which size boiler

sindy 4

Hi all ,can any one advise me on a boiler size.I have got 2 quotes for a glowworm flexicom boiler. one is for sx 18 and the other recomended sx 30.Both same price. My house is 1500 sq ft ,Ihave 5 large double rads and 4 smaller single. The 30 sx guy says i would feel a heat reduction with an 18 as my old cast iron boiler is so big. He says oversize is not an issue as boiler modulating . Is this correct .Would appreciate any advice thanks

pache

Ok so go on to "idhee", follow the step by step boiler size calculator making sure you fill it in correctly and this will give you the required boiler size for your house.

sindy 4

Thanks for that i will

esox28

18 kw boiler is more that enough for 9 rads and hot water. If you in put your house details into the idhee calculator you're. More than likely to get 12kw for a house of 1500 square foot im sure.

whowantstwoknow

Hi All,

I'm having to replace our existing gas boiler, and want to make sure we get the size right as I think the previous one may have been oversized (it was 30Kw). I've used the idhee calculator selecting Northern Ireland, but it doesnt take into account the actual rads we have, which may be over sized too!! Have 9 double rads and 9 singles. With the inputs on the calculator its saying for the detached (approx 144m2) house a size of just over 21Kw. Should I take into consideration the actual rads & windows sizes (for calculator put in 35m2) we have? Installer saying oversizing wont matter (he normally puts a 24Kw in semi-detached), but my last boiler has only lasted 7 years so I want to make sure I get this one right, and like everything at the moment, funds are tight etc.

PS: From an insulation viewpoint the house is 15 years old, though I do have the attic space well insulated from a DIYers perspective!!

Thanks

W.

shane0007

A lot more goes into sizing a boiler than picking from number of rads.
You need to take into account the floor construction, wall construction, level of insulation, windows, number of external walls, etc.

If you just a guesstimate, you could add the total length of all rads in linear meters, taking doubles as 2, add 10% & add 1kw per 50 litres of hot water cylinder volume.

So for example, 9 rads, all 1,000 x 500 but 3 of which are doubles & a 130 litre insulated copper cylinder.
1 x 9 = 9kw
1 x 3 = 3kw for doubles
1 x 2.8 = 2.8 for cylinder

Total = 14.8kw
Plus 10% 1.48kw

=> total system demand = 16.28kw

This does not account for details but would be an average guide.

TPM

whowantstwoknow wrote: »

Hi All,

I'm having to replace our existing gas boiler, and want to make sure we get the size right as I think the previous one may have been oversized (it was 30Kw). I've used the idhee calculator selecting Northern Ireland, but it doesnt take into account the actual rads we have, which may be over sized too!! Have 9 double rads and 9 singles. With the inputs on the calculator its saying for the detached (approx 144m2) house a size of just over 21Kw. Should I take into consideration the actual rads & windows sizes (for calculator put in 35m2) we have? Installer saying oversizing wont matter (he normally puts a 24Kw in semi-detached), but my last boiler has only lasted 7 years so I want to make sure I get this one right, and like everything at the moment, funds are tight etc.

PS: From an insulation viewpoint the house is 15 years old, though I do have the attic space well insulated from a DIYers perspective!!

Thanks

W.

The calculator is an average, it doesnt take into account the rad sizes because it is calculating the heat loss for the building and in this respect the rad sizes are irrelevant.

rad sizes are not a good way to size a boiler as they may be over or undersized for the rooms. Yes the window sizes would have a major impact on the room heat loss.

The fact that your last boiler lasted 7 years is a concern. If the boiler was leaking after this time it would suggest you have issues with your heating system most likely a leak somewhere.

shane0007

Yes, going by rad sizes is only as I said a guesstimate & should only be used as a guide as it is relying on the person who originally sized the rads to be correct.

whowantstwoknow

appreciate the response..and that the devil is in the detail!!!

When I get home, I'll measure the rads and windows.

Any idea of the assumptions/calculations used by the idhee calculator, on a/my house built 15 years ago? I can tell you:

1) Its detached (so 4 external walls!!), its not quite square, but I've sort of squared if off to get the 140m2 figure.
2) Was built mainly (ie part of walls have external brickwork) with cavity blocks (ie not 2 leaf construction), with internal walls battened and insulated with material of the time (reddish fibreglass if I recall)
3) ground floor built from a concrete slab, 1st floor obviously joists
4) Most windows are double glazed PVC, though have some large tripled glazed alu-clad windows as part of an extension.
5) Hot Cylinder: Larger than normal (I'll update with size)
6) Currently zoned for CH & HW with motorized values and TRV on all the rad minus where the temp stat is
7) 3 external doors (1 single glazed, 1 double glazed & 1 triple glazed)

Updates later tonight.....

W.

TPM

shane0007 wrote: »

Yes, going by rad sizes is only as I said a guesstimate & should only be used as a guide as it is relying on the person who originally sized the rads to be correct.

I hadnt actually seen your post when I was writing mine, so by no means was I getting at you.

and the calculator makes alot of assumptions s is probably no more accurate than using the existing rad sizes

shane0007

TPM wrote: »

I hadnt actually seen your post when I was writing mine, so by no means was I getting at you.

and the calculator makes alot of assumptions s is probably no more accurate than using the existing rad sizes

And here was me thinking you were driving a spear through my little ol' sensitive heart

whowantstwoknow

Hi all,

Went around the house with the tape, so adding that to what I know about the house when built:

I can tell you:

1) Its detached (so 4 external walls!!), its not quite square, but I've sort of squared if off to get the 140m2 figure.
2) Was built mainly (ie part of walls have external brickwork) with cavity blocks (ie not 2 leaf construction), with internal walls battened and insulated with material of the time (reddish fibreglass if I recall)
3) ground floor built from a concrete slab, 1st floor obviously joists
4) Windows & Door openings comes to 40m2, of which 14m2 is triple glazed and the remaining double.
5) Hot Cylinder: Larger than normal, external size of insulation is Height=1.3m, Diameter=.66m Using online converters, does 440L seem right?
6) Currently zoned for CH & HW with motorized values and TRV on all the rad minus where the temp stat is
7) Linear length of all rad (@ a .5m height) is 28.5m

So using Shane's calculation that's 28.5Kw + 8.8Kw. I'd say we got carried away with the doubles. Mind you the TRV's arent open on all/most of them. But I'm also taking on board it shouldnt really be driven by the rads but rather the envelope, and in my case I'd say we have "too much" rad capacity.

So in my case should I be concerning anything else than 30Kw, or would a 24Kw be too small?

PS: Havent done a BER on the house but found a similar house in the "sister" estate built by the same builder. Its being advertised @ a D1, so that gives an indication of the insulation properties.

Thanks
W