Boiler Power (KW) setting?

[Madrab]

What is the benefit to lowering it, less gas used?

Yes - When a boiler starts from cold, it will start off in high fire mode where it will run at max until the system heats up and the boiler starts to modulate the burner down. If it's set up @ 24Kw then it's using 24Kw/h of gas @ max, if it's range rated to 10Kw it will run @ 10Kw/h @ max. In short it will use ~ 60% less energy per hour.

 

[JohnD]

In short it will use ~ 60% less energy per hour.

In that example, it would also provide 60% less heat

 

[Madrab]

In that example, it would also provide 60% less heat

Only at max power which would be at start up and just until the systems start to warm up, it would then naturally modulate down as would the system @ 24kw, though it would still be energy saved.

 

[JohnD]

I mean, the only way you could save 60% gas per hour would be by emitting 60% less heat.

Unlikely to happen.

 

[berty3000]

It was nice to have the boiler run for two hours without stopping and starting several times - whenever that happened, the radiators would start to cool, and then presumably the boiler was going into 24KW full blast mode to start all over again. At 10KW, it just hummed away.

 

[JohnD]

I have a different brand of boiler, but mine turns itself down about 15-20 seconds after starting.

Thanks to oversized rads, I have quite a thermal lag, and the digital stat maintains an even room temp.

 

[Harry Bloomfield]

I mean, the only way you could save 60% gas per hour would be by emitting 60% less heat.

Unlikely to happen.

However it will reduce gas consumption, by not going charging up to the stop line.

 

[Madrab]

If 2 boilers were running side by side - one @ 24Kw max rating the other rated down to 10kw, then the one @ 10Kw will use 60% less energy @ fire up as the burner is running @ 40% of max rather than at 100%. So not matter how long it runs for, anytime it runs at max it is saving 60% less energy. If the boiler cycles that could be every time it starts. It's all savings though as it It will reduce/remove cycling though, which then comes back to it not starting back up at max every time.

The Kw per hour figure is the standard that all boilers use energy at, it wouldn't be how much is actually saved every hour of course as that boiler will modulate down to the same level regardless of whether it's range rated or not, I thought would have been a given, guess I should have been clearer. It will reduce/remove cycling though, which then comes back to it not starting back up at max every time.

 

[berty3000]

So what more can I do to reduce return temps?

Max power now reduced down to 9KW, pump head setting lowest, flow temp 55C. Seems to be working fine.

I am finding that the difference at the boiler is about 6C, although this is using a food probe so may not be most accurate.

I am not entirely sure why lower return temperatures (bigger dT) is beneficial in net terms. Whilst it means condensing takes place, doesn't it also mean that more gas has to be burnt to heat the returned water back up? I have insulated the pipework in the attic, would it have been better not to do so for lower return temps?

Radiators have TRVs set to about 3.5, these temperatures are rarely if ever achieved by the time the hearing goes off (I have it timed to come on three times a day).

 

[Madrab]

Be careful, you can't just keep lowering the boiler rating and pump settings etc chasing a nirvana differential. The system needs balanced alongside range rating and close control of space temps so the stat stops calling. Target return temps is reached by balancing the system so the radiators release the required heat into the space to maximise room warm up at the same time reaching 50Deg or lower at the boiler return to maximise condensing, thereby maximising efficiency.

Do note though, it is not a perfect science as there are a whole host of variables that are constantly changing, so there has to be a happy medium being as close to the ideal as possible.

 

[Harry Bloomfield]

Whilst it means condensing takes place, doesn't it also mean that more gas has to be burnt to heat the returned water back up?

The idea is, to remove the latent heat, from the steam, from the boiler exhaust. The cooler the return, the more condensation takes place, the more heat can be removed from the exhaust, and before this passes onto the main heating effect from the flame of the boiler.

 

[berty3000]

Be careful, you can't just keep lowering the boiler rating and pump settings etc chasing a nirvana differential. The system needs balanced alongside range rating and close control of space temps so the stat stops calling. Target return temps is reached by balancing the system so the radiators release the required heat into the space to maximise room warm up at the same time reaching 50Deg or lower at the boiler return to maximise condensing, thereby maximising efficiency.

Do note though, it is not a perfect science as there are a whole host of variables that are constantly changing, so there has to be a happy medium being as close to the ideal as possible.

Thanks for the advice.

Why does it matter if the stat temp isn't reached in a two hour period of having the heating on?

So when you refer to balancing the radiators - you mean the process where the valves are opened up, etc. then you adjus the lockshields? This is the one remaining thing I haven't done. The plumber who installed them should have, but I'm not counting on it. I suspect it'll be one of those fiddly and somewhat frustrating jobs so might see about getting a plumber to do it when boiler is serviced.

 

[Madrab]

So when you refer to balancing the radiators - you mean the process where the valves are opened up, etc. then you adjus the lockshields? This is the one remaining thing I haven't done.

Yes, exactly. Ultimately the whole aim of an efficient CH system is to use the least amount of energy to get the space up to temp in the least amount of time. Once that's happened the controls then turn the boiler off until the temp drops below that set temp and then asks for more heat.

2 key targets for balancing the rads

1) all the rads warm up at roughly the same time, ensure the spaces heat up evenly, which is more efficient.
2) Attempt to obtain the desired delta across the rad and return temp back to the boiler to ensure optimum radiator efficiency and maximise condensing at the boiler whilst it's under load.

A boiler not running is using less fuel than a boiler running even at it's lowest point of modulation. That coupled with a well insulated property means the space will hold that temp for longer before the stat says, boiler, time to add more heat again.

Indeed, balancing is performed by opening all the rads up, closing down the lockshields and then slowly opening them to fine tune the system, well there's a bit more to it than that but essentially that's what's happening.

 

[berty3000]

Thanks pal!

 

[ianmcd]

I've already reduced the pump head setting to the lowest

on a combi the pump setting has to be on max