I am building a new house, 158m2 internal space. I have to meet sustainable regulations as it was on a 'greenfield' site (between two houses, on a road in a village - but it had cows on it), so i am not passive house standard, they call it an 'eco-house' which in reality seems to be only a bit higher than standard building regs.

There is no town gas here so the SAP report says to use an ASHP to UFH with an electric emersion heater for DHW.
….however my preference would be to drop the ASHP altogether and have Calor gas, a PV array and a small wood burner with a back boiler. I have also downloaded all the Accredited Details forms, which i will document as i go, to improve my SAP rating.

Question is how can i calculate if this is possible? Do i go back to the report writer or can i do it myself? I'm concerned that if i wasn't to pass the SAP rating i wouldn't get my VAT back

@treeplanter, ask your 'SAP' assessor to model these changes.... it would take about 30 seconds to tell you the answers you require?
quote-
I'm concerned that if i wasn't to pass the SAP rating i wouldn't get my VAT back -
No, you would not get your house signed off by LABC / AI, and have other issues to deal with as well !
Good luck

Richard is right, you should speak to your SAP assessor.
You are paying him/her to get to a solution that works for you (I say this as a SAP assessor myself).

Your solution would work fine. I've just tried it on one of my projects although I did assume that the bottled gas boiler would run the full heating and hot water.
I entered the wood burner as secondary heating only and ignored the back boiler option because I've never seen it used before. Using the wood burner to heat water rather than just as a room heater should improve SAP performance so this would be ok if you wanted to do it.

By the way it took me longer to write this comment than it did to try out your SAP option.

Thanks for all the replies, its very helpful. I will contact the SAP report firm and ask them to re-model it.

I am project managing the build myself and hoping to bring it in at around £875-900/m2 so having a passive house 'built for me' isn't an option.
And I don't want an ASHP as they are an expensive piece of kit and don't feel as eco friendly as 'proper' renewable sources like PV and solar thermal. I think having the gas boiler and wood burner should be enough back up for very cold weather. I'll see what the SAP guy says.

(...and i'm lucky enough to have access to some space in darkest norfolk to plant some hazel for firewood, so in 5 years it will be free! :)

Oh yes, the £10-15k we were quoted for a ASHP system when we started in 2013, was down to £3k when we fitted one this year. Still not cheap, but prices are down and it is worth ignoring the MCS approved route. The money the gov may give back eventually for an approved system is stuck on the price up front.

I don't mean to sound like a salesman for ASHP, but it has been a revelation for me.

my SAP assessor let me play "what if" games to see what would work. His gut feeling was that I would need a GSHP but we managed to find a solution with PV, direct electric heaters and electric immersion, MVHR and a wood stove that satisfied the requirement. just !

Log boilers, thermal stores and ancillaries are expensive and it's still nice to have a wood stove anyway.

Posted By: treeplanterMy biggest concern with the ASHP is the enormous amount of electric it uses. I spoke to a self build owner today who said hers works beautifully but when i asked about running costs (3 bed detached house) it's using £1500-2000/p.a in electricity, which is terrible!! I've heard similar things about others.

I suspect that is because they are using it to heat water to 65°C+. As the water temperature goes up, it relies more and more on the built in resistance heater, this reduces the CoP to 1. It will also be defrosting more often and for longer.
Ideally you want to set the water temp to 40°C and then use an inline modulation electric heater to lift the temperature up to 65°C when it is needed.
The other advantage of storing water at 40°C is that the cylinder losses are lower.

If you are serious about renting the place out, you are better off with just one fuel source. You only need one safety certificate then.

And do you really want a dirty woodburner with all the pollution issues they cause (see Mikee thread on here about them).

Ed, when you ask for the 'space heating requirement' where is that on the SAP report?

He's put down 59.5 kWh/m2 which is the minimum for a pass, and he used just Calor Gas, a normal wood burner, along with 1.5kw of PV.

I figure on using more PV and Solar Thermal as i have the roof space, and a back boiler on the woodburner.
He's set the fabric targets really low too, i will beat all of them - except the floor which stays at 0.13 U-value.

...does that help?

Posted By: SteamyTeaThe other advantage of storing water at 40°C is that the cylinder losses are lower.

You must NOT store water at 40c unless it is in a closed system, so if you want to do this use a thermal store rather then a normal hot water cylinder.

Forget about Solar Thermal is it not worth the cost, better to use the PV to heat the water when you are not using all the power the PV generates. Not having Solar Thermal save both capital cost, and maintenance cost.
If a property I was going to rent out had Solar Thermal, I would consider removing it, so as to reduce issues with tenants!

Given that tenants often think of heating controllers as being on/off, and ASHP (connected to radiators or UFH) hates this, I would not put a in ASHP if there is any chance the property will be let out!

A LPG (or oil tank) has issues with you having to check how much is in it when tenants change, you are dependent on a letting agent to do this check, but most letting agent’s staffs are chosen on their ability to charm people, so may have no idea what a LPG tank is!

I am coming round to A2A heat pumps in the down stairs rooms and simple electrical heaters (fixed to the walls) in the bedrooms, along with a very good level of insulation. Then put on as much PV as you can, with a DHW tank that is heated by excess PV and E7. See what SAP rating that will give you….

Posted By: SteamyTea: “And do you really want a dirty woodburner with all the pollution issues they cause (see Mikee thread on here about them).”

Lots of threads this link could go on (and maybe it has but I've missed it) but still this thread's about one which can be prevented:

http://www.bmj.com/content/350/bmj.h2757/rr-1

Posted By: treeplanter
Ringi + Ed - Thanks for the advice on the solar thermal and just using PV. It would simplify my heating to just have LPG gas and PV. How much PV would you want to have?

As much PV as you can, as the inverter is a fixed cost. There is a limit to what the electrical board will allow you to have, so ask them.

Can you use a GRF fake chimney to keep the planners happy?

The DWH water tank should hold what you expect to use in a "normal day", so you can mostly heat it all on E7, and only top up on full price electric when you have more people staying. Or put in a thermal store that will hold about 1 or 2 days of PV output, then use the thermal store to preheat the water before it goes into your LPG combi.

Posted By: treeplanterDo i need a massive immersion tank to store warm water in?

This is always the dilemma.

A thermal store can hold different amounts of energy depending on the temperature. So you can have a small store at 80°C holding as much energy as one double the size at 40°C. Or you can have two separate stores in series, one that stores at 40°C and another at 80°C (or what ever temperatures you like).
Ringi mentions having a store that is large enough to hold 1 or 2 days PV output, but which days are those? And would the previous days generation fit in with your lifestyle over the next 2 days. This is why you need to know your normal DHW usage.
When you use the water is important too. Ideally you need a cold store at the end of the day, then it can take full advantage of the PV during the next day. That pattern is the absolute opposite of what I have (being on E7). In some ways E7 and PV don't work too well together, but in other ways they do. So you may have to change your bathing times a bit.

So is it possible to calculate the optimal size. Yes, but you need to know the size of the PV array, your DHW usage, desired temperatures and average daily insolation (Met Office can help there).

It is fairly easy to work out your daily DHW needs. Just a jug, a clock and a thermometer. Work out the flow rate from the taps (jug and clock) and multiply it by the amount of time they run for. Take the temperature (and if your cold is from the mains, take that temperature too). Then you can wok out the amount of energy.
Specific Heat Capacity of Water in kWh = 0.00117 kWh/(kg.K)
So say your shower has a flow of 10 lt/min and last 3 minutes, you like the water to be at 38°C and your incoming mains is at 18°C, then:
kWh = 0.00117 x 30 [kg] x 20 [K]
kWh = 0.7 kWh

Now yesterday my local weather station reported that I had a mean solar radiation level of 136.5 W/m^2 over the 24 hours period and on the horizontal plane. This works out as 213.8 W/m^2 during daylight hours (06:04 to 21:13) or 3.9 kWh/m^2.
Now a PV system looses a lot of that energy in converting it to electricity, people argue over the amounts and get really upset about it (especially if they have forked out a few thousand quid). I work on converting 10%, other may claim 12%, 15%, 20%, but I shall use 10% for this.
So 1m^2 of PV should deliver around 0.39 kWh, for that one 3 minute shower we needed 0.7 kWh, so 1.8 m^2 of PV was needed (just slightly larger than a standard 1m by 1.6m PV module).
Yesterday was not a brilliant day down here, but let us look what the 11th of Jan yielded.
The mean radiance was 15 W/m^2, over just daylight hours (8:34 to 16:00) it was 47 W/m^2, the yield was 0.35 kWh/m^2.
So to generate 0.7 kWh would need 20 m^2 of PV or around 13 PV modules.
On the equinox you would have needed about 3 PV modules

So thermal store size for 1 short shower is 30 lt (think of it as squeezing a bladder at the right temperature) and somewhere between 2 and 13 PV modules to generate all your DHW.

Now that is all a bit silly as your whole plot would be covered in PV, so what is a realistic level of PV and what size thermal store do you need.
A 200 lt store, with an operating range of 15°C (40°C up to 55°C, or 50°C to 65°C) will hold 3.5 kWh.
You can double that by extending the temperature range (40°C to 70°C), but turning on an immersion heater.
7 kWh should give a family of 4 enough how water for the day even allowing for the baths.
To get 7 kWh/day from PV will require somewhere between 20 and 200 PV modules.
If you could fit 20 PV modules (5 kWp), you would generate about 5000 kWh/year, if your water usage is about 3000 kWh/year, then for about 3/5th of the year you are covered for the 7 best months, and you get a saving on the other 4 other months.
And that is with just a 200 lt thermal store (which I think is a bit small).

So it really is a matter of finding out the usage, finding out the expected PV yield in your area and doing the sums.

Watch the capital costs - when we looked at buying a cylinder or TS, and solar panels and a top up instant heater, getting them installed, and building the house several m2 bigger to fit them in - it added up to many many years worth of running costs for a LPG instant DHW heater or combi.