ASHP- struggling to warm house in the cold weather

[ProDave]

I think our house is probably a good ballance. Definitely not zero heating, but very low heating required.  But it's also a very comfortable house, no sudden changes in temperature, no draughts, and good air quality thanks to the mvhr.  

 

I think it has been mentioned before, but when we finally reach state pension age, if they are still paying the winter heating supliment, that will probably poy for most of our annual heating bill.

 

Now if only I could find a way to reduce the council tax by a similar amount to my now low fuel bills.

[newhome]

This is the video I mention (video 2).

 

https://portreepassivhaus.uk/video-reports/

 

Discussion about the heating requirements isn’t included here but the quote I mention “none installed” was from Facebook and the blurb at the bottom of the page says that the previous house has no installed heating. 

 

“designed and constructed to Passivhaus standard, with no installed heating system”

, with no installed heating system

The details of the proposed house start at around 17.35. Most of the earlier stuff is about Skye itself, and we’re all going to hell in a handcart too ?

[joe90]

It also depends on location. @JSHarris found his location very sheltered which is why overheating was a factor which surprised him a little, here we get mild but constant winds and rain which I firmly believe “wash” the heat in the outside walls away. 

[Onoff]

2 hours ago, scottishjohn said:

anything over 6" of pir is probably too much cos extra cost verus extra insulation never works out.

which is probably 10-12" of polystyrene as a comparsion .

 

10-12" of eps in a wall or whatever means deeper studs than if using pir so there's probably no cost saving over using pir in the first place.

 

Paying for extra insulation now guards against the volatility of future energy prices. Taken over the lifetime of a house that will presumably outlive the builder it just makes sense. Just because it might not payback in their lifetime isn't a reason enough I imagine to put most on here off of doing it. 

[Jeremy Harris]

If comparing different insulation types purely on the basis of R value (which determines the thickness needed for any given U value) then here's a list of approximate R values for different insulation types, together with the thickness needed to meet the Passivhaus minimum fabric standard U value for a wall (0.15 W/m².K).  This list ignores all the other components in the structure and the surface resistance, it's just for comparing the relative effectiveness of different insulation materials:

 

Silica carbon Aerogel ~0.0135 W/m.K , thickness for 0.15 W/m².K ~90mm

 

Silica Aerogel ~0.017 W/m.K , thickness for 0.15 W/m².K ~113mm

 

PIR/PUR foam insulation ~0.022 to 0.025 W/m.K , thickness for 0.15 W/m².K ~150mm

 

EPS/XPS foam insulation ~0.033 to 0.037 W/m.K , thickness for 0.15 W/m².K ~240mm

 

Rockwool ~0.037 W/m.K , thickness for 0.15 W/m².K ~246mm

 

Wood fibre ~0.038 W/m.K , thickness for 0.15 W/m².K ~253mm

 

Worth noting that none of these materials need to be more than around 250mm thick to give a U value (just for the insulation) that meets the Passivhaus spec.  When the other elements of the construction are taken into account, along with the surface resistance, then the actual thickness of insulation needed to give a U value of 0.15 W/m².K will be slightly less than the thicknesses given above.

[scottishjohn]

18 minutes ago, JSHarris said:

If comparing different insulation types purely on the basis of R value (which determines the thickness needed for any given U value) then here's a list of approximate R values for different insulation types, together with the thickness needed to meet the Passivhaus minimum fabric standard U value for a wall (0.15 W/m².K).  This list ignores all the other components in the structure and the surface resistance, it's just for comparing the relative effectiveness of different insulation materials:

 

Silica carbon Aerogel ~0.0135 W/m.K , thickness for 0.15 W/m².K ~90mm

 

Silica Aerogel ~0.017 W/m.K , thickness for 0.15 W/m².K ~113mm

 

PIR/PUR foam insulation ~0.022 to 0.025 W/m.K , thickness for 0.15 W/m².K ~150mm

 

EPS/XPS foam insulation ~0.033 to 0.037 W/m.K , thickness for 0.15 W/m².K ~240mm

 

Rockwool ~0.037 W/m.K , thickness for 0.15 W/m².K ~246mm

 

Wood fibre ~0.038 W/m.K , thickness for 0.15 W/m².K ~253mm

 

Worth noting that none of these materials need to be more than around 250mm thick to give a U value (just for the insulation) that meets the Passivhaus spec.  When the other elements of the construction are taken into account, along with the surface resistance, then the actual thickness of insulation needed to give a U value of will be slightly less than the thicknesses given above.

ok so thats answered my questions

 

0.15 W/m².K for all walls ,floors and roof 

Edited February 5, 2019 by scottishjohn

[newhome]

13 minutes ago, scottishjohn said:

ok so thats answered what spec for wall

 

0.15

 

That could be better perhaps? Mine is 0.11 from nearly 10 years ago. 

[scottishjohn]

So maybe the answer to summer overheating if you want lots of floor to roof windows is coating on glass to cut down the amount ,not all of the solar gain it can give ,for a comfortable house

[ProDave]

My solution is natural.  Lots of trees south of the house.  In the winter without leaves, they let most of the sun through.  In the summer in leaf, they screen the sun nicely.

[AliG]

This thread brings up many interesting issues.

 

We weren't in last winter, so the recent cold spell has been the first time that we have been able to test out the heating, check for cold draughts etc.I think we will need this winter to get things properly set up, maybe even another winter. It is almost impossible to do when it is warmer as you don't need much heating at all then. I went round the house looking for draughts when the temperature got below zero and it was windy.

 

@Jude1234 seems to be in this boat.

 

Unfortunately it sounds like a lot of the spec was done by the builder. They don't normally know much about this stuff. Clearly they are talking through a hole in their head. The slab won't still be heating up, the heating has been on for months. Heat rising upstairs will not make the ground floor cold and certainly not with the top floor also cold.

 

I did consider an ASHP for my house, eventually I went with gas as it is as cheap or cheaper. One key consideration for me is that I was worried that the ASHP performs least well exactly when you need most heat, i.e. below 0 outside.

 

Anyway the real issue seems to be the house is using more heat than expected. An ASHP will be fine if the heating load is as expected.

 

Breaking down your 1300kwh a month electricity usage. Most families will use around 3-400 kwh a month of electricity before heating, especially in winter when it is darker. Your MVHR and UFH pumps could be using up to another 100kwh. So you are probably using around 800-900 kwh a month for heating and hot water. Assuming a conservative COP of 3 when it is cold, this means around 2500 kwh of heating load or 85kwh per day.  I use roughly 3x this in a house almost 5x the size and I am disappointed at my current performance.

 

So some suggestions-

 

1. Is the ASHP providing hot water also. What hot water temperature is set? If it is asked for too high a temperature is an immersion heater kicking in and using a lot of electricity?

 

2. MVHR - How aggressively is it running? Can you see what the temperature of the supply air is? I found that as commissioned our MVHR kicked up at 45% Relative Humidity. This had it running fast a lot of the time and changed the setting to 53%. I also set it to run on the lowest speed during the night. Below freezing outside, the supply air may be at around 15. Indeed if the house is too cold, the exhaust air won't be warm enough to heat up the supply air and it may be only 13-14C. If so, with no heating upstairs the MVHR will cool it to this temperature. I worry that unheated rooms and MVHR are a dangerous combination unless the house is very well insulated.

 

2. Draughts/Air tightness - This seems to be the big issue. Indeed I thought you weren't supposed to use MVHR if the air tightness is above 3. There are probably dozens of these leaks that can be cheaply fixed. As that air tightness level it is probably accounting for around half the heating cost. I have not had an airtightness test yet, but two weeks ago I went around finding leaks. When it is cold and windy outside you can feel them with your hand. I would go around the house feeling things such as the bottom edge of skirting boards, pipes through the walls, around doors and windows, sockets in outside walls. Basically anywhere that an air leak is likely.

 

For example I knew that my french had not been sealed up along the bottom edges. They were lifted off the slab to be level with the floor. They have just put mastic and compriband on the outside, but there was still a draught. I have foamed under them and now they will have silicone seal along the inside edge.

 

My front door is well insulated and triple sealed everywhere except the bottom edge. A little thin strip of draught excluder fixed this.

 

I have steel beams in the roof with air tightness membrane. I have just discovered that the membrane has been stuck to the beam not around them and there is a two inch gap that I hadn't noticed underneath.

 

3. Check the loft. In my old house when we had work done someone moved the insulation around and left some areas doubled up and some with no insulation. Is it insulted as expected, 3-400mm of rockwool or 200mm of PIR. Again are there draughts below the insulation?

 

4. Floor finishes make a big difference to how the UFH works. Our double height hall with tiles still heats up faster than bedrooms with wooden floors. You have to fiddle a bit with the flow temperature to get a good balance between heating time and efficiency. But at the moment you will need more heat than your ASHP can supply.

 

 

 

 

 

 

Edited February 5, 2019 by AliG

[scottishjohn]

1 minute ago, newhome said:

 

That could be better perhaps? Mine is 0.11 from nearly 10 years ago. 

no argument there ,

but is it really required --will only cost more  to build  and is the extra £10 or more  per sqm (icf poly block ) really needed?

lest say 150sqm of wall @!£10 sqm = £1500 .

you would need to know extra fuel costs to know if its a good trade off.

getting even cheaper heating cost gets more expensive --its the law of deminshing returns and if low anyway ?

I,m not saying anybody is wrong going above passiv minimum ,so don,t jump on me ,

but as with all things ==there is always compromise

[newhome]

46 minutes ago, scottishjohn said:

the law of deminshing returns and if low anyway ?

I,m not saying anybody is wrong going above passiv minimum ,so don,t jump on me ,

but as with all things ==there is always compromise

 

Ha ha, my house is far from passive sadly. I wish we had thought more about the floor TBH. I love hearing how low some people’s heating requirements are. Mine are sadly a lot more because (I think) the floor isn’t good enough.

 

As you say a cost benefit analysis is required if you are mainly wedded to the view that the construction costs vs payback is the main concern. It’s all a bit finger in the air with these things however and for many they concentrate what they can afford in the here and now so will look for the most cost effective option. Those of us approaching retirement probably think more about how we will afford heating costs when on a pension (back to @Onoff‘s point) so may be more likely to take a longer term view. It’s not quite that black and white but I think life stage does have some bearing on what you build. 

 

The extra cost of the walls was 10k (compared to the standard TF cost) so wasn’t insignificant at the time. 

 

https://sites.google.com/site/sapserviceuk/informationadvice/Building-Fabric/News/supawall

 

 

[AliG]

When you calculate the percentage of the envelope of your house that is wall, changing the U-Value from 0.15 to 0.11 for example has negligible absolute effect on heating costs, but could be very expensive. If your heating cost is low it could be reduced by a substantial percentage but that might still only be £50 a year. More insulation in the roof is probably the most cost effective insulation as it is cheap to add.

 

However, the percentage of your house that is small holes causing draughts will be less than 1% but could account for half your heating cost! Fixing these will likely be very cost effective indeed.

[scottishjohn]

I am already there ,at pension , and trying to sell off my last biz property and starting on self build .my views  are similar to yours 

but tinged with --when i,m dead --do I give a crap  about someone getting extra benefit from  me going OTT on build  spec ?,which will take 20 years to recoup

I am thinking more of economical running costs .but leave enough cash to spend on going to far flung places

my will should read 

"being of sound body and mind --I SPENT THE BLOODY LOT".

and government can go whistle for inheritance tax if at all possible 

 

 

 

[scottishjohn]

47 minutes ago, ProDave said:

My solution is natural.  Lots of trees south of the house.  In the winter without leaves, they let most of the sun through.  In the summer in leaf, they screen the sun nicely.

think that is a 30 year plan though -trees that drop leaves don,t grow as quick as christmas trees.

but i suppose it depends how far away they are as to hieght required

Edited February 5, 2019 by scottishjohn

[Jeremy Harris]

I agree about reducing U values below the Passivhaus fabric standard, it  doesn't give much benefit at all in our climate.  In terms of cost, then if designed to be thermally efficient a house needn't cost any more if built to PH fabric standards, either.  Our build illustrates this, as a local architect who was interested in what we'd done asked if he could see our cost breakdown.  He later told me that the basic build cost (insulated and airtight structure) was slightly less than the average cost to build a just-meets-building-regs house, so better insulation and airtightness doesn't need to add anything to the cost.

 

Our house ended up with roof and floor U values of about 0.1 W/m².K, and walls that are around 0.12 W/m².K.  There's no real benefit in our location to reduce these any further, TBH, as our heat loss is already dominated by that from the doors, windows and ventilation losses. 

 

The main problem is that a lot of builders are looking to just bodge build methods they've used for years in order to improve performance, and then that does add cost.  Take cavity wall construction as an example.  We don't need cavity walls at all now, they are an anachronism that only came about as a way to limit damp ingress through walls, back when walls were leaky solid brick.  There's no good reason for sticking with two masonry skins now.  A single skin wall can easily be made rain and damp proof, and can be made so that it's inherently more airtight and better insulated. 

[Ferdinand]

59 minutes ago, scottishjohn said:

no argument there ,

but is it really required --will only cost more  to build  and is the extra £10 or more  per sqm (icf poly block ) really needed?

lest say 150sqm of wall @!£10 sqm = £1500 .

you would need to know extra fuel costs to know if its a good trade off.

getting even cheaper heating cost gets more expensive --its the law of deminshing returns and if low anyway ?

I,m not saying anybody is wrong going above passiv minimum ,so don,t jump on me ,

but as with all things ==there is always compromise

 

I  think this illustrates that trying to be TOO precise can be a waste of effort.

 

If we look at Celotex, to go from 0.16 to 0.11 is 50mm extra.

 

The current Wickes retail price is £37 for 2.88sq or £12.5 per sqm.

But Self-builders get the VAT back ,(£10.50 per sqm)

and anyone sensible can get 20% off at Wickes across the board. £~8.45 per sqm)

And then they do 5 for 4 sometimes. £~7 per sqm.

 

Or you can just buy from insulation Superstore for £7.50 per sqm anyway.

 

Or buy-and-store from a seconds place for perhaps £6 per sqm if you watch it carefully.

 

The stack I have in my shed cost about £14 per sheet, delivered, or £4.50 per sqm, and that includes VAT. About 2 years ago from Seconds & Co. I think @PeterW did even better.

 

Currently you can buy 110mm 8x4 from Seconds and Co for £32 per sheet, which gives you a 0.9 u-value for <£25 per sqm for the materials - obv foam, alu tape etc needs to be added. Or you could use a 65mm insulated plasterboard on top of one layer for 0.11 to save labour.

 

Ferdinand

 

Edited February 5, 2019 by Ferdinand

[scottishjohn]

8 minutes ago, Ferdinand said:

Or you could use a 65mm insulated plasterboard on top of one layer for 0.11 to save labour.

above the extra insulation .

the noise reduction will be very good with insulated plaster board as i found  out when refurbing  this house and that was only 38mm  screwed direct onto existing plasterboard on north and east walls (outside walls of that room)

[Onoff]

Don't forget a lot of people on here aren't the type to compromise with cost a considered but secondary factor. There are some of course for whom compromise is or becomes a necessity but given the choice they wouldn't have.

 

There's then the pure  satisfaction of being able to say "I did that and it's the very best it can be!" Means a lot to many for them and their family. Tends to be that the best air tightness figures are achieved by them either doing it themselves or paying close attention to their builder / having a builder who understands and is willing to meet their requirements. Simply put many on here at least are prepared to go the extra mile whether it be by DIY'ing, paying a premium or even having the tenacity (often need) to shop or sometimes wait for the best deal.

 

Many will not like to be told they could have done better. Many builds are a labour of love. Hand in hand of course there's the bragging rights such a build gives you. Then there's the feeling as you watch visitors marvel at the warmth, sound and draughtproof qualities of your build that costs a pittance to heat. Can't put a price on that!

[scottishjohn]

everybody makes their own choices  and as long as they are happy with them that's perfect

and no one has the right to say their choice is not  a valid one if it  fulfills their design goals and aspirations 

 

[ProDave]

If you want an idea of the running costs you can achieve with a bit of care, here is ours so far.  Walls etc barely to passive house insulation levels (typical U value 0.14) triple glazed, mvhr, ASHP driving UFH downstairs only.   Bear in mind also we are in the highlands.  These last 2 very cold weeks, we have been using 95KWh for heating, and 35KWh for hot water per week.

 

 

[newhome]

43 minutes ago, JSHarris said:

I agree about reducing U values below the Passivhaus fabric standard

 

I think for us the TF supplier offered 2 choices for the walls; ‘standard’ or the one that was 0.11 and we went with the latter so it wasn’t that we were trying to do better but we did think it was worth the additional cost. I think @Stones used this method too for one of his many builds. Passive Houses weren’t talked about that much back then (2009) and we didn’t have any thoughts about building one although we had heard of them. I suspect that if we were doing it now we would be more likely to as more info is available. Ultimately however I’ve got what I’ve got and if I get round to trying to fix the draft that I can recently feel at the back door I suspect that will do me better than lamenting about what I might have built ?. 

[scottishjohn]

29 minutes ago, ProDave said:

If you want an idea of the running costs you can achieve with a bit of care, here is ours so far.  Walls etc barely to passive house insulation levels (typical U value 0.14) triple glazed, mvhr, ASHP driving UFH downstairs only.   Bear in mind also we are in the highlands.  These last 2 very cold weeks, we have been using 95KWh for heating, and 35KWh for hot water per week.

 

 

thanks for that 

I know my electric bill for year at this house is around 9700kw --thats all energy  input as i,m all electric ,including 24/7 pond pump outside lights  etc  in this house 

to be fair me and her like it 23c everywhere in house and it is running on theromstats 24/7,and probably hotter in years to come

so anything I build close to passiv will be a major improvement  and will suit me fine .

I,m still unsure till I get final design what i will be building and with what system --but plenty of scope .

I,d like to think I have enough money to do both plots  --we will see as costs crystalize

Edited February 5, 2019 by scottishjohn

[NSS]

As I've said before, our build spec was driven by the need to create the most appropriate environment (air quality primarily) for my wife's conditions, and most importantly to be able to maintain that environment year round (or at least to avoid large swings). We seem to have got it about right. Relatively low energy input needed to maintain a comfortable temperature inside when the outside temperature is low, yet little need for active cooling on all but the most sultry of summer evenings, without having to sacrifice the views out to combat solar gain.

[AliG]

Don't forget we are trying to help @Jude1234 get the heating bill down.

 

To summarise my earlier post, the biggest difference will probably be sealing up all the air leaks, unless some idiot has forgotten to insulate your loft.