Cycling Boiler

RelievedSheff

We moved into a new build earlier this year and now the weather is getting a bit cooler it is the first time we have really had to pay the boiler and heating system any real attention.

We have a modest 900sgft detached house with ten double bank radiators coupled up with an Ideal Logic 35 condensing combi boiler. All new obviously with all new plastic plumbing.

We have dual zone control with the master bedroom and en suite having its own thermostat while the rest of the house is on the main system. To keep the other two bedrooms and main bathroom cooler we had turned the TRV's down to number 2 and the downstairs loo and kitchen TRV's down to number 3. Which left the living room toasty warm.

The problem we have had is that the boiler keeps cycling in and out for a few seconds at a time even though the room stat is asking for heat. To try and stop this I turned all of the TRV's up to number 5 last night and knocked the temperature on the boiler up a couple of degrees. This seemed to stop the cycling but does now mean that most of the rooms, especially the bedrooms are getting a bit on the hot side.

Is there anything we can do to stop the cycling in and out and still keep the bedrooms cooler?

jefaz07

It sounds like the boiler is reaching temp and shutting down? Would you maybe agree, it’s hard to judge not looking at the system. If this is the case, do you have a bypass radiator? Sure on an ideal is 10% of the total output.
And you say a couple of degrees, what is it set at?

RelievedSheff

Yes. The boiler water temperature is reaching temperature, the dial has been set on the "e" position which I believe is about 68 degrees. It is now set at 70 degrees, the maximum being 80 degrees but we don't want to crank it up that far.

There are two radiators, one on each system that do not have TRV's and are always on. One in the hallway where the main room stat is located and one in the master bedroom where the zone 2 stat is located. The one in the bedroom rarely turns on as we have the temperature set much lower in the bedroom to keep the room cooler. The hallway radiator gets nice and hot.

With the boiler cycling it was taking the house a long while to get to the desired temperature, in fact some colder evenings this week it didn't get to the temperature set on the thermostat although it was not cold in the house.

With all of the TRV's open last night and the boiler temp turned up a notch the house did warm up a lot quicker and the boiler stopped cycling. But the spare bedrooms and main bathroom were overly hot and we are keen to find a way to be able to keep these rooms cooler while still stopping the boiler cycling.

RelievedSheff

I have done some reading on this matter this morning and am starting to think that the boiler which has been installed by the developers is far too big for the property.

Does this sound plausible?

btr30

To get the efficiency benefit of a condensing boiler the system should have "oversized" radiators, and the boiler operating at a relatively low flow temperature setting of below 60 DegC.
The boiler flame output should modulate as it approaches its set temperature, to prevent cycling.
Might be worth increasing the pump speed to take the hot water away from the boiler quicker.
Should be a rotary dial on the pump with low medium and high speed options

ianto11

I have a Vaillant ecotec 428 system boiler which exhibits the same issues. From a cold start the boiler will run for probably 15mins until the rads are hot and the return flow water temp is up.
After the system is hot boiler will cycle maybe 10times an hour. Like yours boiler temp is set to 70°C

On my boiler there is a parameter in the menu for max output (28kw) in my case I brought this right down to 20 and it seems to have helped as it means the boiler never goes to full chat, the return flow temp takes longer to come up, and the boiler gets time to modulate

While low flow temps are desirable, too low can stall the condensing process ( Link ) and higher pump speeds may not help as they will bring the return flow temp up quicker also

RelievedSheff

It seems to be behaving much better this evening with all of the TRV's on 5 and the boiler stat at 70 degrees. It isn't cycling in and out all of the time.

The boiler is due its first service in February so I will ask the engineer who comes out what they suggest can be done about reducing the bedroom temperatures while avoiding cycling.

Shouldn't complain really. At least the house is warm!

RelievedSheff

Just a quick update on this. The boiler seems to be working much better with all of the TRV's open and the temperature a couple of degrees higher on the boiler stat.

It also seems to be running more economically presumably because of the lack of cycling now. We have used much less gas this weekend although admittedly it has been much milder weather. We will compare the use later in the week when it drops colder again.