Gas Combi Boiler. How to run efficiently, a few questions.

Leon_W

Just had a new Gas Combi fitted which replaced a very old conventional boiler and hot water tank.

I have set the DHW to 45deg.  This seems ample enough so no problems there.

I have currently set the heating flow temperature to 50deg, which I understand will obviously heat the house at a slower rate but ensure the boiler does actually condense as it is supposed to. I am also working on balancing the radiators which is a work in progress !

My main question is about the wireless thermostat.  On the rear is a plug whereby you can switch the "span", or, from what I can gather, change the "hysterisis" of the stat.  . This was set to 1 degree on installation........so........ if I set the thermostat to say 20, am I right in saying that the boiler would be turned on at 19 and then turn off at 21 ?

This seems like quite a big temperature difference. I have the option to change it to 0.5 degrees, therefore if I set the thermostat to 20 it will turn on at 19.5 and off at 20.5 which seems much more sensible to me. 

Does this sound right ? Any other tips or tricks appreciated.

Miser1964

I'd leave the hysteresis set on 1°C as this reduces repeated, short duration firing of the boiler and pumps.

You should be looking at weather compensation so that the controller can determine outside temp and use that to control boiler firing depending on whether it's really cold outside and the system will need full power to achieve  target temp.

That said, most Combis are over-specified (24kW+) for the heat-loss in houses. Some combis auto modulate or reduce burner and pump output, but many also have a setup menu where you can set the C/H output to a percentage of the rated power which can stop short-cycling. 

If you're doing your own balancing, then look at 'range rating' too to get the optimal 20°C difference in flow and return temps and keep the combi in condensing mode.

BUFF

exactly what boiler &  programmer/thermostat?

Leon_W

It's an Ideal Atlantic 30 with a Boss programmable thermostat. (I think the thermostat is a badge engineered Salus from Travis Perkins)

Thanks for the reply Miser1964.  Yes, I was wondering about short cycling....my thinking was this...... I've trialed it today as it was getting a bit nippy ! 

I set the thermostat at 18 today with the "span" (or hysteresis !) at 1 degree.  Boiler heated the room to 19 degrees
, it duly switched off, and then as the day went on it got colder and didn't fire the boiler until the thermostat hit 17.  This seemed like a wide temperature differential and we were quite cold before it fired back up !

I then reset the span to 0.5 degrees. The rest of the day the boiler didn't short cycle and came on twice in 4 hours......now.....I put this down to the fact that it WASN'T that cold today, and the temperature differential between inside and outside was fairly low, and therefore short cycling would be unlikely. 

I can see the reasoning that short cycling would occur in the depths of winter as more heat is lost more quickly and the boiler would keep cutting in and out.  Perhaps, in these shoulder months a smaller span is fine but a larger span midwinter ?

I take your point about "range rating" rather than balancing radiators and will look into this.

Ideal boilers seem to be compatible with OpenTherm so perhaps it'll be worth my while looking at a Smart Thermostat over the one I have.

As an aside, I remember as a kid we had one of those old Tobacco coloured Honeywell mechanical thermostats (everyone in the house smoked !) it must have had a span (or hysteresis !) of 6 degrees ! We were either blue with cold or rosy cheeked !

Many thanks.

Lorian

As it stands without weather compensation then unless you have really well insulated house and large.radiators then 50c flow may be too low when it gets really cold out. You'll be wanting to check your return temperature when it's on and cold out to calc deltaT. It's the return temp that needs to be low, under 55c to make sure the boiler is condensing. Thermostat hysteresis of 1c not too bad at lower flow temps. If you had flow at 80c then you'd notice more temp overshoot.0.5c much more likely to short cycle the boiler in midwinter.

Leon_W

Thanks Lorian

I don't disagree with what you are saying and thank you for your thoughts. The house is a large 3 bed detached old cottage so heat loss will be high, and radiators are probably too small, and a 50c flow will certainly be too low midwinter.

I'll  be checking the return temp in winter to make sure it's condensing ! I've no idea what to set the flow temperature too as yet, i'm guessing it's going to be as low as I can get it without freezing to death !

As for the thermostat hysteresis setting, i'm going with my theory that relatively low temperature differentials at this time of year won't result in boiler short cycling (unless convinced otherwise !)  In midwinter I can see that short cycling could be an issue so will see how the "span" of 1 degree and 0.5 degrees work in practice.

BUFF

Leon_W said:

Ideal boilers seem to be compatible with OpenTherm so perhaps it'll be worth my while looking at a Smart Thermostat over the one I have.

This, although iirc Ideal is one of the boiler manufacturers whose Opentherm is not as open (using wiggle room in the spec.) as you might expect ...
Penny pinching on the controls (which will more than pay for themselves through savings over their life) grrrr ...
& if you had had fitted Ideal's own controls at the same time as the boiler they would have been warrantied for as long as the boiler (potentially up to 12 years on an Ideall irrc depending upon model).

Workerdrone

BUFF said:

Leon_W said:

Ideal boilers seem to be compatible with OpenTherm so perhaps it'll be worth my while looking at a Smart Thermostat over the one I have.

This, although iirc Ideal is one of the boiler manufacturers whose Opentherm is not as open (using wiggle room in the spec.) as you might expect ...
Penny pinching on the controls (which will more than pay for themselves through savings over their life) grrrr ...
& if you had had fitted Ideal's own controls at the same time as the boiler they would have been warrantied for as long as the boiler (potentially up to 12 years on an Ideall irrc depending upon model).

I can confirm my Ideal Logic 30+ (24kw) boiler works flawlessly with Honeywell evohome and its opentherm bridge (Although I accept evohome is one of the more expensive options). It does also tend to be quite aggressive with initial heat demand for the zones before backing off as rooms reach temperature

BUFF

Iirc Ideal were requiring values/IDs in certain fields in the software that were normally left blank. I suspect that most of the large 3rd party controls suppliers will have cottoned on by now & acted accordingly.

Miser1964

>It's an Ideal Atlantic 30 with a Boss programmable thermostat. <

With the unit price of gas now so high, it could pay you to get a true smart thermostat. My 'go to' choice for years has been the Drayton Digistat +3RF. It's not internet connected but infers the outside temp by the rate of cooling in the room and then adjusts the on/off calling for heat.

I've just upgraded to a Nest which has the internet connection to get the actual outside temp and then uses OpenTherm to communicate to the boiler's controller instead of just calling for heat on a single wire.