Getting proper advice on a boiler for a less usual build.

[Curlewhouse]

I'm building my own 3 bedroom (204 m2 floor space total / volume 518m3) house with very high insulation levels (SIPs + 100mm K3 insulation, mechanical heat recovery ventilation, airtightness and so on). There's no mains gas, and for reasons too involved to go into, Air Source Heat Pumps are not a suitable answer for us, so it's an LPG system we want. We will have underfloor heating, so I know it will need to be a modulating boiler, but I'm getting conflicting advice on whether to fit a combi or system boiler. There will be 2 showers in the house, but it is very unlikely that the 2 will ever be in use at the very same time (and I can make the less used one an electric one if necessary if it saves fitting a much bigger boiler than is needed for 99% of the time). Our SAP thing (all 26 pages of it - good God it's gobbledygook!) says that with all our insulation etc our space heating requirement is 5650 kWh/year, or 27.42 kWh/m2/year which I gather is good. For DHW, the family is 2 adults and 1 child in late teens about to go to university. So there will mostly be just the two of us, but occasionally 3 or 4 occupants.

I've had such wildly different answers from 2 plumbers as to what size & type I should have installed that I'm looking for advice from someone who is not trying to sell me something! Any ideas?

 

[slippyr4]

Sounds like your unusual build isn't really relevant - this is the often discussed question of combi or conventional + HW cylinder. A system boiler is basically a combi with the hot water bits removed.

 

[Doggit]

You've been given a figure of 5650Kw/ year, but what's the system requirement. Have they given you a figure for the size of the boiler that you need - okay, I'm old fashioned.

If you're going to fit an electric shower in the least used one, then a combi boiler would be fine, but if you want the ability to run both shower at once, then you'd need a system boiler, and an unvented cylinder.

 

[Curlewhouse]

Hi, no, the SAP doesn't specify the boiler size, that's the issue really. They just tell you the amount of energy the house will "consume" for DHW & heating and boiler size is one of the things I had quite conflicting advice on - how that converts to boiler size. One seemed way too big (and appeared unfamiliar with underfloor heating) and the other said simply "the smallest" .... not exactly confidence building.

 

[Dan Robinson]

Daikin Altherma Hybrid.

 

[Doggit]

There's no way of converting (that I know of) a yearly rate, to a boiler capacity; divide 5650Kw by 365, and you're using 18Kw per day which is pretty low, but what did the plumbers quote and suggest.

 

[muggles]

I'm building my own 3 bedroom (204 m2 floor space total / volume 518m3) house with very high insulation levels (SIPs + 100mm K3 insulation, mechanical heat recovery ventilation, airtightness and so on). There's no mains gas, and for reasons too involved to go into, Air Source Heat Pumps are not a suitable answer for us, so it's an LPG system we want. We will have underfloor heating, so I know it will need to be a modulating boiler, but I'm getting conflicting advice on whether to fit a combi or system boiler. There will be 2 showers in the house, but it is very unlikely that the 2 will ever be in use at the very same time (and I can make the less used one an electric one if necessary if it saves fitting a much bigger boiler than is needed for 99% of the time). Our SAP thing (all 26 pages of it - good God it's gobbledygook!) says that with all our insulation etc our space heating requirement is 5650 kWh/year, or 27.42 kWh/m2/year which I gather is good. For DHW, the family is 2 adults and 1 child in late teens about to go to university. So there will mostly be just the two of us, but occasionally 3 or 4 occupants.

I've had such wildly different answers from 2 plumbers as to what size & type I should have installed that I'm looking for advice from someone who is not trying to sell me something! Any ideas?

5650kW/year for space heating? Based on heating for 4 months of the year, that's 47kW/day, or about 5.8kWh assuming you run your heating 8 hours a day. That's two electric fan heaters

All modern gas boilers are modulating. You need one with a particularly low minimum output to get best efficency. I wouldn't ever reommend a combi for a house your size - a system boiler and unvented hot water cylinder is the way to go, assuming your mains water supply is decent. Specifically, I'd recommend the Viessmann 200-W 19kW system boiler, connected to a Vitocell cylinder via their integral 4-pipe diverter kit. This will give you a minimum output on the heating of 1.9kW, a low flow temperature to your UFH for max efficiency, and a high flow temperature to your hot water cylinder for ultra-fast heat up times (around 20 minutes).

They're LPG compatible, have the option of WiFi app control and can also be fully integrated with solar thermal heating if that's something you're interested in, providing a report in the boiler's "energy cockpit" of how it's all performing. All clever stuff and, in my opinion, perfect for your needs

 

[Doggit]

Thank you muggles, I learnt a lot from that little piece.

 

[Curlewhouse]

That sounds perfect! Thanks.