I am just installing central heating in my house and looking to replace the immersion heater with an all in one combi.
I have measured and sized up my radiators. I will be using 9 in total and these have a total btu heat output of 37,049. From this I wanted to install the vokera compact 24kw which I think is 80,000 btu. Will this be sufficient for my needs?
The person in the shop tried to convince me on the 28kw model (as i had some double convector rads) which I think is 102,000 btu's - I think this is too much for what I need but please let me know.
Go for the higher one every time with a combi. A mate of mine and various plumbers have advised that you need at least 100,000 btu for a combi (roughly speaking).
My mate that has one is very comfortable with his and probably has about the same amount of rads.
Any self-respecting combi will *eat* your space heating requirement.
What really matters is the rate at which you can produce 'instant' hot water when running baths and showers. So, taking a worst case scenario of cold water coming in at 4 degC and being heated to 60 degC, work out how many litres per minute you can produce with various boiler sizes - and then make a judgement based on what you deem to be adequate.
If if have remembered my schoolboy Physics correctly, you'll need about 4kW for every litre per minute of flow - so the 28 kW model will only give about
7 litres/min (assuming the 28 is net - less if gross!). Are you happy for it to take 20 minutes to run a bath? If not - and if the decision to use a combi is not irrevocable - I would seriously consider *not* throwing away your stored hot water facility. [I'm not sure what you mean by replacing the immersion heater - I presume that you are referring to the hot cylinder. If you have central heating, using a conventional boiler, the contents of the cylinder are best heated by the boiler - and *not* by an electrical immersion heater. If the cylinder doesn't already have an indirect heating coil inside, it will need to be replaced with one which does].
Agreed, and worth mentioning that it is perfectly possible to run a hot water cylinder from the 'heating' output of a combi, and use the instant hot water facility for a mains pressure shower, or perhaps the kitchen tap.
Yep, the heating load is not really the issue here, any combi will cope with that. The issue with combi's the Hot water output, and the bigger the heat output the better the HW flow rate (assuming adequate mains pressure/flow rate).
Having lived with combi's in rented accommodation - normally the smallest they can get away with, when we put one in the last house we put in a 28 kW unit, I certainly would recommend that as the minimum.
Even then the flow rate from taps is limited, and bath filling takes an age.
For the OP, be sure that you understand what you are getting when you replace the existing tank setup with a combi. We put one in our old house, and on balance it was still a good decision, though I'd rather not have one if I can avoid it, and maybe if we were doing it again I might look more into alternatives such as a mains pressure tank in the loft etc. that I wasn't really aware of at the time.
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