First year Heating Energy Use in Passive(I hope) House

[Iceverge]

I've been tracking our electricity use in our new build since last March. 

 

 

A little background. 

 

2 story 186m2 (175m2 Treated floor area).

External footprint 114m2.  (150mm concrete slab over 200mm graphite EPS)  Strip foundation. 

External wall area 249.4m2.  (Masonry build 250mm cavity with SS ties and EPS bonded bead insulation.  Wet plastered both sides. 

 

External Glazed area 36.6m2.  3g Veka Softline 82 UPVC. 

I modelled all junctions myself in THERM and did the best I could to eliminate any significant cold bridges, the details were similar to Denby Dale except the threshold's which were I suspect a one off with fiberglass grating. 

Roof was a trussed hip design, copy again of Denby Dale's but with 400mm of blown cellulose. Sealed roof design as per the Tyvek Supro booklet. 

Windtightness continued with all junctions externally being sealed with airtight paint. There is one 150mmx4m area of block which hasn't been done. 

 

Airtightness tested at 0.31 ACH 50. 

MVHR is a Proair pli 600 unit with a passivhaus efficiency of 86%, care taken to insulate the very short run of ducting into and out of the house. The 1 soil pipe was fitted with an AAV.

We installed no central heating. Instead we use a 2kw plug in rad on a timer. 

For DHW we use a 300l UVC direct cylinder on night rate electricity. 

 

 

PHPP predicted 14kWh/m2/year *175m2 = Total of 2450kWh total annual energy for heating. 

Our daily electricity usage was 18.1 kWh/day in the summer months. Of this 10kWh for DHW.  ( 2 adults + 2 preschool kids) 

Our total annual usage was 9876 kWh. Isolating DHW at 3650kWh and everything else at 2956 kWh left 3270kWh for space heating. 

 

It is almost 33% more than PHPP predicted. Although it is still not much energy at 18kWh/m2 I'm a bit disappointed. 

 

 

I have a couple of theories about why this might be the case. Please jump in with more knowledge/info if you can. 

 

1. The house was still drying out. Some say a heavyweight house can take over a year for this to happen. 

2. The insulation manufacturers overstate the K values of their products, EPS ( although it is my preferred plastic insulation!) seems to have dropped from near 0.04W/mK to closer 0.03W/mK in the past few years by changing colour. 

3. The house was often closer to 21 deg rather than the 20 deg in PHPP.

4. Maybe I made a b*lls of creating my own weather dataset from the closest met station ( Cork Airport) 

5. I was using an old version of PHPP which I gather has higher internal gains than the newer ones. 

6. We had locally less sunshine (I haven't checked this)?

7.  Mrs and Kids wander in and out with the doors wide open more often than I calculated for. ( Obviously I never do this!) 

8. I haven't properly balanced the MVHR (a cheapo DIY attempt as usual) 

9. The main south facing open plan living area has a floating ( never again) LVT floor which doesn't allow the concrete slab to soak and store the sunlight as much as a ceramic or glued down floor.  

10. The floor was laid when I wasn't here so I'm suspicious quality of floor insulation fitment. 

11. Maybe I made a mess of PHPP. 

 

 

Overall I feel somewhat vindicated by the choice not to bother with central heating. The house is comfortable without, a heated slab might have been even more comfortable however and would allowed more energy buffering on cheap night rate electricity. An A2A is probably on the cards at some stage but the likely saving might only be €200 per year when a 10 year lifespan is considered. 

 

Better ideas include properly balancing the MVHR, getting a thermal camera to check insulation. Maybe putting towel rads in the bathrooms to have them warmer and the run rest of the house cooler. 

 

Inputs welcome. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Edited March 27, 2022 by Iceverge

[mike2016]

Don't know how you managed to get a BER cert / Completion Cert with resistive heating and no renewables!? If you had an A2W heat pump and a cop of 2.2 it would have closed the gap on your heating/hot water calcs?  If you're air tight you won't really have a draught if one outside door is open, doubt you'd lose that much heat?

How many Air tightness tests did you get? Was one done at the end of the build? Might be very different from before 1st fix? 

Not sure you have enough insulation - smaller houses are much harder to do in PHPP I've been told. I've 280mm pholenic insulation in the floor design and over 300mm in the walls (kooltherm) in a 100m2 house - PHPP was done in 2018. Did you do the PHPP designers course or have you done a few houses before doing yours? It seems complex to me but I'm sure you're still exceeding building regs in lots of areas so that's good. At least you're living in it! I'm still trying to get past planning!! Get yourself something like a sensorpush and bluetooth it to your phone to monitor temp/humidity in various rooms and temperature swings and monitor the internal temps for starters? 

[Iceverge]

We're not technically finished. A2A and PV was always the plan when the budget allows.

 

ASHP wouldn't make any difference to the energy demand, just a more efficient way of using it. The mark up of 100% was too much for me to stomach too. 

 

38 minutes ago, mike2016 said:

If you're air tight you won't really have a draught if one outside door is open

 

It's certainly not as much as a leaky house but it's not nothing. 

 

38 minutes ago, mike2016 said:

How many Air tightness tests did you get?

Just one. It was at scratch coat stage and before the cellulose in the attic so I suspect the reading would be better now. 

 

38 minutes ago, mike2016 said:

smaller houses are much harder to do in PHPP I've been told

 

Yes, however if you play with PHPP the returns get really small beyond a certain stage. To the point of basically not being able to change the heating heating demand by adding more. 

 

There's a good article about small houses here.  Even near passive small houses often need no heating, as the difference in absolute energy required can often be made up by an extra person or dog! 

 

https://passivehouseplus.ie/magazine/insight/the-small-passive-house-problem-a-solution

 

I've very suspicious of insulation manafacturers. Some have been proven liars recently. I would avoid any high tech insulations if possible. EPS and cellulose but use a K value of 0.04. Simple is best. 

 

Natural insulation provides more heat buffering and protection too which is oft ignored. 

 

38 minutes ago, mike2016 said:

Did you do the PHPP designers course or have you done a few houses before doing yours?

 

No, but I've spent ages playing with it. TBH most of the sheets don't make a huge difference and I just read the manual and/or assumed worst case where I wasn't sure. 

 

Monitoring the temp+humidity is almost pointless as it almost never changes. It's been 21deg and 55%RH with ages. 

 

 

 

 

Edited March 27, 2022 by Iceverge

[Gone West]

13 hours ago, Iceverge said:

3. The house was often closer to 21 deg rather than the 20 deg in PHPP.

4. Maybe I made a b*lls of creating my own weather dataset from the closest met station ( Cork Airport) 

When I was playing around with PHPP, using 2010 Version IIRC, my house was 13kWh/m2a at 20C and 15kWh/m2a at 21C. I also found the results varied a lot depending on the climate data used. I plugged in several different sets but all I can find now is the data I created using Meteonorm for my area, when I was in East Kent, which I finally used. I also created an average for SE England from Met. Office data which gave results of 11kWh/m2a. I decided to be cautious and settled for the 13kWh/m2a.

[ProDave]

12 hours ago, Iceverge said:

ASHP wouldn't make any difference to the energy demand, just a more efficient way of using it. The mark up of 100% was too much for me to stomach too. 

What is this 100% mark up you talk of?

 

My own 150m² house used 1400 kWh of electricty heating it in the last 12 months which is 9.3 kWh per square metre, but that is energy into the ASHP so assuming a COP of say 2.5 average that would actually be 23kWh of actual heat per square metre.  But we are not passive house.

 

The only way to be sure of your heating usage is to meter that separately.  I bought an old school dual rate electricity meter, the sort that has a pilot wire input and would have been used with an external timer.  That meters my ASHP usage, one dial meters the usage in heating mode and the other dial meters usage in DHW mode.  the pilot wire is driven from one of the signals to the motorised valves.

[SteamyTea]

1 minute ago, Gone West said:

When I was playing around with PHPP

Have you done the same with your current place.

Be interesting as the climate is quite different, even if the headline mean figures are similar.

[Gone West]

1 minute ago, SteamyTea said:

Have you done the same with your current place.

No, not yet. I don't know if PHPP would be a good model to use for and old solid walled stone building. I have done some basic heat loss calculations and at the moment I've calculated the heat loss as 17414kW with a 24C temperature difference and I should be able to reduce that to around 11237kW with some straightforward improvements.

[SteamyTea]

1 minute ago, Gone West said:

I don't know if PHPP would be a good model to use for and old solid walled stone building.

Could run it through Jeremy's spreadsheet.  Though that would give a different result from a PHPP analysis.

[Iceverge]

1 hour ago, ProDave said:

What is this 100% mark up you talk of?

 ASHPs selling for about €5000 trade and plumbers charging €10k + install. They're practically required here due to our part L regs requiring a certain % of renewable energy.

 

Like all government mandated purchases from the private sector the price is premium.   

 

I don't think PHPP is prefect either. It seems to overestimate the amount of energy the smaller the house is.

For instance when I played around with scaling our house down, same insulation values, same glazing, just less floor area and less external surface area we no longer met passivhaus requirements. Practically this was illogical as we would have had identical solar gains and internal heat gains but less heat loss through the envelope, so the house would have been practically even a truer passivhaus, (able to be heated via MVHR). 

 

https://passivehouseplus.ie/magazine/insight/the-small-passive-house-problem-a-solution

 

 

Given your construction @ProDave ,suspended timber floor (better than passive slab in my research) and continuous external woodfiber insulation layer I think you'll probably have some of the best thermal bridging figures on the forum. This really starts to make a difference when U values get lower.  My suspicion is that houses built from natural materials out perform synthetic ones in reality. Anybody with further reading on this I'd appreciate? 

 

 

Do you have any rough idea how many m3 of wood you used? Is that metering independent of solar contribution? 

[RobLe]

Hi Iceverge - I second the idea of directly measuring the heating electricity.  Knowledge is power!

 

Am I right in guessing your rad on a timer operates during the night cheap rate?  I notice that a simple rad or storage heater on Octpus Go night rate will be a similar cost to an ashp during daytime, as Go has a 4x price difference.  The ashp always saves more CO2 though.

 

As to the EPS - I think the 0.03 stuff is "platinum" EPS, with carbon bits in it.  The blacker it looks, the more carbon, lower the lambda - I've used stuff that was 0.034 (grey), and 0.03 (black).  The 0.03 stuff got noticeably warmer when left in the sun during construction.  There's no clever gasses, I don't think it loses capability as it ages though?

 

[SteamyTea]

15 minutes ago, RobLe said:

There's no clever gasses

I think Freon us the blowing gas.

 

20 minutes ago, RobLe said:

As to the EPS - I think the 0.03 stuff is "platinum" EPS, with carbon bits in it.  The blacker it looks, the more carbon, lower the lambda - I've used stuff that was 0.034 (grey), and 0.03 (black).  The 0.03 stuff got noticeably warmer when left in the sun during construction.  There's no clever gasses, I don't think it loses capability as it ages though?

 

Propane, pentane, methylene chloride, and the chlorofluorocarbons are the usual blowing agents.

[ProDave]

53 minutes ago, Iceverge said:

 ASHPs selling for about €5000 trade and plumbers charging €10k + install.

Can you not DIY install as many of us have?  It was certainly NOT €5000 of work needed.  Or are you forced to have some stupid sign off by some registered scheme like our MCS?

[Iceverge]

Too late now, no UFH pipes or rads!

 

 I could have done. Maybe if I had my time again. Who knows.

 

There's no restrictions, just opportunism.  

[rh2205]

This is interesting we just did a refurbishment with slightly higher insulation on walls than BR & nothing on slab, use radiators throughout. We have used 1684kWh on ASHP in 10 months on heating & hot water. I’m expecting it to hit 1750kWh worst by year end. I initially thought the meters were wrong but actually our entire 2adult 1toddler household is averaging 11kWh a day all in. 245sqm… what are we doing that’s so phenomenal or does Eastern uk bask in way more sunshine which makes a large enough difference for a comparatively poor performing house that hasn’t been air tested?

Edited March 29, 2022 by rh2205

[Iceverge]

@rh2205That's very impressive. Can you elaborate on your house more please? 

 

 

[rh2205]

Cold roof with PIR 150mm, first floor walls are PIR EWI of 70mm on timber frame first floor extension with 70mm PIR between studs, 49sqm of aluclad 2g windows. Existing masonry downstairs walls had grey eps beads filling 50mm cavity & 100mm grey EPS EWI. As you can tell we didn’t have an architect who really knew what they were doing with the insulation (and it was actually the structural engineer who specified the EWI on the first floor extension to deal with the structural fixings so it could of been a disaster!!), we specified the EWI on the lower walls but pretty tied in by the time we figured it out from this website and what was happening on site that it wasn’t actually a warm roof he’d specified on the drawings!! There was no end of cold bridging mistakes from various steels & poor window detailing/builder meddling, plus the mashing together the upper floor with existing structure & even the downstairs EWI had a pretty big issue in the end because our dpc is not the same height across the original house so EWI insulation got raised one brick. We had plasterboard throughout in particular to hide the reoccurring cracks downstairs & the walls weren’t purged first so I very much doubt our house is anywhere near passive air tight though we personally spent a long time taping all the new build insulation at night once we realised our plans were that terrible and our builder was that average but I can’t blame him if not specified. We have standard fans for ventilation & trickle vents. We left the uninsulated slab untouched & added oversized radiators to accommodate the 45C radiator temp (again we made this decision & sized the radiators because by this point we realised no one gave a crap how well your house works), we keep the water temp the same on the 300L cylinder but only heat once a day, we haven’t run out of hot water before and we heat the house to a slightly cooler 19.5C, the heat pump got set up wrong so the immersion heater never comes on (noones been sick yet & our toddler prefers warm bath water to drink every day no joke). Seasonal performance factor is 4.49 on our Grant aerona 3 heat pump. In a different life I would of done about 30 things differently but we were poorly prepared but on the heating front it’s miraculously not been a total disaster but I guess we also heat house to a lower temp & don’t heat as much water which we are fine with. Probably our biggest problem is my husband requested air con in the summer 🤣. Again I have seen all these things could of been corrected with planning. So with that in mind our build was not great on the design front so it must be more to do with the heat pump COP and how often we heat our water & the house temperature?

[Iceverge]

@rh2205Wow, you seem to have had a very good result despite everyones best efforts! 

 

Well done. One really needs to grab these things by the scruff of the neck it seems. Is the house detached? what approximate m2 is it? 

 

Would you describe the location as sheltered. I think this can make all the difference.

[rh2205]

I think the worst thing is I went into the process knowing about energy efficiency vaguely from a couple of short free workshops I’d done on retrofitting years back without fully appreciating how complicated a retrofit was at the design stage so it was a half baked attempt to make the house better based on cost (hence leaving the 120sqm uninsulated slab) if you don’t actually have someone who knows what they are doing and actually checking the building can do that in real life you are screwed and then as I went along I started researching detailing myself & could see all the mistakes but realising it was already too intertwined with the design to correct half of it. We are 245sqm detached, it’s an oversized half house half dormer assymetric roof, with 3.5m ceilings upstairs. SE facing on the large glazing side on the longest profile of our 16m long rectangular house, very flat farm land round here and not coastal or in the mountains so I guess sheltered?

[rh2205]

Just to add on this on the driving rain index of exposure east anglia/middle England is the lowest rating so I guess we count as very sheltered so maybe that’s the difference. This is all anecdotal of course, you must be in the most exposed area or similar? So maybe some more usage from others in similar areas would help with the reality check.

[Gone West]

Well done, my guess is that the biggest factor is the low house temperature followed by DHW heating. With low temperature radiators you are using the ASHP at near it's most efficient. I'm glad it's working out well for you.

[joe90]

I think location and local weather patterns affect houses more than we realise and very difficult to measure (unless you built a weather station on site fir a year prior to building!). My build is not fully passive and the heating is on fir about 2 or 3 months a year, and then not for long, we have a large south facing conservatory and unless very cold and cloudy it heats the house a lot. I was warned about overheating but with large windows and doors it’s easy to mitigate by opening them.

[Iceverge]

@rh2205do you intermittently heat your house or 24/7?

 

 

[Stones]

@rh2205  Do you know the breakdown between DHW input energy and Heating?  50/50 ? Your figures seem quite remarkable, and from a bill paying point of view most welcome.

 

I'm up in Orkney, and very similar to @ProDave in terms of input energy for heating, albeit we do not have a wood burner / secondary heating source.   Not as cold here but as you would imagine, very windy, which in effect, drives as much heat loss as colder temperature does.  Looking back at my energy usage, year to year there can be up to a 15% variation in the amount of heat energy required, with the total figure ranging between the calculated energy demand for average outdoor air temp and minimum outdoor air temp.  We heat the house to 21.5C, with the ASHP coming on as required to maintain that temp. Within the house the impact of household gains are evident as occupation moves between different parts of the house +/-0.5C.  The ventilation rates set in specific rooms also impact on the room temp by perhaps +/-0.5C

 

We meet some passive house criteria, but certainly not all, and our house design, for want of a better description, two interlinked bungalows, means we have a lot more exterior wall standing up to the elements.  We have 155m2 internal footprint with one upstairs attic room.  We have additional areas of vaulted ceiling as well meaning our house volume is around 473m3, so roughly speaking the equivalent of a 200m2 standard sized room building.

[SteamyTea]

5 hours ago, joe90 said:

unless you built a weather station on site fir a year prior to building

Are while building as most on here take several years.

Should be cheap enough to knock up a very basic unit that can run off a battery.

[AliG]

Part of the error will be the heat capacity of your internal wall leaf and other fabric in the house.

 

Assuming aircrete blocks and plaster, your outer walls will weight around 30 tonnes. This would require over 100kWh to raise the temperature by 15C. The concrete floor will weigh a further roughly 40 tonnes requiring around 170kWh to increase the temp by 15C.  Including floor coverings, tiles etc you may need 3-400kWh. On top of this there is the drying out energy.

 

I am less sure about the energy needed to evaporate water, but it looks like it is around 1kWh per litre, so this could be a few hundred kWh to dry out the house. (Note I quickly googled this, it is a lot less than the energy required to boil water)

 

I would also be careful of talking about the percentage error in the calculation when you get to quite small numbers. Your house uses around 800kWh more heating than expected. Using gas this would have cost £25 a year until recently, even now it is only £64 of gas. Cost on an ASHP would be similar. Its a pretty small discrepancy which will probably be even smaller after it has dried out and got up to temperature.

 

I'd guess that you are within a few hundred kWh in the second year after the house is dried out and up to temperature.