NEW BOILER SIZE:

RONCHAM · 6 Nov 2009

This might be easier to answer remotely than some of my previous questions??!!

I've decided to renew my boiler plus other works. Guy who quoted said off the top of his head (and based on experience) I need a 24KW Worcester Bosch regular boiler for CH and HW. W/B's own site has a rough calculator which gave 12KW (for 1-3 bedrooms) and a more detailed site actually gave similar answer. I'm in a "standard" 1930s 3-bed semi. (30' x 18' plan) although extended living room is now 18'x15', double-glazed throughout. There's not a great price difference for different outputs but I thought boilers were best when working hard? So, although boiler is modulating, is 24KW excessive and does it matter?? (I also read somewhere these boilers are set by default anyway to 15KW??).

Agile · 6 Nov 2009

He is one of the idiots who fits over sized boilers! You dont need much experience to know 24 kW will heat a five bedroom detached house!

A typical semi needs about 11-12 kW for heating and you add 2 kW for the hot water cylinder.

So a 15 kW boiler is fine.

Its not just the maximum power thats relevant. If the boiler is too powerful then it does not modulate down far enough to stay on and then inefficiently cycles on/off.

Tony

RONCHAM · 6 Nov 2009

Thanks for your quick reply. I agree - in fact, I reckon my CH need is possibly 8-9KW (there's no rad in kitchen) plus 2-3KW for HW but 15KW would allow for house extension. W/B site jumps from 12KW to 24KW for 4-5 bedroom property although they make 15KW and 18KW boilers. Do you know if it's true that factory default setting is 15KW anyway for the 24KW boiler? Or could boiler temperature be reduced if output is found to be excessive? Although I believe there is a possibility of it not working in condenser mode if reduced too much?

Obviously reluctant to tell this guy his job!! - but I think his other proposals seem sound - Honeywell valves/stats/programmer and a Grundfos pump.

D_Hailsham · 6 Nov 2009

Use the Boiler Sizing Wizard and you will get a reliable answer.

kevplumb · 6 Nov 2009

real good that

2 up 2 down mid terraced

32 kw

doitall · 6 Nov 2009

A 12-15Kw boiler will be plenty big enough and should modulate down to 6-7KW.

D_Hailsham · 6 Nov 2009

kevplumb said:
real good that

2 up 2 down mid terraced

32 kw

If you are referring to the Boiler sizing wizard, then you have used it incorrectly. I have just recalculated my son's 1960's 3 bed mid-terrace and it works out at 10KW and that assumes no insulation and located in Scotland.

RONCHAM · 6 Nov 2009

Thanks everybody. Thanks D_Hailsham - that's the one I used!! It gave 11KW. So I'm quite sure 15KW or at very most 18KW would have ample capacity for expansion/extension. About £70 difference in price on boilers (15/24)which are physically identical - wonder if I can get that lopped off bill??!! This guy has premises and 5 plumbers, 50 years experience plus he's accredited by W/B - why would he stray so far on high side??
He's got an 08 Merc to support but surely that's not relevant!!

doitall · 6 Nov 2009

Very relevant.

Someone had to pay, or should that be everyone has to pay a premium rate so he can make lots of profit, and buy nice cars.

Thats why we go to work.

What boiler has he quoted for, and dread I ask how much.

D_Hailsham · 6 Nov 2009

RONCHAM said:
Thanks everybody. Thanks D_Hailsham - that's the one I used!! It gave 11KW. So I'm quite sure 15KW or at very most 18KW would have ample capacity for expansion/extension.

Have a look at the Broag Remeha Avanta open vented Heating boilers. There are five boilers, ranging from 12-30kW. The 12, 15 and 18kW versions are identical, they have just been pre-set to the stated output. If you get the 15 or 18kW version you can easily drop it down to 11kW; and when you extend the house, you can up the output again.

They come with a five year parts and labour warranty if fitted by an approved engineer.

RONCHAM · 6 Nov 2009

Thanks again for advice. In fairness to the guy, if he keeps 5 plumbers gainfully employed he probably deserves his Merc. Quote was £2,200 + VAT for a W/Bosch 24Ri. Probably on high side but does include new Honeywell MVs, stats (1 wireless), programmer and pump, and some rationalising of electrics/new wiring.

In fact, have just this minute received an almost identical quote from a 2-guy outfit, they suggest an 18Ri which is nearer the mark - he said it's better to have the boiler working hard.

Not worried so much about extra cost of a bigger boiler (which is minimal) - more concerned about getting it right.

RONCHAM · 6 Nov 2009

Thanks D_Hailsham.

They seem to be a similar price to W/B (which also has 5-year guarantee). I imagine W/B have been around (in this country) a bit longer? Don't often hear people speak badly of them so perhaps a matter of personal preference/experience.

bengasman · 6 Nov 2009

D_Hailsham said:
Use the Boiler Sizing Wizard and you will get a reliable answer.

Unless you are planning to change your rads, the program is absolutely useless. Not surprisingly, it comes from a householder who has neither the qualifications, nor any experience in domestic heating issues.

If you are keeping the existing rads, there is a very simple way to get very close to what you need.

Measure the length of all the radiators in metres, and add them up; double rads count as twice the actual length. This gives you the size of the boiler in kW.
This calculation is based on a standard height of 600 mm; for different heights, you adjust proportionally.

If there are any rooms that are often not warm enough, add a liberal amount to the rad before you do the calculation.

Did you mean by "regular" boiler, one that has a cylinder?

Gasman77 · 6 Nov 2009

Personally i'd go with the worcester, put alot in with no probs. Installed 2 Remeha's and had problems with both so won't be going down that road again.

bengasman · 6 Nov 2009

Second that. Have yet to get a client with less than 100% satisfaction.

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