Boiler firing timings

BiliousGreen

Hi, Trying to get to where I am asking the correct question based on stuff I have been told in other posts. 

Been timing the boiler fire patterns the last 2 nights  and once rooms are at temp and the boiler is just maintaining whatever the stat is set too,  it fires 5mins on 5 mins off. Flow rate doesnt seem to affect it much. Goes more like 6 on 4 off if I lower it to 55 from 60 but 5 on 5 off at 60 or above. So 2 questions......

1 Is boiler being 5 on 5 off to maintain room temp normal and expected, or is that suggesting an issue? shoudl it be going off for longer?
2 Someone said about boilers modulating the gas so is th gas cost of the boiler running solidly for 2 hours when heating the room to temp the same as teh gas cost when boiler is  left on for 4 hours (burning for 2 hours) maintaining heat or is it more efficient in these 2 hours due to being at temp as such

Ie are the 2 hours costing the same. 

Not sure if these are better questions and closer to what I have being trying to figure out??

Many, many thanks.

InvertedVee

By boiler 'being on' do you mean

- you can see that the flame is burning (boilers often display this), or
- the boiler is making a noise?

BiliousGreen

Good question, boiler fires and you hear the flame ignite and you can see the temp go up so I am assuming its burning as it makes the same sound then as when its on constant heating teh house to temp initially.....

chris_n

Without knowing what your boiler is, what the control system is, how (if) the whole thing is programed, the actual internal and external temperatures (throughout the cycles) and the actual heat loss calculations for your house any answers you receive are based on personal experiences and (sometimes educated) guesswork. The more of those variables you can state the better the answers are likely to be.

My own rental with a non condensing boiler and a fairly basic programmable thermostat / timeswitch runs for 10 to 15 mins every 2-3 hours depending on set temp, how long the house has been at set temp and what the external temperature is. This boiler does modulate but as I said doesn't condense. As you can see this is very different to yours.
The house has an EPC of D but close to C (changing to a condensing boiler will make it a C).

BiliousGreen

So 5 on 5 off could be normal as such then? Its a c30 I mini combi boiler heating a small 3 bed detached. 

So would I better having it run 6 on 4 off with flow rate set to 55 degree or 5 on 5 off with flow rate set to 60? All other things being equal

To be honest I think I am better off letting it heat house to 18 and then turning off until it drops to 16 and then running it for 30 mins straight to take back to 18,  rather than leaving it to maintain at 18 where it seems to be on a lot. House stays reasonable warm and takes a while to drop 2 degrees so the boiler would be on a lot less letting the house cool rather than tryinb to maintain. 

peter3hg

Sounds like you have a thermostat with TPI set to 6 cycles. In theory this should be more efficient but it does put more wear on the boiler.
You probably can disable TPI on the thermostat so it will instead overshoot the target temperature and only fire again once it is below the target.

InvertedVee

BiliousGreen said:

So would I better having it run 6 on 4 off with flow rate set to 55 degree or 5 on 5 off with flow rate set to 60? 

I'd go for the lower flow temperature and not worry about the cycling time. It's not excessively fast.

BiliousGreen

Ok so the 5 on 5 off is relatively normal then 
peter3hg said:

Sounds like you have a thermostat with TPI set to 6 cycles. In theory this should be more efficient but it does put more wear on the boiler.
You probably can disable TPI on the thermostat so it will instead overshoot the target temperature and only fire again once it is below the target.

BiliousGreen

M0KBJ said:

BiliousGreen said:

So would I better having it run 6 on 4 off with flow rate set to 55 degree or 5 on 5 off with flow rate set to 60? 

I'd go for the lower flow temperature and not worry about the cycling time. It's not excessively fast.

So like the other poster said, you dont think those on/off times are excessive. Will the boiler be burning gas during those times at the same rate as when its heating the house from scratch ie are 5 mins maintance the same cost as 5 mins when heating up? Is it all equal, or would the fact that its maintaining mean its modulates, so I am not comparing apples to apples by just looking at the time it is on for?

InvertedVee

BiliousGreen said: Will the boiler be burning gas during those times at the rate as when its heating the house from scratch ie are 5 mins maintance the same cost as 5 mins when heating up? Is it all equal or would the fact that its maintaining mean its modulates so I am not comparing apples to apples just looking at the time it is on for?

No, when maintaining an even room temperature, the boiler will be modulating so burning less gas than when it's warming up the rooms from a cooler temperature.

Different boilers modulate at different ratios but there's always a lower limit. When this is reached, some will cycle intermittently, which may be what you're noticing.

BiliousGreen

Thank you for your help.
So in summary 5 on 5 off is normal or at least not a sign of something being wrong, and I am better off accepting the 6 on 4 off at a flow rate of 55 than letting the house cool and firing up the boiler manually for 20 mins every hour or so. 

What was the TPI issue that another poster raised. Is this something to be concerned about and if so would it be a case of changing my room thermostat?