System boiler sizing?

[jp1*]

Hi,

I'm looking at getting a new system boiler and have had 4 quotes but three different recommended size boilers, leaving me confused!

I have a 4 bed house that has 10 Rads + 2 two heated towel rails. There are no plans to extend the house etc and to add rads.

What size KW boiler do I need?

Thanks

 

[Dan Robinson]

18kw

 

[jp1*]

That's the first I've had for an 18kw - thanks though!

 

[Dan Robinson]

My 4 bed detached has 15kW

 

[Dan Robinson]

And that's got a massive ground floor extension.

 

[D_Hailsham]

Baxi Whole House Boiler Size Calculator will tell you. (It is product-neutral.)

 

[vulcancontinental]

Are you warm enough at the moment? If so go with that boiler size. If you want a different route and you're warm enough measure each rad and get the outputs from a chart, add 'em up and there you go plus at least 3-6KW for the hot water. In my limited experience of whole house calculators they're oversized because of the unknowns; wall construction, insulation, glazing, roof insulation, ceiling heights room and furniture layout, curtain lengths, geographic location and exposure. Get a boiler with wide modulation then if it's too large you will not lose efficiency with cycling.

Better than that, which of the installers seemed the most competantt? Go with him, he's your professional advisor. You have doubts, why?

 

[Agile]

The EST recommends just 2 kW addition for hot water !

 

[Dan Robinson]

The EST recommends all sorts of boollocks.

 

[MikeyGas]

18-24kw depending on if you have an unvented cylinder, hot water priority or standard open vent cylinder etc.

 

[D_Hailsham]

measure each rad and get the outputs from a chart, add 'em up and there you go

How do you know the rads are not oversized? If you did that for my house you would say a 14kW boiler was required (ignoring HW), but in reality you need about half that due to insulation improvements since the rads were first installed.

In my limited experience of whole house calculators they're oversized because of the unknowns; wall construction, insulation, glazing, roof insulation, ceiling heights room and furniture layout, curtain lengths, geographic location and exposure.

How many installers do you know who will take all those factors into account?

 

[Agile]

I have always thought they were oversized because they only calculated the standing losses.

So more heat is required for the first heat up from cold.

 

[DP]

I have always thought they were oversized because they only calculated the standing losses.

So more heat is required for the first heat up from cold.

If a cylinder is to reach stipulated hot water delivery, cannot see 2kw achieving that. Cylinder spec often quotes figure like 70% delivery in 17 minutes etc which 2kw will never deliver especially when heating is also running. If 2kw was what the cylinder needed, can imagine primaries to cylinder needing no more than 8mm pipe.

 

[JohnD]

Cylinder spec often quotes figure like 70% delivery in 17 minutes etc which 2kw will never deliver especially when heating is also running.

You're assuming, then, that the CH is running flat out all the time? Is that usual where you are?

Here it is very rare.

 

[vulcancontinental]

measure each rad and get the outputs from a chart, add 'em up and there you go

How do you know the rads are not oversized? If you did that for my house you would say a 14kW boiler was required (ignoring HW), but in reality you need about half that due to insulation improvements since the rads were first installed.

In my limited experience of whole house calculators they're oversized because of the unknowns; wall construction, insulation, glazing, roof insulation, ceiling heights room and furniture layout, curtain lengths, geographic location and exposure.

How many installers do you know who will take all those factors into account?

We're in the age of condensing boilers, existing oversized rads are no problem and in fact are an advatage, especially in light of improvements to insulation etc subsequent to the original (and probably oversized at the time) installation. That plus a boiler with a wide modulation range will work well and efficiently. One advantage the poster has is she or he knows whether they are warm enough or not at present ergo the rads are not undersized or there are other circulatory problems. That's a good place to start.

As for the number of installers who take those factors into account probably thousands but just in case I'm wrong, at least one, me. Pointless question anyway, I might just as well ask you to quantify the number you know who do not.