Boiler Sizing Question

Hi
We currently have a 24kW Vokera combination boiler which was installed
in the early 90's.
I'm looking to replace it for many reasons not least of which is that
it is completely underpowered for our house.
The house is a four bed detached with two bathrooms. There are nine
radiators and two towel rails connected to the current system.
Current problems are:-
1) The boiler is extremely noisy - needs a new / refurb fan
2) It can't heat the hot water to an acceptable temperature in winter
without reducing the flow to an unacceptable rate
3) The response time on the heating system is appalling - it takes
several hours for the house to reach an acceptable temperature.
Now I know I haven't gone into details about the sizing of each
radiator but my gut feeling is that 24kW - even if it was a brand new
boiler - is not adequate for a 4 bed /
2 bath house.
I am leaning towards either Worcester Bosch or Vailant. There is a
calculator on the WB website that recommends I get either a 37kW or
42kW boiler. Does that sound reasonable?
Thanks,
Steve
Reply to
stevelup
In an earlier contribution to this discussion,
If the house is decently insulated, 24kW should be more than adequate for space heating. If the current one can't cope are you sure that it's actually producing 24kW, or is there something wrong with it?
If you want a lot of hot water in a hurry, *any* combi boiler is a bad idea - but if you *must* have one, get the biggest one you can accommodate/afford.
Don't forget to check the adequacy of your gas supply and internal pipework - 42kW needs a *lot* of gas!
Reply to
Roger Mills
"stevelup" wrote
Hi
From info I have read on this group, I would expect that the size of a combi boiler in this situation is likely to be governed by the flow rate of hot water you want rather than its ability to heat the place.
I have a 1970s 4 bed property 1 bathroom 1 en-suite but opted for a conventional system boiler install (retaining hot water cylinder - teenage kids etc etc). Mine is a Worcester Bosch 24Ri which seems well upto the job. Took the opportunity to zone upstairs and down as part of the re-work.
HTH
Phil
Reply to
TheScullster
insulation, 100mm roof insulation, and a 18KW conventional boiler. The house is always toasty. Are you sure there isn't another problem?
Reply to
DIYdisaster
You current boiler is behaving really badly. A 24kW boiler should get all 11 rads/rails untouchable in 20 minutes from a cold start.
I'd try not to go over board on a 37kW boiler. I think you'll find a 31kW will be very acceptable and even a 24kW in working order will be a welcome relief.
Reply to
Ed Sirett
Thanks everyone for your replies. The hot water flow rate is probably more important to us than the heating performance but unfortunately we do not have space for a storage system.
It is possible (indeed likely) that the problems keeping the house warm are more to do with the fact that it is poorly insulated and has draughts coming through some ill fitting doors. I think that in the new year I'll focus on getting the gaps plugged in the fabric of the building!
I still intend on replacing the boiler because of its weak hot water performance and noise, but I'll moderate the power output to something more sensible.
The entire heating system has been constructed (not by me I must add!) using Hep2O pipe, not copper. Will this be affecting the performance significantly?
In terms of gas connectivity, there is a brand new 22mm supply pipe running directly from the meter to the boiler (we have no other gas appliances) which is approximately nine metres in length. I really don't want to replace this - will it be adequate for a 31kW boiler?
Thanks,
Steve
Reply to
stevelup
No. Not even you know who thinks Hep2o affects performance.
Is that 9m allowing for the bends and joints? If so you should be OK, just.
Reply to
Ed Sirett
I would recommend that you do this. Then you will know for sure what power you need.
M
Reply to
Mark
Thanks - I thought it was worth asking. As the rest of the system was already Hep2o, all the bits I've added have also been done the same - it seemed pointless to do otherwise. Literally the only bits that are copper are the radiator tails and the first meter or so of pipe leaving the boiler.
The total length of pipe is 9M but there are 3 x 90 degree elbows:-
2M > 90deg > 2.5M > 90deg > 4M > 90deg > 0.5M.
Steve
Reply to
stevelup
It's a little difficult to do though as I cannot directly identify the radiators. If you think it is a worthwhile exercise, I'd could find the nearest equivalent to each radiator in the Screwfix catalogue and tot up the values.
I'm not convinced it is necessary though as the overall maximum power output of the boiler is being selected to ensure the instant hot water flow is adequate rather than the heating? Surely whatever boiler I choose will be adequate from a heating point of view as much more power is needed for the hot water than is needed to efficiently heat the house.
If this is flawed logic, I'm willing to reassess.
Thanks,
Steve
Reply to
stevelup
In an earlier contribution to this discussion,
It is certainly true in most cases that if you size a combi boiler to give an adequate water heating performance it will be more than adequate for space-heating.
If there is a flaw in your logic it is in your assumption that even a new 24kW boiler wouldn't be adequate. It's a bit like convincing yourself that you need a car with a 4-litre engine because your current 2-litre model will only achieve 40mph, while ignoring the fact that the current car is only firing on half its cylinders!
It would certainly do no harm to calculate the space heating requirements to see what they *really* are. To do this you need to calculate the heat losses for each room based on the construction, required temperature etc. - which has nothing directly to do with the radiators which are currently fitted, although you *will* need to assess their adequacy once you have calculated the heat losses.
If your house is properly insulated (cavity wall insulation, double glazing, thick loft insulation) I would be very surprised if the total losses exceeded 15kW. If a 24kW boiler can't keep up with this, there's either something drastically wrong with the boiler or else the radiators are far too small and not utilising the boiler output properly. In my opinion, you need to resolve these issues *first* - otherwise rushing out and buying a new boiler may not solve the problem.
Reply to
Roger Mills
The message
from stevelup contains these words:
It doesn't matter how powerful the boiler is if the radiators are too small. Other factors being equal doubling the size of a radiator will double the heat output.
Reply to
roger
It also depends on the house size and insulation
A masll house or flat with good insulatin may only require a Kw of heat to keep it warm.But up to 30KW to run a shower.
OTOH houses that need 30KW to heat in winter are not uncommon. A case for more insulation tho :-)
Mind you, such a house would be foolish to run the expected several bathrooms of one 30KW combi...
Reply to
The Natural Philosopher

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