Boiler Output Question

[Alfie25]

Looking at a 30kW combi boiler, I note that the heating output can vary from 8kW to 27kW. So what happens if the heating demand falls below 8kW? Is this when the boiler begins cycling?

My radiators total 10kW, however once the TRVs start closing some of them off as the temp for each individual room is reached, then the output required will fall to below 8kW.

Of course, if that's oversimplifying things, feel free to put me right.

 

[mogget]

So what happens if the heating demand falls below 8kW? Is this when the boiler begins cycling?

Correct. Once the total heat lost (from the rads and pipework) drops below the minimum output of the boiler, it has no choice but to cycle.

 

[Alfie25]

Thanks for confirming that.

I'm being recommended 28-30kW boilers. It's a 3 bed detached house, but I just have the one bathroom with an electric shower so my HW demands are just the same as a small flat really! i.e. kitchen sink, bath and basin.

I see some 24kW boilers have a lower output range (and so do some 28kW boilers). Given my HW requirements (which dictate the combi boiler size), would it be be better to go for a smaller boiler or one which can achieve a lower output? I'm guessing not, or the RGIs would have recommended one, but I just wonder why. HW flow rate for 30kW boiler seems to be 12 litres vs 9.5 for a 24kW. Doesn't seem like an appreciable difference to me.

 

[mogget]

I'm not an RGI, and my advice would be to speak to your installer about which boiler to install. Everyone has their favourites.

Flow rate / output - are you in a rush to run a bath, or need to run two showers at the same time? If not, most people would find 24kw quite adequate... but bear in mind in the winter you may have to run your bath even slower. Flow rate is given for a certain temperature rise, so if the water going in is colder, it will need to stay in the boiler longer.

 

[Alfie25]

Yes that's a good point. I've only once lived in a house with a combi boiler and noticed how I had to leave the bath hot tap partially closed to get sufficiently hot water out in the winter time when the water coming in is a lot colder. Saying that, I'm in no rush to run a bath.

However, another thing I've thought of is that my bath pipework is currently plumbed in 22mm (as it's gravity fed from a cylinder at the minute). At this stage I'm not planning on any bathroom renovation so it would stay in 22mm. With a lower powered combi I imagine this wider pipework would hamper an already weaker flow rate. I'm starting to see why 28-30kW boilers are being recommended.

 

[mogget]

The 22mm will not hamper the flow rate, quite the opposite in fact, but as it contains more water than 15mm, the hot water will take longer to reach the taps. It shouldn't be a problem in practice though.

I must admit I prefer a hot water cylinder to a combi boiler, but both systems have their advantages / disadvantages.

Main disadvantage on a combi being that you're without hot water if it breaks down.

Main advantage of combi - saving space.