Does a say 30kw boiler use more gas than say 24kw?
Okay - I understand it can draw more power when needed, but wouldn’t it just heat the house up more quickly then draw less power when up to temp.?
Thanks
Heating loads are much closer to 10kw and less. Check what the boiler can modulate down to on the CH side.
The 24/30kw rating is to do with Domestic Hot Water (DHW). Higher rating will deliver hot water at a higher flow rate for given temperature rise, and hence use more gas, but give a nicer shower.
What he said. Usually modulation ratios are around 1:6 to 1:10 - so a 24kw boiler with a 1:10 ratio could modulate down to 2.4kw.
That’s great. We actually went for a 30kw to replace a 22kw, thinking being we’d had a decent sized extension done and could do with extra oomph at times. You’ve put my mind at rest that this wasn’t a seriously bad idea.
We have a Worcester Bosch (non Combi) which I think has 10:1 modulation ratio
You can put the biggest boiler in you like but unless you have suitably sized pipework then it won’t deliver that output
We put in a 40KW combi when doing our bathroom (needed a new boiler already).
We have good mains pressure. Absolutely the right decision, shower is excellent and huge bath fills very quickly.
I recommend go as big as you can.
Sorry for slight thread hijack - what does a higher rating do for system boilers? Does it just mean the hot water tank fills up faster?
I'll go against the flow. Wish I'd put a smaller one in when I did the extension, it can't modulate low enough and the crap dual zone controllers do TPI or some other nonsense so it bounces between anti cycling shut off and full bore. Now it's out of warranty that may be solved with an opentherm adaptor from the EU.
Sorry for slight thread hijack – what does a higher rating do for system boilers? Does it just mean the hot water tank fills up faster?
I’ll take this, mostly to clear some
Confusion I may have caused above. When OP said 30kw I assumed combi boiler. Which does usually have a higher output on the DHW side to instantly heat mains water, and a lower output on the CH side.
For a high output system boiler, that would be to supply a large radiator system. An indirect hot water cylinder will only take a few kw. A higher output boiler won’t affect the rate of water heating, the specifications of the cylinder will usually limit it.
Edit: My info on cylinder ratings is a bit outdated (based on my outdated cylinder) but more modern, larger, megaflo cylinders can absorb heat from the boiler at rates up to 25kw. So yeah, big boilers gonna help fill that with heat.
If you’re not worried about efficiency then the being too big is not the end of the world. If you are worried about efficiency /cost then go smaller. I think a cylinder is of the order of 20kw but presumably decreases as the temperature of the water in it rises.
I wish I’d ignored my installer and paid a few hundred more for a boiler with better modulation. Mine rarely runs at full bore even when in theory the sum of the parts calling heat is equal to it - down to pipe work limitation as noted above I suspect.
I went for a higher power combi (42kw I think) so it could deliver sufficient hot water for two showers simultaneously. It can modulate heating flow temperature down to 35 degrees so I don't think there's any loss of efficiency. I installed a smart thermostat with weather compensation.
The minimum it will modulate to is far more important. You're very unlikely to be able to shift 40kW of heat, I don't think, unless you have a huge house. A really big double layer radiator with double vanes inside (type 22) is something like 1600W of output when it's red hot. So you'd need 25 of them to shift that much heat, or more like 35 if you ran at a sensible temperature. And then only first thing in the morning.
I dunno if it would heat the water up that much faster than a sensible size. But if it modulates down to 2.4kW then it'll probably do that pretty sharpish anyway. Mine is 12kW in a 3 bed house, but it only goes down to 9kW. But even that's far too much, as it can't keep the flow and return temps down to an efficient level without cycling.
My info on cylinder ratings is a bit outdated (based on my outdated cylinder) but more modern, larger, megaflo cylinders can absorb heat from the boiler at rates up to 25kw. So yeah, big boilers gonna help fill that with heat.
Only when the water in the tank is stone cold though.
Timely thread. There was me wondering the other day if putting in a larger kitchen radiator would cause an issue for our combi boiler.
Now I'm wondering if turning the flow temperature down to 50 is causing problems with cycling. How would I know?
