Gas combi boiler

Grandad2b

@FreeBear can you explain: The one thing these graphs do not show, is how inefficient a boiler is when it fires up - You could be as low as 75% for the first few minutes. One reason you should try and limit short cycling (some boilers have a setting limiting the number of burner starts each hour).

Is that due to the thermal mass of the heat exchanger and are there other factors?

jj_home_80

Yes, that’s normal. Most modern boilers modulate their output to maintain efficiency, so a slight fluctuation around the set temperature is expected. It’s just the boiler adjusting to demand. If it’s not causing issues (like the house feeling too cold or hot), I wouldn’t worry.

FreeBear

Looking at data from my boiler, the fan kicks in at high speed when the burner first fires up, and then drops to a relatively low level within about one minute.

When comparing multiple short burn times of say 20mins to a sustained single burn of 2 hours, the heat output is similar, but gas consumption is noticeably higher.

Modulation curve from earlier today showing the spike at startup.

Grandad2b

You've lost me. Fan speed is an analogue of gas consumption? Or do you mean that because the fan speed is high heat is being pushed out of the flue rather than going into the heat exchanger? 

FreeBear

Grandad2b said:

You've lost me. Fan speed is an analogue of gas consumption? Or do you mean that because the fan speed is high heat is being pushed out of the flue rather than going into the heat exchanger? 

Fan speed & modulation (the green line in the graph above) are closely linked. So yes, high fan speed means more being burnt, and some of the heat initially is going straight up flue.

Another analogy is the car - Driving around town, all the stop/start motoring severely reduces efficiency and you might only get 30mpg. Get out on a nice long road and drive at a steady 56mph, and watch fuel efficiency climb to 45mpg or more. Start/stop driving wears the clutch & brakes out fairly quickly, but not when you go for a long drive - Boilers are the same, although the difference in performance will be hardly noticeable to most people.

Grandad2b

#Topic drift#

Well, of course I don't know what kind of car you have but my worst fuel consumption between fills is 48.3 and best 60.1. 

Though I have been known to take my wife's car out and come back with more range remaining than I started with  

FreeBear

Grandad2b said:

#Topic drift#
Well, of course I don't know what kind of car you have but my worst fuel consumption between fills is 48.3 and best 60.1.

Best was 50+mpg when steady cruising over a long distance. Worst, 25mpg going round in circles at Castle Combe.

moneytalks

I rang a gas installer about high pitched whistle from some of my TRV on rads. He told me to turn them ALL to 5. I said that will mean that the radiator will be constantly on and using a lot more gas.
He said if too warm, turn off the boiler.
I spoke to two people I know with gas and they also have them set to 5. I could not believe it

ThisIsWeird

moneytalks said:

I rang a gas installer about high pitched whistle from some of my TRV on rads. He told me to turn them ALL to 5. I said that will mean that the radiator will be constantly on and using a lot more gas.
He said if too warm, turn off the boiler.
I spoke to two people I know with gas and they also have them set to 5. I could not believe it

That is clearly not an efficient setup.
It would be interesting to check the temps in all the rooms by the time the 'wall stat' room turns the boiler off. Any room at a higher temp than desired would clearly be a waste.

moneytalks

Following on from keeping TRV at 5, if the flow rate was set at 50-55 and heating run for a couple of hours, would the boiler condense and therefore be efficient, but would the rooms heat quicker?