PHPP assessment

[vk108]

Hi, As part of my PHPP calculations for my self build, The model indicates space heating demand of 15.1 kWh/m2a, peak heating load of 10.6 W/m2 and overheating of 1.4%.

 

I am new to PHPP and don't understand the spreadsheet calculations, Can some one suggest what kind of heating enough for us (radiators, UFH)... any suggestions would be greatly helpful for me

[Dreadnaught]

How many m² will your self build have?

[vk108]

Hi @Dreadnaught,   Treated Floor area is 226 sqm over 2 floors with flatroof

[A_L]

Heating load of 10.6W/m²  x 226 sqm = a heat demand of 2395.6Watts at -10°C so a few 'micro' oil filled radiators would do ?. More important is your intended heating regime, a 24/7 one only requires to meet the heat demand but if you want an on/off regime you will need to allow for more to have a quick 'warm-up' but probably not to much (say 50%) as PH level houses (and yours just misses the requirements) do not lose much temperature of periods of a few hours.  The 15.1kWh/annum just means you will need 15.1 x 226 = 3412.6kWh per year, not particularly useful. The overheating % is acceptable.

Edited April 16, 2019 by A_L
typos

[Dreadnaught]

Personally think those are pretty impressive figures, not quite PH but close.

 

If you heated the house with just electric resistive heat (not an ASHP) you will be paying about £40 a month on average across the year (ignoring standing charge). With an ASHP, and a guesstimate CoP (coefficient of performance) of, say 3, that cost could be less.

 

And your peak heating load will be 2.4 kW, so a smallish ASHP heating to UFH could be a good match for space heating, assuming you have a some sort of slab in which to run the pipes.

 

Domestic-hot-water heating could also be supplied by the ASHP to an unvented cylinder @joe90-style.

It sounds to me like you house design is close to some of the near-PH houses built by those on this site so solutions similar to the ones they are using could match your circumstances well.

 

Do you have access to mains gas?

Edited April 16, 2019 by Dreadnaught

[vk108]

yes @Dreadnaught, we do have Mains Gas

[Dreadnaught]

Sadly for fans of the latest tech, having sensibly-priced access to mains gas usually means you should use it for heating. An old fashioned gas boiler and unvented cylinder.

 

You just won't use very much of it.

Edited April 16, 2019 by Dreadnaught

[vk108]

and my designer said Radiators on Ground floor and just heated Towel rails on First floor bathrooms. we like UFH on ground floor, but he said its not necessary to have UFH with existing design

[Dreadnaught]

1 minute ago, vk108 said:

and my designer said Radiators on Ground floor and just heated Towel rails on First floor bathrooms. we like UFH on ground floor, but he said its not necessary to have UFH with existing design

 

He/she's probably right.

 

Are you having an insulated reinforced-concrete raft foundation?

[vk108]

yes we are having insulated reinforced-concrete raft foundation

[Dreadnaught]

The advantage of ASHP, UFH and pipes in the raft is that it can easily be used for summer cooling, if that is needed (ASHP in reverse). Gas will likely be the sensible choice. ASHP will be tech-lovers choice for flexibility.

Edited April 16, 2019 by Dreadnaught

[Dreadnaught]

Personally, I would try to get your 1.5% overheating risk down a bit. Its way lower than the PH requirement (10%) but I would aim for less than 1% myself. There are many examples of 0%.

 

As you may have read on this site, with a near PHs, summer overheating can often be more of a challenge than winter heating.

[vk108]

Yes its due to south glazing, But resistance from family not to reduce glazing further ?.....hopefully slab cooling will be the best option

[Ed Davies]

1 hour ago, Dreadnaught said:

If you heated the house with just electric resistive heat (not an ASHP) you will be paying about £40 a month on average across the year (ignoring standing charge). With an ASHP, and a guesstimate CoP (coefficient of performance) of, say 3, that cost could be less.

 

Bearing in mind that heating for hot water is likely to be in the same ballpark (e.g., 3 people x 3 kWh/peep·day x 365 days/year = 3285 kWh/year) you'll get almost as much reduction on that with an ASHP. Obviously depends on the number of people and your water usage. At a COP of 3 on an ASHP and assuming £0.17/kWh your total heating bill would be

 

>>> (3 * 365 * 3 + 226 * 15) /3 * .17
378.25

 

Take off the standing charge and boiler servicing costs and work out how long the payback on gas connection would be. It seems to me this is one of the places where it really doesn't cost that much financially to be a little less un-green.

[Conor]

I've looked at costs (very roughly!) On this and gas always comes up on top. The payback on ASHP is quite long over gas... And that assumes nothing goes wrong with the unit and service costs are comparable to gas. I just feel that ASHP tech just isn't there yet. I can get a gas fired system installed for half the price of an ASHP system and running costs are about the same (5p/ kWh assuming COP of 300%). My last boiler service cost £45. Am I missing something obvious? I'm all for decarbonising but doesn't seem to make sense if you are on natural gas network.