25kW or 30kW combi boiler?

FataVerde

Bendy_House said:

FataVerde said:

I talked to a local plumber, who lives across the street from me and he said the pressure and flow rate in the area is bad. He also said because I am in a conversion, I actually share my mains with downstairs, which obviously halves my flow rate because ground floor has priority. He also explained the diff in the vaillant ranges and said the Plus is a more expensive but better combi than Pro. He advised on getting the 32kW with the idea of later getting my own mains when extending into the loft. Reading the online specs, the 32kW has modulation at 1:6 and can get down to 5.5 (min CH output), but I will see when the quote comes.

I would have to be confident that my flow and pressure will be seriously improved before entertaining a larger combi. And, when you extend into the loft, anticipate a further ~0.2bar reduction in pressure from what you currently have due to the extra height above ground level. This will equate to a lower flow.

But, if you are planning a loft conversion, then you will most likely want a better flow too, so that's what you should be planning for.

My previous suggestion of a Salamander would be problematical if you literally share the same rising mains pipe with the flat below, 'cos you'd be sucking away their water, and possibly even introducing air into your pipes if they open a tap at the same time!

So, Qs I'd be asking include - if you fit your own rising mains pipe, what P and F will you have? And, do you have room for a booster tank - the size of a hot cylinder - if necessary?

I don't have the funds to get my own mains, that's for sure. Not for a while. I already had and Ideal 24kW combi and it worked decently. I had hot water on demand with relatively low pressure. I had no problems taking a shower, just had to wait a minute for the water to warm. I put this down to the old boiler but it seems it's the mains water flow. think it helps that I have a different schedule than my downstairs neighbour and there is a single person downstairs. Everyone I know on that street in coversions has combi boilers (with much larger families of 2 to 4 in 1 BR flats) so not sure what else I'd get but I think I've determined I don't really need a 30kW. 

No, no space for a tank. I will ask my neighbours with loft extensions how they dealt with water flow and pressure.

FataVerde

jj_43 said:

24kw is fine, 30kw is overspending. Your in a flat, the heating requirement is low, and a 30kw would cause your boiler to cycle on and off all the time increasing costs. Hot water requirement can be meet. Pressure issues not sure. Don’t get Hive it does not do load compensation.

Would an oversized boiler cycle on and off even with 1:6 modulation, where it can go down to 5.5? The label of 24 and 30 is slightly misleading. The 32kW boiler only has 31max output for DHW. Its max CH output is 25kW. The 25kW model has a max CH output of 19kW. Would this be enough?

Bendy_House

FataVerde said:

jj_43 said:

24kw is fine, 30kw is overspending. Your in a flat, the heating requirement is low, and a 30kw would cause your boiler to cycle on and off all the time increasing costs. Hot water requirement can be meet. Pressure issues not sure. Don’t get Hive it does not do load compensation.

Would an oversized boiler cycle on and off even with 1:6 modulation, where it can go down to 5.5? The label of 24 and 30 is slightly misleading. The 32kW boiler only has 31max output for DHW. Its max CH output is 25kW. The 25kW model has a max CH output of 19kW. Would this be enough?

Yes. Most domestic 'heat-only' boilers are only 15, 18 or around 20kW output. That's enough for most up-to-4-ish bedrooms houses, to heat the rads and the hot tank.

Think about it - even 15kW is around seven 2-bar electric fires. How often would you have that running in your house?!

Combis are rated at the max they'll deliver to the DHW, as that is what they are about - how much hot water they can deliver on demand.

BUFF

The average heat loss for UK properties is 6-8kW. Obviously need more when a cold start but once up to temp that is it.