Boiler Sizing

[Malc]

I've done the calculations for my house and come up with 7.1kW of heating load, 13.4kW of radiator surface. So far so good. I now have a choice of a 12/15kW or a 15/20kW boiler (HRM wallstar).

The current coal fire is rated at 11kW to water and 1.8kW to room. The later is free standing and mine's not so say about 12kW. But my feelings are that last winter the coal system struggled. I guess it could be that the fire is not producing 11kW to the water as it was probably rated on Welsh anthracite, not the imported stuff we get these days. Or my calculations could be off but I've checked these (it's only plugging numbers in a program after all).

So, the 12/15 would seem to be the one to choose but on the other hand the 15/20 would give more in hand, for say a future conservatory. Also, the 12/15 is only just bigger the current coal fire so it feels too small. The 15/20 matches the kitchen cupboards better perhaps as it is 700mm high like they are. The 12/15 is only 600mm high.

Are there any disadvantages to choosing the 15/20? With so much radiator I can't see it cycling significantly more that the 12/15 as the cycle would be mainly the room stat I would think, yes?

 

[ChrisR]

" 7.1kW of heating load, 13.4kW of radiator surface."
eh?
heating load?

HRM = oil so one for others.

Add 20% or so to total requirements to get boiler size - allows for pipe losses, a bit to aid quick heating up, etc.


Have you made allowances for air changes, extra for North walls, & non-ideal rad placements?

 

[Malc]

Heating load = heating power not including hot water heating, so 9-10kW with water.

Figure is from Heatloss manager. Enter room sizes, windows etc. Choose wall construction, window type, air changes, etc. and out pops the figure. Air changes are kitchen, bathrooms = 2, bedrooms = 1, Lounge etc. = 1.5. Living room = 21C, bathrooms=22C, rest =18C.

No allowance for north walls (the program does not have this). Non ideal rad position I would have though just changed the rad size, not the boiler. Likewise the pipe loss should aid the radiators although as some of my pipe are in the floor slab I suppose that's not totally true.

 

[oilman]

What size pipes are you going to connect the boiler with?

 

[Malc]

oil pipe - 10mm
Water - 22mm to HW cylinder, then a run of 28mm (old gravity feed), then 22mm, then split to 15mm to each rad/ rad run.

I've tried a couple of the online calculators: http://www.idhe.org.uk/calculator.html and http://www.est.org.uk/bestpractice/boiler/ The first is probably the most useful as I can enter my true window size (windows are probably on the large size compared to today's houses). Also I can play the figures on the first. The results are 13kW, if my walls are filled and 17kW if not (or 12kW on est). Some walls are new so to latest standard. Some are old but with a polystyrene pellet fill (so perhaps not quite as good a U value).

I'm tending towards the idea of the 15/20 set to 15 (which of course has scope to be set higher) rather than a 12/15 set to 13, which might not be enough.

 

[Malc]

Spoke to HRM, what a world of difference. Extremely helpful and knowledgable. I was speaking to someone who really knew his stuff.

I'm now going for the 12/15 since as my radiators are oversized I should get high efficiency, i.e. max power >15kW, because my return water temperature can be lower for the same radiator output. Of course I need to make sure that the return flow is >55C so I don't get condensing.

I was going for a grundfos alpha pump as I have a lot of TRVs. Current system has no bypass and when all TRVs close I get a lot of vibration. The alpha slows down to keep a more constant head and thus a slower flow. But now I think I would be better with a standard pump and a bypass valve. I was going to fit a bypass valve anyway but it sounds like I should feed some of the hot water back to keep the return water temperature high rather than slowing the pump down since that would give an even lower return temperature.

 

[oilman]

Your limit is the 22mm pipe. You can take 13.5kW through them so any more than this will result in the boiler cycling on its thermostat. Most boilers work like this, too big for the pipes, so they start and stop a lot.

There are some calculations you are supposed to do to give you the right size boiler for the house, but that all goes out of the window when you connect the pipes.

 

[Malc]

I've actually done the pipe flow calculations. Your 13.5kW must be a run of thumb and assumes a 10C difference. A 20C difference would give twice the through put of course although the radiators are down to 80% of rating.

At 10C with my calculated load of 9kW I only get 0.77m/s on 22mm pipe. The recommended speed range, I read, is 0.5-1.5m/s so that would suggest an 18kW limit. But since my rads are oversized I would expect >10C diff and so a larger limit. In fact most of the pipes are <0.5m/s, all if I use a 20C diff, so my problem is rather that sludge could settle.

 

[oilman]

Your 13.5kW must be a run of thumb.......

It is, and it is for 11 deg C. Most other things in heating are rule of thumb too. How many days a year is the temp <0 deg C? Yet calculations are done for this instead of say 8 to 10 deg C which is much more common and look to supplementary heating, like jumpers, or fires etc.

 

[Malc]

Yes I notice that sometimes it is 11C difference and sometimes 10C difference. I suspect this harks back to when it was 180F in, 160F out, 100F above room temp and 20F boiler difference. 20F is 11.1C. I guess some are rounding this to 10C.

 

[BAD_BOB]

Thanks for posting the links.

On that IDHE calculator site do you put down 'two walls front to back' if you have a semi detached house? The difference is 2.8Kw!

B