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Update on Timeshifting to Minimise Heating Costs.


TerryE

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When we first decided to self-build in 2014, Jan and I visited quite a few passive house builds and talked to various experts;  we soon decided that a low energy approach was broadly the way to go for our build.  One of these experts, a passive-house evangelist called Seamus O'Loughlin, emphasised that a conventional heating approach (where boiler demand is based on some central thermostat set point) doesn't work well in a passive house, because the time constants of a high-thermal capacity low energy house are a couple of orders of magnitude longer than those anticipated by conventional CH control systems.  

 

At the time this seemed a controversial assertion, but because I have done some mathematical modelling professionally, I was able to and decided to do some time-dependent heat-flow modelling and control strategy simulation of how our designed house would behave and this very much supported this assertion.  I have already covered a lot of detail of my CH approach in previous posts and discussions, but it’s probably worth summarising some key headlines to set the context for my changes to our heating strategy: 

  • We were cash-flow limited during the build phase, so had to make various cost-benefit trade-offs on our build, like most members here. I based these on a general net 10-15 year payback, and it was clear that we wouldn’t be able to achieve a true zero-input passive house largely because of design compromises owing to planning restrictions and our plot size and orientation.
  • However, we would be able to build a low-energy house that would need generally low levels of supplemental heating for maybe 6 months a year, with overall heat losses an order of magnitude less than a conventional build, and the thermal capacity of the heated fabric be many factors more.  
  • We decided to go all electric in the house with wet UFH embedded in the ground floor slab only.  Cost benefit trade-offs didn’t even support installing an ASHP, though I did future proof the installation to simply the later addition of one if the cost numbers changed.
  • I decided to adopt a simple but unconventional strategy for heating the house: calculate the total heating requirement for the coming day daily at midnight; this is based on actual averages for energy use, average house temperature and forecast average external temperature for the coming 24 hrs. This allows me to dump as much of this heat into the house fabric as practical at the cheapest electricity rate, and for us this is in the 7 hour overnight off-peak window on our E7 tariff.
  • We used to get some spill-over into peak rate top-up in the coldest months, but a year ago I added an oil-filled electric radiator on my 1st floor landing, and one in my son’s 2nd floor bedsit controlled by my Home Automation System, with these scheduled to come on in the overnight E7 window to dump extra heat in the upper floors.  This simple addition reduced the thermal layering from ground to second floor, and almost  eliminated  the need for daytime slab top-up.
  • In practice we have roughly a 1°C daily ripple on overall winter house temperature. Because using a daily forecast computation does have some intrinsic prediction error, this can add typically less than  0.2°C day-to-day ripple on top, but any longer term drift can be corrected by the daily feedback.
  • I have RPi3B running NodeRED attached to some digital thermometers and 4 GPIO controlled solid-state relays (SSRs) to control the time of the UFH pump and Willis heater, plus the 2 × SunAmps for DHW.  This was very cheap to implement, and basically has no monthly or annual maintenance. 

With the current Electricity price hikes, we have decided:

  • To trim our house temperature set-point back from 22.3°C down to 21°C  
  • To hard limit automatic heating of the slab to the cheaper 7-hour off-peak window.  (We can still do peak by request in one hour chunks if we want to.)
  • To use electric oil-filled radiators overnight to do any additional top-up.  I can automate this through my Home Assistant (HA) that runs on a separate RPi4 and do this using MQTT via WiFi connected powered/metered sockets.

This strategy currently limits heat into the house to: 

  • ~21 kWh through the slab and 
  • ~7 kWh through the two radiators.

28 kWh is enough to maintain overall house temperatures so long as the external temperature is at ~7 °C or higher, and it clearly isn’t the case at the time of posting.  The house needs about 2½ kWh/K, so with the average daily external temperature at zero today this is 17½ kWh too little to maintain house temperatures.  The long term Dec / Jan average where we live is about 4°C, so to maintain temperatures in this case we would need an extra 7½ kWh/day. (This last year, we had 26 days where the average external temperature was 4°C or below and only 2 where  temperature was below zero or below.)

 

So what happens when we underheat our house?  Simple: it slowly cools down, and very slowly.  For example, in the last 5 days of cold-spell, capping the heating has dropped the average house temperature from 22.3 down to 21.3°C, and given an average of -1°C for today, it will be down to our new target of 21°C by tomorrow .  At this point I will need  to add more heat or to accept that the house temperature will fall further.  I will definitely need to add another 7kWh or so extra radiative capacity for overnight topup.  We will play it by ear over the next week or so.  I can either accept that I will be paying £0.38/kWh for extra peak period top-up during these really cold spells, or let the average temperature fall a little further if we find it comfortable enough (wear a thicker jumper, etc.)

 

This approach works well for us because our house is so insulated and it has a huge amount of thermal capacity within the heated envelope.  If we accept a small heating ripple then it really doesn’t matter that much when we heat within the day and so we can time-shift our demand to make use of the best tariff rates: currently over 85% of our electricity use is at the off-peak cheap-rate price. This latest exercise of clamping the heat output to 28 kWh when the maintain level is closer to 40 kWh underlines that the heat budget for and given day can be off by 30% or so and the net temperature drift is still on 0.1 °C or so; the time constants of the system are of the order of a week rather than days or hours.

 

By way of a contrast my daughter lives in a pretty large but conventional 1990s house.  When her heating goes off in the evening, the living room temperature drops maybe 4-5°C within an hour.
 

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My current house living room is sub 9oC tonight (below 0cC outside) but I rarely use it. I heat an hour in the morning, and another in the evening with possibly an occasional third hour somewhere in between on demand with a gas boiler. When working from home I use an oil-less radiator to top up heating in my bedroom/office. That often drains the 6kWh Battery which I fill each night in winter using off peak. 

The Data above is really inspiring as I'm hoping to build my passive house next year and having those comfort levels in a 24 hour period is my dream! I try to warm the person as much as possible rather than the room but I find that if my feet get cold at all I find I come down with a sudden headcold for the rest of the day. I can only dream of being in a house that isn't frigid for parts of the day anymore!! 

My house is 25 years old. Anyway, thanks for sharing! 

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Resistive heating is completely off the table now as far a I'm concerned. The only use for it in my reckoning is for cycle-by-cycle dumping of excess Solar PV. A2A now fulfils the roles I had for electric convectors although I've kept one as an overflow dump load in the workshop for when the HW immersion divert max's out. I doubt this will happen because in the summer the A/C units will be cooling at the hottest times of day.

 

13 minutes ago, mike2016 said:

My house is 25 years old

So's mine and it's been a nightmare heating it with gas. The A2A units are in an outbuilding that was recently extended and has cinema and hobby rooms. It's been tempting to shut down the house and move in over there since the 60m2 can be kept at 18oC all night and day for just 10 to 20 kWh whereas we've been sinking 150kWh of gas into the house on a couple of days this week.

 

It's kind of odd really as the house has EPS beads sandwiched between lightweight concrete blocks and EPS sheet insulation under the floors. But it is kind of big. Even odder is the rental property we have - the tenants had me over the day before yesterday to look at some damp on their walls. I had a good look around and the house was very cosy (cosier than mine). It was built in the mid 80's - barely any insulation and a few badly fitting windows but not too shabby. I asked about how they heated it and the answer was just an hour first thing in the morning and three hours in the evening. Jeez - I wish! Mines on pretty much 24/7. Turns out the condensation was in one room they keep cooler for their kid who was complaining his bedroom was too hot!

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59 minutes ago, Radian said:

A2A now fulfils the roles I had for electric convectors

Is the noise an issue or do you have them placed out of the way? I'm thinking about PV divert and have been looking at posts on these with interest.

 

Useful info thanks @TerryE, trying to think how well this will work in the 90's house you mention at the end (but mine!). High thermal capacity still but larger temperature swings needed due to the much higher energy input.

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9 minutes ago, S2D2 said:

Is the noise an issue or do you have them placed out of the way? I'm thinking about PV divert and have been looking at posts on these with interest.

 

 

It's so subjective. I used to build PC's with passive cooling chassis just to eliminate the last vestiges of noise that disturbed me when trying to think. Yet now we're in an energy crisis (of multiple makings) I seem to be fairly oblivious of the noise the indoor units make. Although they're very quiet and have multiple options for fan control) It's actually a sound I now find soothing as I'm being warmed for minimal cost. Air movement is probably the biggest difference that has to be reconciled but the units that have thermal occupancy sensors do a good job of keeping the airflow away from you. Placement is worth thinking about very carefully. The most comfortable area seems to be at the furthest point from the wall unit. Pretty obvious except it also seems the warmest (not quite so obvious).

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1 hour ago, Radian said:

Resistive heating is completely off the table now as far a I'm concerned. The only use for it in my reckoning is for cycle-by-cycle dumping of excess Solar PV. A2A now fulfils the roles I had for electric convectors although I've kept one as an overflow dump load in the workshop for when the HW immersion divert max's out

I do the same with a wireless operated 600W convector heater, for those times when the surplus PV is more than my immersion heater can absorb which seems to be about 2.8kW

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1 hour ago, Radian said:

whereas we've been sinking 150kWh of gas into the house on a couple of days this week.

I better not say my ASHP  used 50kWh in the last WEEK heating the house.

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1 hour ago, ProDave said:

I better not say my ASHP  used 50kWh in the last WEEK heating the house.

But you went and said it anyway didn't you. How's it been doing over the weekend?

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@S2D2, the rads that I have are in practice silent.  Any convection is passive.  And yes they are ~100% efficient -- that old entropy: everything ends up as heat.

 

ASHPs are at a CoP / efficiency of maybe 400% if you have enough buffer to avoid cycling and keep the O/P flow to under 40°C.  But as I've said before you only fall within permitted development if you use an MCS certified installer and installation, so in our locale this would work out at perhaps £10-15K plus we'd need to find space for a buffer tank because using a slab as the buffer (as we do currently) is not in the MCS playbook.  Not worth it for maybe £1K savings at the current hiked electricity prices.  My view is that ASHPs are still on the early-adopter end of the engineering experience curve.  They will significantly fall in price over the next few years and there will be more variants to cover different use cases.  So there are advantages in waiting this one out for a few years: the real price savings will fully offset any short term increased energy costs.  

Edited by TerryE
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Hi Terry - just picked this up - an interesting read! (as usual)

 

I've been monitoring our heating since the prices went up. As you know, we have a conventional gas system boiler rated at 18kW. The heat loss calcs undertaken during the build showed we needed 10kWh for DHW and around 5.7kWh for total heating losses. 15% overhead was allowed in the boiler sizing, giving 18kW.

 

Our house isn't as well insulated as yours - our energy rating is at the top of B at 89 (we could only get into A with by adding some renewables, or ironically going all electric). We also have 2 large (7.5m2 and 8.3m2) triple glazed doors and windows, with a U value of 0.88 W/m2K and another 24m2 other doors, but overall I think an 18kW boiler is pretty good for a 224m2 house.

 

We have GF rooms with UFH set at slightly different temperatures, between 21 and 23 deg. Upstairs we only heat the bedrooms we use regularly (2) and one bathroom. The landing gets borrowed heat from the UFH.

 

Our daily gas consumption for both heating and DHW is typically around 80-100kWh during this cold spell, when outside temperatures have been below zero. So I think your 28 + 17.5kWh is excellent for a 3 storey house, where there must be some distribution losses to get the heat to where its needed.

 

The big difference in terms of cost of course is gas is 'only' 10.24p/kWh while your electricity is over 3x as much at 38p/kWh. We are spending around £8-10 a day during this cold spell, purely because of the difference in energy source.

 

Interesting to hear about your daughter's 1990s house. Our GF slab takes over a day to noticably cool down. The GF rooms only lose a couple of degrees overnight after the heating has gone off at 10.30. The upstairs rooms lose more as we are mostly in the roof. I'm going to experiment to see if it's worth keeping the boiler running 24/7 to do as you are doing - maintaining everything at a constant temperature.

 

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On 11/12/2022 at 20:43, ProDave said:

I better not say my ASHP  used 50kWh in the last WEEK heating the house.

 

Amazing numbers, especially given your location.

 

Back of a fag packet calculations:

 

50 kWh at COP of 3 = 150 kWH total energy provided for heating

 

150/7 = ~21 kWh/day

 

That's an average of less than a kW over that period, which even in a Passivhaus would be impressive.

 

What temperature are you keeping your house at?

 

What sort of weather have you had? We've been mostly very cloudy, so very little solar gain, meaning the ASHP has been working harder.

 

You mentioned some supplemental heating with your diverter - do you think you had much from that?

I don't know exactly what we're using for heating at the moment, but I do know that I've never seen a weekly electricity cost as high as last week's (admittedly we were doing loads of driving, so the electric car was the cause of a lot of those costs).

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6 minutes ago, jack said:

50/7 = ~21 kWh/day

 

That's an average of less than a kW over that period, which even in a Passivhaus would be impressive.

 

What temperature are you keeping your house at?

That's last weeks figures before it got colder.  I worked it out as an average of about 0.9kW of heat into the building.

 

I normally read my meters once a week on a Friday, but did a mid week reading yesterday and in this cold snap the ASHP is consuming 13kWh per day space heating so witht he same assumed COP of 3 (might be a bit less now it's colder) that would be about 39kWh of heat per day or an average input of 1.6kW

 

House temperature is 20 degrees downstairs, unheated upstairs currently about 18 degrees.  Night temperature has been down to -7 and day temperature not above 0 since the weekend.

 

A few days like today has been good sunshine so probably some useful solar gain, and "as good as it gets" PV generation with the sun at almost it's lowest.

 

Actual heat input per square metre as measured suggests it is just about reaching passive house levels, but it would fail passive house certification if we had tried as the air tightness was only 1.4

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15 minutes ago, ProDave said:

Actual heat input per square metre as measured suggests it is just about reaching passive house levels, but it would fail passive house certification if we had tried as the air tightness was only 1.4

Luckily wind has been notably absent coinciding with this cold snap.

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1 hour ago, ProDave said:

Actual heat input per square metre as measured suggests it is just about reaching passive house levels, but it would fail passive house certification if we had tried as the air tightness was only 1.4

 

To be fair, even if you'd had the required airtightness, it's extremely unlikely you'd have managed to get certification after the house was built. It's technically possible to start the PH certification process after building, but there's a huge amount of record-keeping required to prove that what was designed was actually built. For example, every junction would need to be thermally modelled at design stage, which you can do/show after the fact, but then a full photographic record of every junction is needed to show that what was designed was actually built.

 

The only building I've heard of managing this feat in the UK was that amazing arched house that was on Grand Designs:

 

https://images.squarespace-cdn.com/content/531e5ec2e4b02c4ce263c44c/1456106421390-RYSUKC6L8BHZ75UJJ1BY/uk-passive-house?content-type=image%2Fjpeg

 

I really don't know how they managed it tbh. The arch is exposed internally along most of its length, and I can't see how they managed to insulate it at all, let alone to the sort of value that would been needed to meet the Passivhaus energy consumption standard.

 

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9 minutes ago, jack said:

I really don't know how they managed it tbh

Being on TV?  There are plenty of other with multiple whole walls of glass, where it make you wonder how the calculation adds up

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5 minutes ago, JohnMo said:

Being on TV? 


It's certified though. I can't imagine the PassivHaus Institute certifying a house that didn't pass the standard.

 

7 minutes ago, JohnMo said:

There are plenty of other with multiple whole walls of glass, where it make you wonder how the calculation adds up

 

In this case, there's loads of glass AND an uninsulated masonry arch that makes up the vast bulk of the non-glass fabric.

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One thing that I have noticed is that we lose maybe 10-20% of our slab heat because of a partial thermal bridge that I discussed in an earlier post.  Space heating using the rads doesn't have this loss, and so is slightly more efficient.

 

@ProDave has done a fantastic job, hasn't he?

 

@RandAbuild, about 80% of our electricity use is at the cheap which is currently 17.96p / kWh so our energy cost during this cold snap is around £12 / day.  Do whatever you think works for you.

 

@jack, I remember that episode, back from the days when Grand Designs was watchable. I always wonder what the actual as-built performance of these builds is.

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I never wanted to get mine certified as a passive house, I didn't see the point. I just wanted to do the best that I could and know it was using a lot less energy than most new builds.  I was a little surprised when I gave the as built EPC to building control and they remarked it was the first A they had seen.

 

And I do wonder if I had built the same house in a much less cold place, just how low the heating bills would be.

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Just checked mine, over the last two weeks the ASHP has used 105 kWh on heating (155sqm internal footprint, 50% full vaulted ceilings with an additional 15sqm attic type room - footprint is laid out as two interlinked cottages, so a lot of exposed exterior wall). This has delivered 387 kWh of heat to the house (so a CoP of 3.6). I have the master thermostat set to 19C - we are trying a lower set point this year to see what the energy saving is compared to previous set point of 21C. So far, so comfortable and the house sits around 19.5 - 20C. It can get higher, but that depends on the sun appearing.   Ambient temp wise, not as cold here as elsewhere, but higher wind speeds, which ultimately have much the same effect in determining our heating requirement.

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59 minutes ago, ProDave said:

I was a little surprised when I gave the as built EPC to building control and they remarked it was the first A they had seen.

 

The guy I used was very experienced and had never seen anyone achieve an EPC of 100 before. To be fair, I think a lot of his work was with developers, so perhaps not that surprising he wasn't see much in the way of low-energy builds.

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36 minutes ago, jack said:

 

The guy I used was very experienced and had never seen anyone achieve an EPC of 100 before. To be fair, I think a lot of his work was with developers, so perhaps not that surprising he wasn't see much in the way of low-energy builds.

Although I was disappointed with my air test result of 1.4, the tester was almost having kittens at the result, being the best he had yet tested.

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14 minutes ago, ProDave said:

Although I was disappointed with my air test result of 1.4, the tester was almost having kittens at the result, being the best he had yet tested.

 

Ha. My air test guy did loads of MBC tests, so he wasn't that impressed with our result!

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42 minutes ago, jack said:

lot of his work was with developers, so perhaps not that surprising

And that  is with prearranged units being tested, so some quality control.

I can't remember the detail unfortunately but a bco once told me that a big volume can have falsely good air pressure tests. That was because we were testing a warehouse of some 100,000m3 with a very big fan and the result was far better than required.  Subsequently kit has got smaller and they want to close off sections where possible.

Operators have asked me 'what figure are we hoping for'.

 

Also i have observed some constructions with interest and been dubious about the published  pressure results and epc. No further  comment.

 

Re the arched house (which I have driven past after a wrong turning) (the turf had slid a bit at that time)  : the external area will be relatively small which helps A/V.

And it is covered in turf which won't insulate so well when sodden or frozen as when dry. 

It would be interesting to see the real life energy figures.

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31 minutes ago, saveasteading said:

Re the arched house (which I have driven past after a wrong turning) (the turf had slid a bit at that time)  : the external area will be relatively small which helps A/V.

 

While a curved surface does help with that, it's still quite a complex shape as a whole when you include the boxes that stick out from the arch. I remain unconvinced!

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13 minutes ago, jack said:

still quite a complex shape.

Agreed...i looked at the picture again.

When brand new, the building was available for industry visits, but I never managed to go. 

Since then, I think silence. 

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On 14/12/2022 at 12:46, jack said:

 

To be fair, even if you'd had the required airtightness, it's extremely unlikely you'd have managed to get certification after the house was built. It's technically possible to start the PH certification process after building, but there's a huge amount of record-keeping required to prove that what was designed was actually built. For example, every junction would need to be thermally modelled at design stage, which you can do/show after the fact, but then a full photographic record of every junction is needed to show that what was designed was actually built.

 

The only building I've heard of managing this feat in the UK was that amazing arched house that was on Grand Designs:

 

https://images.squarespace-cdn.com/content/531e5ec2e4b02c4ce263c44c/1456106421390-RYSUKC6L8BHZ75UJJ1BY/uk-passive-house?content-type=image%2Fjpeg

 

I really don't know how they managed it tbh. The arch is exposed internally along most of its length, and I can't see how they managed to insulate it at all, let alone to the sort of value that would been needed to meet the Passivhaus energy consumption standard.

 

 

Had a little dig into this, and rewatched the episode. As it happens they also redid their EPC this summer.

 

The episode discusses that the design is passive. And the epsiode includes an air test @ 33 minutes below.

 

His "High Five" accent reveals a Borat fan, I'd say ! 

 

Basically, the roof u-value is 0.13 (EPC = 92), and the arch is insulated on the outside under the green roof. It overhangs at the sides and that is where the uninsulated bits are - must be an interesting joining detail there.

 

Timber frame box walls are 300mm of newspaper clippings.

 

image.thumb.png.2e65eef6fadad4ac1352ac6c424ec5bb.png

 

One point I did not remember is that the outside 2 layers of tiles (from 3) are laid in cement, whilst the inside layer is in plaster of Paris.

 

Drainage via a layer of terram or similar, filled with 20 tons of gravel:

 

image.thumb.png.61dc953f62bcc3b2746187b713126b74.png

 

Main source of heating is a biomass boiler.

 

image.thumb.png.efc30c67d22a4ef9b7767c3e33e448f7.png

 

Has solar PV (with a very nice payment per unit) and also solar thermal. 

 

I'd say they knew exactly what they were doing. Well done all round.

 

And, yes, there was the obligatory baby.

 

Links:

EPC:

https://find-energy-certificate.service.gov.uk/energy-certificate/9676-0036-6304-9432-7200

Programme:

 

 

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