[MattB]

I have around 3 years of gas data and it has been very useful. Using the Bright app I can drill down to hourly usage. This has been interesting as even during the coldest months the peak hourly usage is pretty much always less than 20kWh - in fact mostly less than 12.
I am looking at replacement boilers - currently a 24kW for a detached 5 bedroom house with 2 baths and 3 showers but mostly 2 people, up to 4. THe house is modern, very well insulated and easy to heat particularly with 5 zones of ground floor underfloor heating.
Specifying on traditional criteria, or using online calculators I would need a 25 or even up to 35kW boiler. But since I know exactly what my gas consumption is - not just annually but by day and hour, can I use that data to calculate or even estimate for the replacement boiler? Also my usage has decreased year on year so I dont anticipate major changes. System boiler with unvented cylinder - not combi.
I am not concerned about the additional cost and I appreciate that with modern modulating boilers they will essentially never be working at full output, so I could just choose a larger boiler and rely on the temperature and flow rates. However I have Tado setup for just on/off so wonder if I could land on say a 19kW boiler on the basis of my usage. Or is that not the case?

[geoffers]

@MattB - just googled for max Vs actual capacity, and this comes up if your boiler is larger than actually required

A boiler that is too large for your home will run inefficiently by cycling on and off frequently, called "short-cycling," whereas a correctly sized boiler will run more steadily and efficiently.

[rwh202]

With the thermal ‘inertia’ of the system, looking hourly is too granular.
You’d be fine looking at the daily values - say 120 kWh on worse days as an example, so 5kW continuous.
Now, that would probably be a bit extreme, but a 10kW on a 50% duty cycle seems perfectly reasonable.
That’s more akin to the sizing applied to heat pumps than traditionally to boilers where higher outputs are cheap and easy.

[meldrewreborn]

My peak gas is normally about 100 kWh (smaller house but all rooms heated). But when I’d been away and left the thermostat down to 8 degrees in my absence the day I returned the consumption was 150 kWh ( snow on the ground!).

So it’s wise to have some extra capacity in hand for when you need to heat up from a low level and perhaps more quickly than normal. You also need to allow for extreme weather events which we haven’t many of recently.

The boiler will be operating at well below peak capacity virtually all the time, because it’s wise to size it for the worst scenario.

i imagine that the current boiler copes OK with your requirements so I’d start by identifying its output and if necessary scaling back a little bit. Personally I’d rather over size than under!

[POIuy354]

Can't comment on the OP.

Would suggest that you should (in addition to considering the maximum output of the replacement boiler) pay attention to the minimum output and, everything else being equal, go for the boiler with the lowest minimum. My last gas boiler had a minimum of around 5 kW and this was part of the reason for excessive cycling.

[I now have a heat pump that, at its minimum, consumes about 400 W so equates to an output of somewhere around 1.5 kW. Even this is annoyingly high for much of the year.]

[Indyk_EONNext]

Good Morning @MattB

I hope all is well?

I can see the members have shared some great insights on replacing your boiler. Have you had a chance to explore them and find the best option for your needs? 😊

[MattB]

@POIuy354 Have just been quoted for a Viessmann which is 3.2-25kW. I think this will be low enough for us, particularly as our current does not go below 24(!)?

[MattB]

@rwh202 & meldrew thanks for the advice. I am consistently below 120 per day so your suggestions work. I think part of the problem is that engineers were concerned about the time it takes to heat the hot water. With a 300L unvented tank this could take time with an underpowered boiler (and provoke complaints from our daughters) but 90% of the time there are only two of us taking 1 short shower per day. The tank fully heats (even currently) on 30 minutes, which I extend to 60 in winter. If we had an issue with our family or guests staying we could easily heat for longer. It just goes to emphasise how different everyone's requirements are.

I have just downloaded 3 years of data from the Bright website with gas consumption per day and put it into Excel to analyse and graph. Am I mistaken that in the future this will be useful for heat loss calculations and heating design? The engineer looked a bit startled when I showed him and asked him if it would help.