Does this boiler calculation sound right?

[spacecat]

I am about to move into a 4 bed 1930's detached house and have done the boiler calculation website form thingy and its coming out at at 16,500 BTU

This seems is low? or is that about right?

 

[PEDANTICVINDICTIVEMAN]

In a nutshell no.

 

[Andygasman2010]

Yes that's way too low. You sure it wasn't 16.5kw?

The house you describe i'd say wants about 30kw (105000btu)

Obviously that's a very rough guess.

 

[PEDANTICVINDICTIVEMAN]

I was thinking more like 18 to 24Kw 60,000 to 80,000btu, but then without seeing the property etc it's always tricky.

 

[spacecat]

16.7 KW is what the sedbuk calculator says

so is it pointless doing this just wait for an engineer to visit

the length is 9 metres width about 7 metres . height of rooms about 9ft -2.7 metres
2 floors, pvc double glazed, brick walls , pitched roof

 

[spacecat]

another site came out at 17.68KW

 

[Andygasman2010]

I was thinking more like 18 to 24Kw 60,000 to 80,000btu, but then without seeing the property etc it's always tricky.

This is probably more acurate, difficult to say without seeing the house. A decent engineer will tell you for sure. You could get a boiler that you can range-rate which means you can taylor it to your houses exact needs

 

[ChrisR]

the boiler calculation website form thingy

WHere is that?
Perhaps you forgot air changes or something?

 

[Agile]

Not many people take a heating calculation with them when they first move into a property.

Most concentrate on curtains, furniture carpets and things like that!

I do have to agree any heating work should be done before any carpets.

Heating work is best done during the summer when we are less busy.

Tony

 

[D_Hailsham]

16.7 KW is what the sedbuk calculator says

the length is 9 metres width about 7 metres . height of rooms about 9ft -2.7 metres
2 floors, pvc double glazed, brick walls , pitched roof

I also get 16.7kW. That is assuming you have solid walls and over 75mm insulation in the loft. Don't forget that the answer includes 2kW for HW, ie. heating is 14.7kw

Most 1930's houses have cavity walls. Are you sure yours does not? If you have cavity walls it will reduce to 12.8kW if unfilled and 8.4kW if filled (both plus 2kW for water). Your surveyor will be able to tell you if it has cavity walls.

I did a detailed heat loss calculation on my house and compared it with Sedbuk whole house; the difference was negligible.

What you have to remember is that Sedbuk assumes a cold temperature of -1C and a room temperature of 21C, i.e a 22C difference. If you want to use different temperatures you have to make allowances. Fortunately its directly proportional. So for -5 and 21 (difference = 26) you multiply the heat required by 26/22. So you would get 14.7 x 26/22 = 17.3 +2 = 19.3kW

I'm not sure if you are just replacing the boiler or installing a complete new system.

If its a new boiler, do you know the total output of your radiators? If not, Stelrad Elite Catalogue to get an idea. They should total the same or more than the calculated heating requirement, i.e 14.7kW

If it a new system, the rads should total about 20% more than the heating requirement. This will allow the boiler to run at lower temperatures, which is more efficient.

 

[spacecat]

EThanks for the responses

The house has 225ml thick brick/solid walls

I will be replacing an old fs potterton boiler and the rads hopefully using existing pipework

I am still considering:combi, gravity or system

I am leaning towards combi

 

[D_Hailsham]

I am leaning towards combi

If so, you need to check the cold water flow rate and pressure. The number of simultaneously running hot outlets is also significant. More than two can be a problem.

You can check the flow rate at the cold water tap in the kitchen using a bucket and a watch - most buckets have litre markings on the inside. You need at least 15 litres/min.

If you have an external tap straight off the mains, you can also check the pressure. Turn the external tap on full and repeat the bucket test on the kitchen tap. The less the flow rate drops in the kitchen, the better.

The size of combi boiler is determined by the flow rate. You need 2.5kW for each litre/min flow rate. This gives a temperature rise of 35C, which is more than adequate. Installing a larger boiler will give a higher temperature, it won't improve the flow rate.

 

[spacecat]

Does this mean a minimum combi is 37.5 KW?
15 litres per min x 2.5KW=37.5kw

 

[D_Hailsham]

Does this mean a minimum combi is 37.5 KW?
15 litres per min x 2.5KW=37.5kw

As combis can be as small as 25kW, I suppose I had better revise that to 10 litre/min.

Obviously when showering/filling the bath you are using both hot and cold water so the combined flow rate will be much higher. Check the rate from the bath taps and compare it to the cold water tap in the kitchen. I suppose it all comes down to what is acceptable when you are showering, filling the bath etc.

The number of outlets running at the same time is important. If you have two and want a flow rate of 10 litres/sec from each of them, then you need 20 litres/sec coming into the house and a 50kW boiler.

 

[Agile]

Most 1930's houses have cavity walls. Are you sure yours does not? If you have cavity walls it will reduce to 12.8kW if unfilled and 8.4kW if filled (both plus 2kW for water). Your surveyor will be able to tell you if it has cavity walls.

In the London area most pre 1940 houses have solid walls !

Tony