Ralph

I could do with some advice - Biomass, ASHP and the C02 regulations

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Hi all,

 

We’re struggling a bit to come down on a choice of green energy for our new build in north east Scotland.

 

According to our SAP calcs to meet the Scottish government C02 regulations we’re looking at either:

 

  • A biomass boiler with 0.5 KWp PV on a ground array (roof is too flat)
  • An ASHP (Mitsubishi Ecodan 8.5kW is being recommended) again with PV on a ground array

 

We’re not self-builders it will be a main contractor.

 

We’re trying to keep the build cost down as low as possible. To be honest I would have been happy putting in an oil boiler with radiators instead of underfloor but that would require something like 4 KWp.  A pretty big ground array and quite expensive.

 

I’m leaning to the ASHP plus PV but any comments would be welcome.

 

The house is 2 story, about 200sqm with a fair amount of glass.

Thanks in advance.

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Biomass is frowned upon by a few of us on here.

ASHP, if set up correctly is probably your better choice.

0.5 kWp of PV is not worth bothering with, is that the size someone has said or is it a misprint/mistype.

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Welcome to the forum Ralph. Has it been designed with a good level of insulation? Adding more insulation would reduce CO2 emissions and ASHP would be better than biomass IMHO.

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That is the size that comes back on the SAP calcs, I'm assuming it's the minimum required to meet the regulations. It makes me wonder if there is something else we can do to offset it and void the PV all together? As you say not worth bothering with if it's all you're going to do.

 

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Hi Peter,  yeah the insulation is good and I'd be happy to increase it if it would mean no PV. To be clear I don't have any issue with installing green energy at all. I'm just trying to get the build cost down.
I'm not sold on biomass though, it seems expensive and to be honest overkill for a house our size. I also think the green credentials are suspect.

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What sort of U values are the walls and roof coming in at?

 

It sounds like the basic structure is barely good enough and the computer has spat out that by adding 0.5KW of solar PV you would scrape a pass.

 

ASHP would be my choice, actually got my new house it IS my choice.

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The Energy Efficiency Rating is 82 and the Environmental Impact (C02) Rating is 82 with the ASHP and 96 with Biomass

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what are the u values of the walls/ roof/ floor?

not the same as energy efficiency rating

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Sorry, still trying to get my head round the 48 pages of calcs I've been given, 
----------------------------------------------------------------------
2 Fabric U-values
Element        Average                Highest
External wall  0.22 (max. 0.22)       0.22 (max. 0.70)          OK
Floor          0.17 (max. 0.18)       0.17 (max. 0.70)          OK
Roof           0.12 (max. 0.15)       0.14 (max. 0.35)          OK
Openings       1.40 (max. 1.60)       1.40 (max. 3.30)          OK
----------------------------------------------------------------------
 

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I have a biomass system and it takes a fair bit more work to maintain. It needs cleaned out every 3-5 days depending on how much you use it. It will have some sort of ash tray that will need cleaned out. Every month it gets a proper cleaning which takes maybe 20 mins work. 

Then you have the storage of the wood chips/pellets. They have to be kept bone dry or it will wreck the auger. The more you order the cheaper you get them but the more room you need to store them. I can fit about 1.5t on a pallet but they are stacked maybe 7ft high.

Who is making you choose between the two options?? 

Is gas not an option or even oil???

Your u value for your openings is high. Most good 2g will be lower than that.

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Hi Declan, Kw

We're pretty rural so no gas. We could do oil but I'm being told that would require a 4KW of PV cells to offset the C02 (Scotland has different regulations from elsewhere) As they would have to be ground mounted it will take up a load of room and be expensive.

Excuse my ignorance here but by openings does that mean external doors and windows? The windows we are looking at are 1.3 although there are a lot of them.

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18 minutes ago, Ralph said:

Sorry, still trying to get my head round the 48 pages of calcs I've been given, 
----------------------------------------------------------------------
2 Fabric U-values
Element        Average                Highest
External wall  0.22 (max. 0.22)       0.22 (max. 0.70)          OK
Floor          0.17 (max. 0.18)       0.17 (max. 0.70)          OK
Roof           0.12 (max. 0.15)       0.14 (max. 0.35)          OK
Openings       1.40 (max. 1.60)       1.40 (max. 3.30)          OK
----------------------------------------------------------------------
 

 

Hi Ralph,

 

Those numbers could be significantly improved. Our timberframe has the following and is nowhere near state-of-the art.

- walls 0.13

- floor 0.1

- roof 0.07

 

I would push back and ask for better insulation before adding green-wash to the build

 

Did they quote you an expected air tightness value ?

 

 

 

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That's the what I took it to mean. You also need to ask are the window u values the centre pane value or the complete window.

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The only air values I see are
Air permeability at 50 pascals: 5.00 (design value)

We were going to use Scotframe Val-U-Therm pacakge which has a certified 0.13 U-Value for the walls . It needed quite a bit upfront so we went with a built on site option which I being told has to be calculated with Scottish Accredited Details which results in poorer u-values.

 

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Welcome ..!

 

What is the floor build up ..? You can easily get to 0.11 with 300mm EPS or 150mm or PIR done properly. Roof and walls are ok, but that glazing doesn’t get close to English regs from memory never mind Scottish ones ..! As others have said, good 2G is 1.2, and 3G can be below 0.9 however a large expanse will be expensive. 

 

I’d also look at that air figure as that is standard not anything done “properly” and Timber Frame is easy to get airtight. £3-400 of tape and membrane will get you well below that and the SAP will be much better.  

 

Do you have plans you can post ..?

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Hi

 

My business is biomass boilers and personally, on a well insulated, airtight new build property I d probably suggest ashp. I think biomass boilers are more suited to applications with higher heat loads.

 

@Declan52 Out of interest, what boiler do you have?

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Hi Peter, 

 

The windows are supposedly pretty decent quality by Nordan although admittedly 2g.

 

I've attached some plans and I would be delighted if anyone wants to take a look. You've all been very helpful.

 

Regards

Ralph2016 AHI 05 Plans Elevs.pdf

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hi @Ralph

welcome to the forum, looking at the plans there is a big expanse of glass and at 2g this is going to be a big cold area if sitting near it. as others have said u values are ok but could be better and how you're managing to get everything to add up with that amount of 2g is going some, suppose that's the pv. you may be better looking at pvc and 3g unless you must have timber. i'm going with that ashp and ground floor ufh only

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1 hour ago, Trw144 said:

Hi

 

My business is biomass boilers and personally, on a well insulated, airtight new build property I d probably suggest ashp. I think biomass boilers are more suited to applications with higher heat loads.

 

@Declan52 Out of interest, what boiler do you have?

It's a Musa hydro 15kw pellet stove.

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A yes, the MCZ, I know it

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Hi @Simplysimon, Yeah the glass is coming back to bite us slightly. I wonder how much weight 3g would add and would it impact the structural elements, probably not. 

I'll have a look at pvc, thanks for the advice.

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Posted (edited)
2 hours ago, PeterW said:

Welcome ..!

 

What is the floor build up ..? You can easily get to 0.11 with 300mm EPS or 150mm or PIR done properly. Roof and walls are ok, but that glazing doesn’t get close to English regs from memory never mind Scottish ones ..! As others have said, good 2G is 1.2, and 3G can be below 0.9 however a large expanse will be expensive. 

 

I’d also look at that air figure as that is standard not anything done “properly” and Timber Frame is easy to get airtight. £3-400 of tape and membrane will get you well below that and the SAP will be much better.  

 

Do you have plans you can post ..?

Hi Peter,  I have just had a preliminary SAP done and looking at the elements in the construction spec for walls and roof they have not included any breather membrane or vcl. I  am spending money and time on fitting the vcl and taping, would this effect my Sap calculations?

 

Edited by JamesP

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Think it was about 4-5 years ago that we had to make our final choice. Either the mcz stove or an ashp. Didn't know a sinner who had an ashp but knew a few who had the mcz stoves. The choice was pretty limited. Either 15kw or 22kw in a few of the mcz range or the much bigger pellet units upwards of 45kw. If I had to make the same choice today would I still go for biomass. Honestly I doubt it because I now know through here and ebuild much more about ashp so could have adapted my system better to suit one. But you have to make decisions along to way of every build based on what you have infront of you at the time so it will do me till it's time is up.

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16 minutes ago, JamesP said:

Hi Peter,  I have just had a preliminary SAP done and looking at the elements in the construction spec for walls and roof they have not included any breather membrane or vcl. I  am spending money and time on fitting the vcl and taping, would this effect my Sap calculations?

 

 

It will if you get the air infiltration rate down. We’ve spent about £250 on Gerband tape and it’s made a massive difference. There is no perceptible draught anywhere in the build, and the 3G uPVC windows are holding their own. In fact we have snow sat on the outside pane and the inside is reflecting the warmth of your hand. 

 

It was time consuming and fiddly job but it’s made a “standard” brick and block build remarkably warm and comfortable - even with no heating ..!! 

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@Declan52 Most of the builders round here don't have great things to say about ASHP, I assume it's because they have been poorly specified. A lot seem to be saying the weather here is not suitable, too cold or changeable. I hear some are starting to fit a hybrid ASHP that has small oil burner backup.

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