Overnight temp drop 0.3/hour in my MVHR+SIP house, is this good?

[haddock]

Don't know if this is a sensible way to measure the effectiveness of insulation, but I'm interested to know how this compares to, say, a conventionally-built new-build UK house and a new-build passivhaus.

 

It's 17.5 degrees inside right now at 11am, was 21 at midnight when the heating went off. So that's about 0.3 degrees of temp loss per hour. It's been an average of about -1.5 outside during that time.

 

The house is very airtight (it was measured when constructed ~15 years ago), but at least one of the skylights has a thermal bridge which I think causes a lot of heat loss.

[Nick Thomas]

In my ~10-year-old new build box scraping by on contemporaneous building regs, I lose about 0.5°C/hr in the daytime when it's cold outside (say, deltaT=15°C). No idea how valid the comparison is though 😅

[jack]

27 minutes ago, haddock said:

Don't know if this is a sensible way to measure the effectiveness of insulation, but I'm interested to know how this compares to, say, a conventionally-built new-build UK house and a new-build passivhaus.

 

It's 17.5 degrees inside right now at 11am, was 21 at midnight when the heating went off. So that's about 0.3 degrees of temp loss per hour. It's been an average of about -1.5 outside during that time.

 

The house is very airtight (it was measured when constructed ~15 years ago), but at least one of the skylights has a thermal bridge which I think causes a lot of heat loss.

 

That seems like a lot to me.

 

For comparison, I have a 289 m² house with PassivHaus levels of insulation and airtightness. We aren't blessed with good solar gain, but we do have a lot of glass, much of which doesn't have any internal coverings like curtains to help keep the heat in. I think we use more energy per m² compared to others with similar insulation etc.

 

As it happens, I was mucking about with the ASHP settings recently and accidentally turned off the heating for over 24 hours. The heating went off in the early evening on Sunday (red arrow). This is a graph of the temperature on top of the (polished concrete) floor downstairs:

 

 

The first yellow arrow is the temperature peak, which was about 6 hours after the heating turned off. The second arrow is 24 hours after the peak. The difference is 1°, so about .04° per hour.

 

This is the air temperature upstairs in the landing:

 

 

Still a 1° difference albeit at lower overall temperatures (no heating upstairs).

 

It's been really cold the last couple of days. The last 24 hours, in particular, have been absolutely freezing (-6 overnight, I believe, and only -3 even now).

 

I turned the heating on for a couple of hours last night and have turned it on again this morning. Because the UFH pipes are buried so deep in the slab, it takes several hours for the heat to appear at the surface - another reason why low, continuous heating makes more sense with such an arrangement.

[MikeGrahamT21]

I've a 1960's bungalow, temp in living room at midnight was 17.5C and by 6am was 15.5C giving a heat loss of 0.33C per hr. Room is very airtight, but still has double glazed window (approx 2.2-2.4Uw) and no external wall insulation on the external wall, approx u value of 0.45.

 

I still need a further 50% energy savings to get to enerphit standard, so by no means a passivhaus.

Edited January 17, 2023 by MikeGrahamT21

[SteamyTea]

I have found that with my house, a small terraced place, that the major influence is temperature difference.  Wind direction and Solar Gain have very little effect.  This is not surprising really as when it is windy, it is predominately from the South West, which is warm, but with cloudy skies.

 

Looking at my data for the cold spell last year, 27/11/2022 to 17/12/2022 and grouping the means by hour of day, this is what happens.

 

 

Then looking at all the negative slopes I get this.  Added a linear trend line as it makes the arithmetic easy.

So Midnight to end of 4 AM the temperature drop is -0.180°C.h^-1 

 

9 AM to end of 11 AM, slope is -0.119°C.h^-1 

 

 

And finally 3 PM to the end of 11 PM, the slop is -0.104°C.h^-1 

 

 

This gives a mean temperature drop of -0.134°C.h^-1 

 

This is only part of the story though.  As I know that the major influence is eternal temperature, I can plot and calculate the slopes against temperature difference by hour.

This again shows three periods when the temperature slope is negative, with the final one being only two hours.

Note that the y-axis has changed from temperature to temperature difference.

 

 

So Midnight to end of 4 AM the temperature drop is -0.180°C.h^-1 

 

 

 

8AM to end of 12 PM the temperature drop is -0.477°C.h^-1 

 

 

And finally 10PM to end of 11PM, slope -0.227°C.h^-1 

 

 

That is a mean difference of -0.296°C.h^-1 

 

 

I am not sure which is the most important number -0.132 or -0.296°C.h^-1 as it really depend on the internal temperature range, which has to be within the comfort range.

I aim for a mean room temperature of 20°C, but during this cold snap, the mean temperature dropped half a degree to 19.51°C, with a range of 2.05° (min 18.37°C, I can live with that).

The external temperature, from the local WeatherUnderground station (reports higher than the new probe I have put in by ~1.1°C) is a mean of 5.86°C, a range of 2.07°C and a min of 5.20°C.

 

This can be plotted two ways again, x-axis as external temperature or as external temperature difference.

 

 

 

The slope changes with a value of 0.635°C increase in room temperature for every degree rise in external temperature, or 0.409°C rise for every degree rise in external temperature, the inverse is also true, so the same drops in internal temperature for every degree drop on external temperature.

 

Not sure if any of that helps any, just pointing out that thermodynamics is never as simple as we first think.

 

 

 

Just noticed I did not change the times on the third chart down from the top.  Is is 3PM to end of 11PM, and I also have a typo that makes it hoours, which is just me pretending to be French.

Edited January 17, 2023 by SteamyTea

[haddock]

Absolutely fascinating to see the different heat performance results. My house is exactly the same size as @jack's but over one floor. So it seems it should be retaining heat much better than it is. I wonder what the best DIY approach to finding the heat leak is, some kind of heat camera?

 

We currently have forced-air gas central heating, which has the advantage that it can heat the house from cold very quickly. I have it set to switch off automatically when the burglar alarm is armed. But want to replace or augment this with ASHP in the coming years.

[SteamyTea]

50 minutes ago, haddock said:

Absolutely fascinating to see the different heat performance results

Let's tabulate it.

 

@jack -0.04 °C.h^-1 

@SteamyTea -0.132 to -0.296 °C.h^-1 can call that 0.21 °C.h^-1 

@haddock -0.3 °C.h^-1 

@MikeGrahamT21 -0.33  °C.h^-1 

@Nick Thomas -0.5 °C.h^-1 

 

So my old 1987 house, with some minor modifications, is petty good.

I am not sure how much of that is to do with being a terrace, or small, or at a colder internal temperature, or a higher external temperature.

My place is TF, so can we knock that 'thermal mass' nonsense on the head, again.

 

Cooling is proportional to the exposed surface area, the temperature differences, the shape, and the exponent function e

 

Worth looking up Newton's Law of Cooling as it can nicely shows all that in a single chart.

 

https://en.wikipedia.org/wiki/Newton's_law_of_cooling

 

 

Edited January 17, 2023 by SteamyTea

[jack]

15 minutes ago, SteamyTea said:

@jack -0.4 °C.h^-1

 

You missed a zero - mine's 0.04

[SteamyTea]

Just now, jack said:

 

You missed a zero - mine's 0.04

Top of the list for you then.

Realistically, at the external temperatures you have had recently, just normal living is heating your house.

 

Because I knew there was a cold snap coming, I pumped an extra 10 kWh or so into my night storage heaters, house is a little warmer. Tomorrow I expect to see a reduction in energy usage a some of that 10 kWh was getting the storage heaters up to a higher temperature.  I have kept the heating window (4 hours) the same, so will be interesting to see when the night stores actually switched off.

[MDC]

Is there an answer to @haddock's question?

 

Is .3 an hour any good? I tested two bedroom's last night. The temperature dropped from 18.8c to 16.0c over 8 hours with no heat input. I suspect rooms on the north side of the house would have dropped by more.  Our house is made of playdoh and yet has a tested air permeability of 3.3 @ 50pascals or whatever. 

 

The comparisons are interesting, but is .3 an hour okay?

 

 

[SteamyTea]

3 minutes ago, MDC said:

The comparisons are interesting, but is .3 an hour okay?

All our data is in that general ballpark, so probably pretty good.

Yours works out as 0.35°C.h^-1 

If there was a temperature difference of 15°C, then I think that is reasonable.  Would not be the case if the difference was 3°.

[larry]

Just so you can all feel smug (and warm), I can see in one of my uninsulated rooms (solid walls, suspended floor, large badly fitted DG window), it dropped from 16 to 14'c in two hours after the heating went off. So that's what, -1.5°C/h.  So some work still to do, then!!

 

https://phpionline.co.uk/news/the-alarming-truth-about-heat-loss-from-uk-homes/

 

 

[Conor]

Our passive ish house is about 1c over 14 hours when it's hovering around 0c, so less than 0.1c. hard to really know tho as the UFH comes one every night regardless. I'll have to get some monitoring setup.

[Nickfromwales]

3 minutes ago, Conor said:

I'll have to get some monitoring setup.

Why?

Spend the money on beer or whatever other happiness-inducing activity puts a smile on your face!!!

[SteamyTea]

4 minutes ago, Nickfromwales said:

9 minutes ago, Conor said:

I'll have to get some monitoring setup.

Why?

Spend the money on beer or whatever other happiness-inducing activity puts a smile on your face!!!

Monitoring makes me happy.

 

This topic has made me look back at my historic data, going to take me hours to get it all filtered, but should be able to see what improvements have happened.

 

12 minutes ago, larry said:

So some work still to do, then

At least you know it, and where the problems are.

 

[Radian]

43 minutes ago, SteamyTea said:

If there was a temperature difference of 15°C, then I think that is reasonable.  Would not be the case if the difference was 3°.

 

Looking at last night's figures, @dt 21°C (17°C in, -4°C out) my house was loosing 0.3°C.h^-1

Going back to Jan 1st when the outside temperature was pegged at 10°C all night, the loss was 0.05°C.h^-1

The delta used when measuring this is massively important as both you and Sir Isaac tells us.

[Nickfromwales]

4 minutes ago, SteamyTea said:

This topic has made me look back at my historic data, going to take me hours to get it all filtered, but should be able to see what improvements have happened.

.......soaking up valuable pasty-munching time........

 

You like maths, so;

You'll see me tag you elsewhere shortly .

[SteamyTea]

2 minutes ago, Radian said:

The delta used when measuring this is massively important as both you and Sir Isaac tells us.

Yes.

I intend to group the next analysis by delta T.

1 minute ago, Nickfromwales said:

You like maths

Well I like statistics, much more informative.

 

[Radian]

7 minutes ago, SteamyTea said:

I intend to group the next analysis by delta T.

 

The cold hard reality of the seemingly innocuous 0.3 loss is driven home by how much energy is consumed to make up those losses. I plotted a selection of average daily deltas against the day's gas consumption for the house before and after getting CWI installed at the end of 2022. 

 

WIth gas at 11p/kWh some days were costing us £20 during December even with the CWI. Winter 2021 was a lot milder hence the lack of data points at the top end. The deviations from the trend lines reflect the days when DHW was used more or less.

 

At least the CWI shows up a bit.

[SteamyTea]

12 minutes ago, Radian said:

The cold hard reality of the seemingly innocuous 0.3 loss is driven home by how much energy is consumed to make up those losses

Yes.  This is especially true if you do not know your air infiltration rates.

 

The form of a building also makes a difference.  Using two connected, but separate, metrics may shed more light on it.  Volume to Total Area and Floor Area to Exposed Area may shed light on it.  Just thought that Exposed Perimeter Length may be useful.

Generally small houses perform badly on a Floor Area metric.

[Radian]

14 minutes ago, SteamyTea said:

 

The form of a building also makes a difference. 

Hence all the talk about 'boring boxes' 😄

Our house is a 'T' shape with the biggest, two-storey, part being the top of the T and the tail being rooms in roof. Rear Loggia and front porch add to the complicated coastline. Volume to exposed area would quantify the situation but the measurements for our house are additionally complicated by having some rooms in a pitched roof. Insulation standards are also 'all over the place' in the various regions - even from new.

[SteamyTea]

7 minutes ago, Radian said:

Volume to exposed area would quantify the situation but the measurements for our house are additionally complicated by having some rooms in a pitched roof

Only trigonometry.

8 minutes ago, Radian said:

Insulation standards are also 'all over the place' in the various regions

Should not make a difference to the overall losses, would make a difference to the room by room though.

 

Half dome is the best shape for a house, not sure how well a wardrobe would fit.

There is a cafe in Tehidy Woods that is a hexagon (6 sided), it is not open much so don't make a special trip.

I have often thought that as it is getting closer to a dome, it could make for an interesting house.

[Radian]

6 minutes ago, SteamyTea said:

I have often thought that as it is getting closer to a dome, it could make for an interesting house.

 

 

This house near us is one storey too high. But pretty close to a cylinder with one domed end.

 

[SteamyTea]

3 minutes ago, Radian said:

This house 

I know it.

 

If the walls were angled to 15°C or so, the upper ones what face the sun (NE to NW) could be covered in PV, get good winter generation.

 

I would feel a nob living in a place like that, but would make for a good business premises.

[Radian]

40 minutes ago, SteamyTea said:

If the walls were angled to 15°C or so, the upper ones what face the sun (NE to NW) could be covered in PV, get good winter generation.

 

But in the winter there's no sun on those faces? Cloud bounce maybe? I've often thought that  EWI clad in vertical solar PV could be a justification for the optimisation of winter generation by offsetting the cost of an alternative type of cladding. But only on SE, S, SW elevations.