Boiler Sizing Help

[Antony Robinson]

Hi,

In the middle of an house extension and would appreciate some guidance on boiler sizing.

The house is a 3 bed detached two storey house built in 1960s so has small 50mm filled cavity walls. Width = 7m, depth = 8m, double glazed & insulated, room height = 2.4m.

Extending two storey 3.2m on the side, On the back one storey across the whole width = 10m.
Will have 1 bath, 2 showers, 2 basins.

All downstairs will have underfloor heating. Upstairs will have radiators.

Current boiler is a conventional Gloworm Flexicom 18hx, less than 5 years old. It is in one of the bedrooms so has to be moved when the new extension is water tight.
Bits in the loft look well thought out (cold storage tank, immersion tank with elecrtic heater element, radiator filler tank, magnet filter)

I don't need the space in the loft so was planning to keep all the tanks in the loft and just go for a conventional boiler. Was hoping to place the boiler in the loft in the gable end of the new two storey side extension and just route the two pipes that go into the existing boiler approx 5m.

I've used a few of the boiler sizing calculators. One of them suggest that I only require a 15kW boiler. British Gas man said that I may be able to get away with just moving the Gloworm 18kW but I would rather not pay once to move it and then again to replace with a biger boiler.

Any advice on boiler sizing when half the house will have UFH would be greatly appreciated.

Ant

 

[Dan Robinson]

The current boiler is plenty. Just move it.

 

[Antony Robinson]

Dan,

How can I be sure that 18kW is enough? I can't seemd to find a any decent online calculators that allow you to provide enough detail.

 

[Razor900]

The current boiler is plenty. Just move it.

Defenitely!!

 

[Twenty Four]

Anybody else feel this thread is going to go wrong soon?

18KW is ample for your needs.

 

[Antony Robinson]

Thanks for the input.
I'm not going to push for a bigger boiler if not needed. I've seen how long some of the other forum threads go on for.
Another online calc came up with 15.2kW. We'll have big sliding doors & 3 big velus windows so maybe it should be more like 16kW.

 

[Dan Robinson]

If you saw the size of the house I'm fitting a boiler into tomorrow and then heard that I wanted to fit an 18kW unit then i think you'd be surprised.

 

[Razor900]

Thanks for the input.
I'm not going to push for a bigger boiler if not needed. I've seen how long some of the other forum threads go on for.
Another online calc came up with 15.2kW. We'll have big sliding doors & 3 big velus windows so maybe it should be more like 16kW.

I'd say 16.35 kW would be perfect

 

[Antony Robinson]

Now have to decide whether to move existing boiler or buy new one with a decent guarantee. One plumber replied back to me saying that he doesn't move boilers as they have more chance of a leak.
Would rather keep old one as it's less than 4 years old.

 

[Dan Robinson]

At that age I would probably agree to move it subject to a new flue being fitted and being in decent conditions (and full service with gaskets).

You'd also be getting some caveats as part of the work.

 

[PullerGas]

Is the OP related to you Dan??
FFS I know it's not wise working for relatives, but at least give him the size of the Intergas he should be fitting. Perhaps it'll be less of an atmosphere at family gathering, just a thought.

 

[Dan Robinson]

Only one of my immediate relatives on gas and he owns a Faillant Turbomax.

 

[Antony Robinson]

Two local plumbers both say moving 5 year old gloworm boiler a bad idea.
Think I'll play it safe and buy a new one with a warranty for about £700 + flue kit. By the time I've bought a new flue kit and paid the extra in labour for uninstalling old boiler I think there will be about £500 difference
Whole extension is consting near £75k so would rather spend the extra for a new boiler.
Online cals suggest just under 16kW so do you think 18kw would be enough to cover any losses, errors in the estimate?
Does UFH require more output from a boiler or less due to it being more efficient in spreading heat in a room?
Anyone suggest efficient and reliable 18kw heat only boiler. Already have the tanks in the loft.

 

[Agile]

It is a common problem for people who come to this forum not to understand how boiler sizing works and what should be done and what should not be done.

Not helped by most builders and some installers who like to seriously oversize boilers.

The full boiler output is only required when it is -1 C outside. That's pretty rare with global warming.

Even if the boiler was slightly oversized then all that would happen is that it would take a little longer to heat up. At the extreme it would only achieve say 20 C or 19 C instead of the rated 21 C.

These calculators oversize the boiler by 2 kW to cover water heating and allow about 20% for heating up from cold. So there is a lot of spare output available to cover cold spells which we hardly ever see now. When did you last throw a snowball in the UK ?

Boilers generally last about 10 years. The G-W is seen as a rather budget boiler and as less reliable than most. Regardless, my view is that it would be worthwhile to move it and keep the £900 cost of a new boiler for later when it needs to be replaced in another five years or so. There is the same cost to move a boiler as to fit a new one.

The old boiler still needs to be removed even if you buy a new one!

Tony

 

[Antony Robinson]

1st plumber today recommended 24kW!