo]

Even without room stats or any radiator valves the following happens.

 

WC flow temp is closely matched to heat loss, adding just enough energy toto keep up with overall heat loss.

 

The boiler is doing a couple of things to manage flow temp, trying to stay to target flow temperature and maintain a fixed delta between flow and return. So steady state it just ticks away. Start your fire, the follow occurs 

1. Room temp and eventually house temp goes up.

2. The delta between room and radiator or UFH decreases, so heat output reduces 

3. Return temp to boiler is higher

4. Boiler modulates the output to return the dT between flow and return.

5. As things get warmer the boiler cycles off.

6. Boiler circulation pump continues, as fire goes off and room cool, boiler will cycle back on.

 

So you have a flow temperature target of 30 degs. The boiler will have a hysterisis around the 30 degs to help stop cycling to much. So say 5 degs above and below. So it allows the flow temperature to go up to say 35 and then it stops the heating cycle, when the flow temperature gets down to 25 it starts it's heat cycle again. Remember the boiler is also managing the flow return delta. So your room temp goes up because the sun's out or your fire is on.  As water flows through a radiator it gives off heat and has an expected dT say 5 degs. Water goes in at 30 and out at 25 normally. If the room gets hotter from your fire, the radiator cannot give it's heat off as easily so the dT reduces to 3. Now boiler will slow it's self down and add less energy to get it's delta back in to equilibrium.

 

The above only occurs well on a low temperature system, if your expected flow temp are above 50. You will need to think about TRVs on most radiators (not all). But you don't want ones that will just go open or closed you want modulating ones.

 

This is what my boiler manual says on the subject.

 

 

 

[sharpener]

On 21/07/2024 at 22:54, Sarah29 said:

I understand that we don't have any thermostats in the house if the weather comp system curves are setup correctly. All sounds great. But what happens if we have a separate log burner too. On a cold day when we have the log burner on, surely the boiler would keep putting heat into the room despite the room already being up to temperature?

 

The short answer is that in addition to the outside air temperature sensor needed to drive the WC, you also need to add a thermostat or TRVs in the room with the log burner to control the heat emitters (UFH or rads) in that room only. Same applies to any rooms with a source of heat, AGA, floor-to-ceiling windows etc.

 

If as is sometimes done it is the living room and has a thermostat for the whole system then as the log burner heats the room up the boiler will turn down and the rest of the house will go cold. So you would need to move it to a room away from the log burner or remove it entirely.

[SteamyTea]

8 hours ago, sharpener said:

remove it entirely.

Yes, best to forget all about burning logs.

Just put a fishtank there.

[sharpener]

5 hours ago, SteamyTea said:

Just put a fishtank there.

 

Or a flat screen TV. This also gives you the possibiity of a fish tank or a log burner.

[SteamyTea]

6 hours ago, sharpener said:

 

Or a flat screen TV. This also gives you the possibiity of a fish tank or a log burner.

Or a CRT screen and get some heat.