Boiler sizing and SAP

[cully]

A couple of questions re new build and heating requirement
Should I have all the info I require to size my central heating requirement from a new house SAP calculation information?
The house, having five bath / en-suite rooms plus the rest will require a big DHW tank, because of this I would imagine the boiler would be sized to suit the DHW. Given the DHW priority over the central heating the boiler sizing would be to the cylinder water modulating down to suit the central heating requirement. Is my thinking correct?
Thanks

 

[D_Hailsham]

Forget SAP; use Baxi Whole House Boiler Size Calculator.

If it's a new house, built to current standards the heat loss will be low - say 15kW.

 

[MJGas]

Forget SAP; use Baxi Whole House Boiler Size Calculator.

If it's a new house, built to current standards the heat loss will be low - say 15kW.

Hahahahahahahahahahahahahahahahahahahahahaha.

Im doing a new build at the moment where the heat loss has been worked out at 6.5kw for a laugh I thought I would try the Baxi calculator just now, Answering as best as that POS would allow it came in at 38.1kw

 

[Agile]

The advice on sizing of boilers is to add 2 kW to the heating loss of the building for heating hot water. Most programs already include that, but it does need to be confirmed.

Hot water storage should be sized at around 50 litres plus 50 litres per person. So the important question is how many people will live there.

The rationale is that the hot water will be timed to be heated before the central heating is set to be on. It will be reheated as required during the day but the storage will be sufficient to ensure that with normal usage the hot water will never run out.

The heating demand is the worst case when its -1 C outside, something rarely encountered in England. The allowance is already generous to allow for heat up etc.

Tony

 

[cully]

This is a big 6 bed house (580 sq mtrs.) and working on your 50lt / person calc is going to be a 500lt cylinder
Underfloor heating to ground floor other two floors over sized rads to heat the rooms using the 60* flow temp of the underfloor heating circuit.
Is there any gain in using two small heating boilers (oil) in this house instead of one big one thinking about the size of the cyl. water to heat and most of the rads turned off creating a problem for a big boiler to modulate down far enough to be efficient heating the ground floor only?
Thanks for the response
Paul
ps MJgas which way did you go with your 6.5kW to 38 kW requirement conundrum.

 

[D_Hailsham]

Answering as best as that POS would allow it came in at 38.1kw

Then, with the greatest respect as I don't know you personally, you have input the data incorrectly.

Remember GIGO (garbage in; garbage out)?

 

[Agile]

Oil boilers don't normally modulate.

Unless they all use water at the same time then a 300 li cylinder would usually manage with 9 people although a larger boiler capacity available would be needed to ensure faster recovery. But the 500 li would be safer.

 

[MJGas]

Then, with the greatest respect as I don't know you personally, you have input the data incorrectly.

Remember GIGO (garbage in; garbage out)?

With the greatest reapect that whole house calculator is bullshit and gives very limited options for insulation valvues. If a property is a new build workinb to current standards its going to be a big property needing even 15kw as the current standard is 50w/m2 due to go down to 35w/m2 soon.

 

[MJGas]

Current house we are looking at has a heated space of 573m2 and a membee on here kindly did a full heatloss calc and it came in at 20kw. Hope that helps OP.

As for your other question the house with a 6.5kw heatload will be getting a 12kw heatpump. But thats more for the water recovery.

 

[D_Hailsham]

the current standard is 50w/m2 due to go down to 35w/m2 soon.

Where are those standards published?

OP's house is 580m² (post #5). So at 50w per m² that is 29kW.

580m² over three floors, assuming the house is square, gives a wall length of 13.9m.

Putting this into the calculator and assuming a detached house with best insulation and living in Scotland, I get 23.3kW (excluding HW). That's 43W/m².

Bullshit? I don't think so.

Down in the South West it would be 19.9kW. The regional difference is because the calculator takes into account the anticipated minimum temperatures in each region.

 

[cully]

I mentioned in the previous thread where there is any mileage in utilizing two boilers in this house, now knowing that oil boilers don't modulate (didn't know that) then two smaller boilers should reduce cycling.
Am I on the right track??
Paul

 

[Agile]

In theory a small and a large boiler with a good cascade controller would be good.

But for most normal domestic would be total overkill.

Tony