I am going to have to change out my boiler. I had my whole house heating design done by underfloor heating company.
They sized boiler at 28kW ... (so currently have 30kW boiler) Think it prudent to check sizing before I order replacement.
I know there are a plethora of ?rad sizing? programs, anybody able to recommend a good one (Stelrad, Castrads etc)
That would be pretty basic approach ... anybody aware of something that looks at whole house.
If you want the full monty the Stroma SAP 2012 software has a free download
- link here:
Theo
There are two things to decide first
1/. how much power you need to keep the house warm at the coldest outside temp and wind you are likely to encounter 2/. How fast you need the place to warm up.I sized my boiler at 1/. and forgot 2/. This winter it was on 20 hours out of 24...
Basically an oversized boiler won't hurt - but an undersized one will struggle
Fit the biggest that will physically fit
Just fit a really big one.
Bill
I'm going through the process of replacing my 26yr old boiler (14.5kW). Of the 4 quotes I have, one recommends an 18kW, two 21kW and one 24kW replacement. 4 bed detached house with loft conversion and 17 rads. My preferred vendor had gone with 21kW (their working assumption was 1kW per rad plus 3 for the HW tank). When asked about the price difference between 21 and 24kW models it will be £50. The only further consideration for me is will the new boiler run from the 15mm supply or will it need an upgrade to 22mm. They will install the boiler and if it needs an upgraded supply they'll put in the 3m external run (£220).
One point. I am not clear if the OP is using underfloor or radiators but underfloor has a couple of limitations which may impact on boiler choice.
The system has a maximum running temperature in the 40-50 deg. C range. Excellent for operating a modern boiler in condensing mode but you still need 65-70 for HW.
Underfloor manifolds may have temperature management to deal with this but consideration might be given to choosing a boiler with dual capabilities.
Our house is normally occupied 24 hrs so the underfloor system is permanently on during the Winter months. Mostly insulated to current standards, the consequence is that our 30kW boiler runs at the lowest of
5 possible burner settings but at a system temperature of 67 deg.C
There is the manual option:
Getting the "right" match of output power to demand is less important these days since modern boilers have modulation - i.e. they don't have to run flat out, and can vary their output (and hence gas rate) to match the load.
Also there was a trend in the past to specify the heat input into the boiler rather than output. So 14kW might only mean 10kW of output. Whereas 18kW these days, probably means that as an actual output)
The 3kW for DHW is a bit suspect these days... it was probably about right for an old C-plan system with a slow recovery cylinder, but for a more modern setup with S or W plan and a fast recovery cylinder, the cylinder will usually be able to absorb the full output of the boiler for a sizeable part of the reheat (and hence do it in minutes rather than hours)
Yup there is not much difference in pricing incrementally. In some cases there won't even be any hardware changes to the boiler itself.
Thnx
Yup, when researching my system some years ago it was not actually that easy to find many boilers that supported split temperature operation to allow different flow temps when running the rads from that used for DHW recovery.
(I also wanted weather compensation, and multiple heating zones to make it a bit more interesting)
I find with weather compensation, in the milder bits of the winter, it will run flow temps through the rads in the 40's for much of the time. However it switches to 65/75 for DHW reheat (the higher temp on Thursdays for an "sanitising" DHW heat to 70 rather than the normal 60.
It is UFH on both floors. Only rads are heated towel rails in bathrooms (fed off DHR which is mains pressure)
House is well insulated.
My Thermal store has Max temp of 60C though stat is low down, and given the horizontal baffle .... top temp for DHW draw off is much higher. Heating loop comes off low down so cannot exceed 60C (and likely a lot less.
I gave a wrong figure in my OP .... I said Heating company stated boiler requirement was output of 28kW ..... just checked it was only 16kW ... in any event I have a current 30kW Baxi installed.
The boiler is fully modulating so combined with store it is working in condensing mode. Run at about 60% of Max temp setting.
It's not as simple as that. Most boilers have a finite range of modulation and the last thing you want is an idling boiler.
It is possible that the larger heat exchanger may well mean a more efficient boiler at part loads, but I haven't seen any numbers confirming this.
14kW is the output (18kW input). As you have said with modulation and similar pricing for 18/21/24kW boilers I can't see any reason not to go large (worst case having to upgrade to 22mm). Minimum output of the Greenstar 24Ri is 8kW vs 6kW for the 18Ri model which should not present any issues.
We have a 250l Megaflow pressurised HW tank with a fast recovery coil. The extra output from the larger boiler should as you say help with quick re-heating. Still on a Y plan system which means all the output going through one 3 way valve. Wonder if moving to S plan would improve flow to the tank (or rads) to take advantage of the additional 10kW output that would be available?
IIRC, the minimum output on my Vaillant 624 (24kW) system boiler is about 6kW
The 3 way is ok, but you probably want the timing set so that you don't run CH and DHW at the same time - the the full output (or close to it) of the boiler can go to the cylinder. I installed a 210L Unistor unvented cylinder, and that can consume heat at up to about 22kW - so the boiler will run close to flat out into that appart from the last 10 mins or so.
The Natural Philosopher explained :
Apart from in your pocket when you pay extra for it and again in poor gas efficiency, because it is too big and constantly cycling, then the extra wear and tear from cycling.
very true oversized boiler is not a good idea
I still have the Myson 1998 Heatloss application if anyone is interested. It looks like it belongs on a Win 3.1 machine but it seems to work (AFAIK) on a Win10 32 bit m/c
the zip file holds a file HeatLossManager98r105.exe
Myson now have it on-line
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