How Does My Boiler Work?

[DIYWell]

Our house has a modulating combi boiler feeding multiple rads. Each rad has a TRV except for the bathroom one and there's no ABV or roomstat. The boiler and radiators seem to have been installed about 2004, before we moved in, and do the job.

My question is, what triggers my boiler to fire up the second time?

I'm guessing when the timer goes on, the boiler senses the flow temperature as low so there is a demand for heat, boiler fires up, and as the rooms come to temperature, the temperature of the return increases. When that delta hits some minimum value, heat isn't needed and the boiler and pump cut out.

So what triggers the boiler to come back on and go round the cycle again?

 

[elite123]

Flow temp is low?

I think as part of part L all new installs would have to have room thermostat now

 

[DIYWell]

It sort of makes sense, but that would rely on a pipe of stagnant water to radiate enough energy for the temperature to drop sufficiently. Ok if the boiler is in a cool garage or attic I suppose.

I wonder if any heating engineers have an authoritative answer?

 

[CBW]

ABV are often built into Combi boilers. Room thermostat’s provide what’s called boiler interlock, which prevents unnecessary firing, and more control, but because one isn’t fitted, the slightest drop can trigger the boiler to fire up.

Also if any hot tap is used then this can bring the temperature down from the system water.

 

[ianmcd]

Flow temp is low?

I think as part of part L all new installs would have to have room thermostat now

He is in Edinburgh, there is no part L in Scotland, it only applies to England and Wales

 

[ianmcd]

I'm guessing when the timer goes on, the boiler senses the flow temperature as low so there is a demand for heat, boiler fires up, and as the rooms come to temperature, the temperature of the return increases. When that delta hits some minimum value, heat isn't needed and the boiler and pump cut out.

No the timer tells the boiler when to come on, the burner will modulate and cut out, the pump wont it will constantly run, depending on the boiler the pump speed will also modulate, to suit your requirements, if you state which boiler make and model you have, and what your actual concerns are we might be able to help more

 

[DIYWell]

Well there's no ABV built into my boiler. Also, when there's a call for hot water, the central heating pauses until the hot water isn't needed. It's very little impact on the overall state of warmth.

He is in Edinburgh, there is no part L in Scotland, it only applies to England and Wales

The Scottish equivalent is Part J

 

[DIYWell]

No the timer tells the boiler when to come on, the burner will modulate and cut out, the pump wont it will constantly run, depending on the boiler the pump speed will also modulate, to suit your requirements, if you state which boiler make and model you have, and what your actual concerns are we might be able to help more

I don't have concerns, just curiosity triggered from replacing the boiler in my mother's house which has a ridiculously over complicated roomstat controller (but that's possibly a different thread).

I don't think the pump runs constantly while the timer is "on" but I might check later. In any event, it must still be down to a temperature sensor(s) within the boiler on flow and/or return to determine if there is a demand for heat.

 

[ianmcd]

Well there's no ABV built into my boiler. Also, when there's a call for hot water, the central heating pauses until the hot water isn't needed. It's very little impact on the overall state of warmth.

The Scottish equivalent is Part J

Still no requirement for a room thermostat in Scotland

 

[Harry Bloomfield]

I don't think the pump runs constantly while the timer is "on" but I might check later. In any event, it must still be down to a temperature sensor(s) within the boiler on flow and/or return to determine if there is a demand for heat.

It should continue to run for a few minutes after the boiler stops, to remove the heat from the boiler.

Well worth fitting a room stat, if you don't have one. Normally you should fit it in the space where the rad has no TRV, or leave it with TRV set to Max..

 

[DIYWell]

Again, I'm not reading anything that tells me why a room stat has any benefit.

 

[Harry Bloomfield]

Again, I'm not reading anything that tells me why a room stat has any benefit.

It will automatically prevent your boiler continually cycling, irrespective of the need for heat and relying entirely on the TRV's. The stat just says 'enough' and your boiler stops cycling.

 

[stem]

If you think of it along the lines of: Would you leave your car engine running whilst your car is static and parked on the drive just in case you might want to go out in it later on?

It's a similar situation with a heating system without a thermostat. When no heat is required and the radiators are all off, the boiler will still be ticking over keeping itself hot, just in case one of the radiators might come on.

The room thermostat is like the ignition switch that turns the engine off when you don't need to move anywhere. In the same way the room thermostat turns the boiler off when you don't need any heat.

Today for example, in my home, the heating came on first thing this morning, the house warmed up by 9am and the room thermostat switched off the boiler. As the weather is mild, and we have autumn sunshine the house has stayed warm all day and the thermostat hasn't switched the boiler back on, and it's now 4:30pm. So the boiler's not been on all day. If I didn't have a room thermostat, then the boiler would have been running all day just ticking over to keep itself hot. So, the thermostat saves energy and wear on the boiler.

 

[DIYWell]

For the sake of argument, assume I've fitted a roomstat in my lounge. Heating comes on in the morning, and by 9am, the roomstat turned off the boiler. The lounge has a large SW facing window and doesn't tend to loose heat particularly quickly. at 10am I go to my study in the NE corner. It has cooled. If I want heat, I have to go to the lounge, turn up the roomstat. The lounge cooks, unoccupied but I can work in my study.

In the evening, the heating is on, but we fancied burning a few logs in the stove. The lounge is nice and cosy, but the rest of the house is unheated because the roomstat has shut off the heating.

An alternative would be to look at the new boiler fitted in my late mother's house. It is boiler+ compliant, whatever that means. It has a 520RF programmable thermostat; an overly complicated device. It doesn't programme ons and offs, but temperatures. 8am 20C, 10am 15C, 2pm 20C, 4pm 15C, 8pm 21C and 11pm 12C. The boiler is never off. It just maintains the temperature profile.

 

[denso13]

The lounge has a large SW facing window

The lounge is nice and cosy, but the rest of the house is unheated because the roomstat has shut off the heating.

Then you wouldn't put the stat in the lounge. Systems need designed properly with correctly sized radiators in each room and the stat fitted in the correct place.