Insulation, Heating, time constants etc. Am I expecting too much?

[ProDave]

So our house is pretty well complete from an insulation and airtightness perspective. There are still a few outstanding items that will improve it later, but it is basically there.

 

With the trees to the south of us in leaf, there is little solar gain, perhaps some in the afternoon through the west facing windows.  Consequently it seems to sit pretty much averaging the outside temperature (over many days) and only changing temperature inside slowly. In the spring we had a hot spell and the inside of the house remained cool without overheating, remaining at below outside temperature for many days. I took that as a good sign.

 

Summer has ended abruptly here. It's 10 degrees, cloudy most of the time, wet and windy and has been for some time. the house remains at about 15 degrees inside (still cooling down)

 

Now (and this is where I think it's not performing as expected) I decided to introduce some heat to the house.

 

Before I post any details, I have input the house size, U values etc into Jeremy's spreadsheet and with a 5 degree difference between inside and out, we should have a heating demand of 376W just to maintain the present temperature.

 

The permanent heating (ASHP and UFH) is not ready yet so I have a little plug in fan heater in the house now. With no thermostat on it I only run it when I am in the house. It's only a 1KW heater and from the spreadsheet 376W of that is just replacing the heat lost, so 624W should be heating the internal fabric of the house up.

 

Over the last 24 hours that has been on a total of about 10 hours, so 6.2KWh of heat has been pumped into the house. That has only raised the internal temperature 1 degree.

 

I read on this forum if Jeremy runs his hoover too long the room overheats. If someone lights a 3KW WBS then in an hour (3KWh of input) the room has overheated and can't be used for days.

 

So why has 6KWh of heat input to my house had negligible effect?

 

I am not concerned. Yet. It is the heat required to maintain equilibrium that matters, not how much it takes to heat the fabric of the building when it is cold. but it is not "ringing true" with what I read here about modest amounts of heat input seriously overheating a well insulted house very quickly.

 

For further information. the house is plasterboarded upstairs but no doors are yet on. Downstairs it is not plasterboarded so in effect one big room.  The MVHR is on, and appears to be working. Outside air is 10 degrees.  Extracted air from the rooms (measured at the plenum box) is 15 degrees, and supply air also measured at the plenum box measures at 15.2 degree so somehow very slightly higher than extracted air. This at least shows the heat recovery is working.

[gravelld]

What was your air test result and how much air is the MVHR actually moving?

 

Is this timber frame or masonry?

 

What type of insulant?

[Guest Alphonsox]

Panic not - your structure is absorbing the heat. The specific heat capacity of your structure works both ways. Something that "remained cool" in the summer long after the outside heated up isn't going to change it's behaviour just because you have reversed the hot and cold sides.

Our place was exactly the same - once it reached temperature it needed very little energy input to maintain that temperature but it seemed to take a lot of input to get it there. From the other threads you could work out how much energy it would take to raise the total amount of plaster board you have by 1 degree.

[ProDave]

My Very rough DIY air tightness test was about 0.8ACH  I now find that test was done with a 40mm waste uncapped, so open to the vented stack, so should be better now?

 

Timber frame, wood fibre outside then rendered, average wall roof and floor U values about 0.14 average window UW values about 0.9

 

So how many KWh will it take to raise 79 sheets of plasterboard by 1 degree? then there is the 2 layers of OSB on the inside of the frame before the heat even reaches the insulation.

 

What is bugging me is okay the house takes a lot of heat to warm it up.  Why then do people find 3KWh of input from a WBS overheats the room so it can't be used for days?  It would seem to me if my house had been allowed to cool down, it could quite hapilly absorb many hours of input from a WBS before it got too hot.  Why is my house so different to those that reported that issue?

[MikeSharp01]

I blame thermal mass - Jeremy does not believe in it so it doesn't happen to him. You have not perhaps so strongly espoused the true path so it is still happening to you. Simple answer - keep the faith.

Edited September 13, 2017 by MikeSharp01

[jack]

13 minutes ago, ProDave said:

What is bugging me is okay the house takes a lot of heat to warm it up.  Why then do people find 3KWh of input from a WBS overheats the room so it can't be used for days?  It would seem to me if my house had been allowed to cool down, it could quite happily absorb many hours of input from a WBS before it got too hot.  Why is my house so different to those that reported that issue?

 

Because (and I speculate):

• The fabric of house with the WBS is already not far off the target temp (21 degrees, say), so all the heat goes into overheating not heating up from cold.
• How many people do you know who have a 3 kW WBS? I've heard of a 4 kW model, but that's at the lowest setting, and I suspect (but don't know) that you'd probably have to get it up higher to start with.
• You also have residual heat from a WBS. You don't burn it for an hour then have it fall immediately to ambient. Even a small WBS will probably put at least two or three times its kW rating (in kWh) into the house on a minimum length run (ie, a 3 kW stove would likely burn for at least a couple of hours, then take a least another hour to dump its stored heat, amounting to more like 8-10 kWh of energy).
• The fabric can only absorb heat at a certain rate. The WBS puts out more than that, and the radiant heat it emits quickly makes nearby surfaces very warm (albeit not to much depth). The result is an unpleasant warmth near the WBS. 

The situation you're describing in your own house is more like running an air heater in a cold room. The air feels warm quite quickly, but if you turn the heater off it quickly gets cold again because all the surfaces in the room are cold.

Give it time.

[Guest Alphonsox]

79 sheets, each 2.4m x 1.2m  x 0.012m = 2.7 m3

Density of gyproc = 668 kg/m3 

total mass = 668x2.7 = 1823 kg

heat capacity = 1090 j.kg.K

So around 2Mjoules to raise the lot by 1C which is 0.5 kWh 

Now do the same calc for the rest of the structure ( particularly the slab) and you'll find things are as pretty much as expected.

 

 

[Ferdinand]

28 minutes ago, ProDave said:

So how many KWh will it take to raise 79 sheets of plasterboard by 1 degree? then there is the 2 layers of OSB on the inside of the frame before the heat even reaches the insulation.

 

The Specific Heat Capacity of plasterboard is around 0.95 kJ/kg.°C.

 

79 sheets of 12.5mm plasterboard has a mass of around 23kg * 79 = 1817 kg.

 

So for a 1°C rise 0.95kJ * 1817 kg = 1726 kJ of energy. 

 

Divide by 3600 to convert to kWh = 0.48 kWh per °C.

 

Which is not a lot unless I have something wrong :-).

 

Or it is still driving moisture off.

 

Quote

So around 2Mjoules to raise the lot by 1C which is 0.5 kWh 

I feel better now.

 

Ferdinand

Edited September 13, 2017 by Ferdinand
Reassurance

[ragg987]

I suspect your energy is going towards drying out the house, it has been exposed to the elements for a while prior. Think of all the surfaces and materials that could have soaked up moisture. Not to mention plaster, paint and other finishes.

 

When we first moved in we used up a lot of energy, it seemed to take a couple of months or more to stabilise.

[SteamyTea]

Once the plasterboard is 'up to temp', say 19°C for an internal air temperature of 21°C, it then starts to fully transmit energy.

So you can think of it as a change from absorbing energy from the internal air to just allowing the energy in the internal air to escape to the outside (which is your insulation/frame/render and finally the great outdoors) only being hindered by the R-Value (or U-Value).

 

I suspect what other have said about moisture is correct.

A kg of water takes 4.2 kJ to heat up 1K.

But to evaporate 1 kg of water takes 2,260 kJ, 538 times more energy.  This is also affected by the relative humidity, I think the higher the ambient RH, the more energy is needed (why things don't dry well in the tropics)

Now I have no idea how many kgs of water is in your plasterboard, but for every kg, you need 0.63 kWh extra.

Edited September 13, 2017 by SteamyTea

[ProDave]

Just to add the building should be "dry" the walls have not yet been plastered or painted. It has been wind and watertight for well over a year. No slab, it's an insulated suspended timber floor. Yes there will be residual moisture in the timber but that's all.

[TerryE]

Dave, our slab is around 10 tonne in total and I suspect that yours is similar.  That's an awful lot of thermal capacity.  And as Nick says, if you have above steady state levels on residual moister then evaporating this off takes a lot of energy.  Having a large thermal capacity is good.  It means that once your house is up to target, then it will "resist" temperature variation.  Consider an analogy, an oil tanker: once it is up to its cruising speed, then a very small amount of power is needed to sustain the speed on a straight course, but getting it there, stopping it or changing direction takes a of of power. 

[Stones]

@ProDave We experienced this very phenomenon in both our last and current build. Up until the house was brought up to temperature it would remain cool, sometimes colder the ambient - after all we are talking about giant well insulated boxes (stick a refrigeration unit in instead of heating and we would all have fancy cold stores)

 

Once up to a steady temperature, not much energy required to keep it there, and any extra input - work tools, solar gain, WBS, would rapidly push us into overheating territory.

 

 

[Jeremy Harris]

FWIW, I agree with pretty much all of the above. 

 

Our house warmed up fairly quickly after it was sealed up and insulated, but only because we had at least two people working indoors all the time, plus two 400 W halogen work lights.  By the time we got to plastering, the structure was already pretty warm, and the plasterers had a problem with the halogen lights giving off too much heat and overheating the house (they were working in shorts and tee shirts in February).  We then had a very high humidity inside the house, and had to bring in a dehumidifier to run overnight, and keep the windows open during the day (the latter was far more effective.  The house then cooled down again, but within a couple of weeks of the windows being closed it was back up to temperature again, but that was mainly because we had two or three of us working in the house, decorating, fitting the internal joinery, laying the flooring, fitting the kitchen and doing second fix electrics and plumbing.  By then we were into early summer.

 

 

[jack]

Our experience is different to Jeremy's, partly because of the differences in our microclimate and situation. We get relatively little solar gain due to trees, plus we don't have his nestled-into-a-hill microclimate, so our house is nowhere near as naturally warm as his.

 

On the flipside, we don't have the summer overheating issues he's had in the past - the house has been fine in summer even without using cooling mode on the ASHP.

[ProDave]

I will see how this goes.  Yes we are in a very much colder and windier location to Jeremy.

 

I think this is a case of my expectations were too high, talks of 800W of lighting and stripping off to your shorts by the end of the day and I am asking why is my 1KW heater not doing the same?

[Jeremy Harris]

I'm sure you're right, Dave, but there is also a seasonal difference, too.  Our house was airtight and sealed up for about two weeks over the Christmas period, with no ventilation or heating at all, and no one in the house.  As a consequence, it gradually came up in temperature inside, due to a bit of solar gain and very little heat loss.  By the time we started first fix, the house was comfortable to work in without heating.  I took a fan heater over there but we found we didn't need it, but then we did have two 400 W halogen work lights running, plus at least two people working hard inside, during  the whole of January and February, and a part of March.

 

Another factor may have been that our insulation was pumped in slightly warm.  The act of pumping cellulose does warm it up a bit, so the walls were filled with 300mm, and roof 400mm, of pre-warmed insulation that has a pretty high mass heat capacity. (a lot higher than brick or concrete block).  There are many tonnes of this stuff in our house, and we closed it up for the Christmas break not long after the insulation was pumped in and the air test was completed.  My guess is that this may well have helped raise the internal temperature a bit, too.

[TerryE]

12 hours ago, JSHarris said:

and had to bring in a dehumidifier to run overnight

 

Dave, this is one to note.  If you are driving off moisture then you either need a dehumidifier or your MVHR (not in bypass mode) or both.  As Nick explained converting water to vapour sucks up a lot of heat so the best thing to do is to convert it back to water before it leaves the air-tight envelope.  We had a small domestic dehumidifier which we ran 24 hrs a day when we were drying out the build.  Better to dump water by the bucket load than as moist air.

[ProDave]

We have a dehumidifier so I will bring it into the house and see how much moisture it finds.

[SteamyTea]

Do you have a couple of temperature and RH sensors rigged up to a logger.

Then you can see what is really happening

[Dudda]

Along with all above which I agree with another thing is you've only a fraction of your internal heat gains currently which all add up to a huge amount. No cooking, no hot water or hot water pipes giving out heat, no showers and steam, nobody living in the house full time giving out heat, no fridge or freezer giving out heat, no TV or computer even in standby, no hairdryer in the mornings, no lights late into the night until bedtime, no washing machine or dryer. This and loads more will probably add up to about half your overall heating requirements so until these are all in you need to input additional heat. 

[ProDave]

I am re visiting this with some more meaningful measurements.

 

I have just received a few cheap elect convector heaters, something I am happy to leave on unattended.

 

So I plugged one in last night and it's been on about 20 hours so far, just on a low heat setting of 1250W (just one element turned on)

 

In that time the internal temperature of the house has risen from the 10 degrees it was sitting at with no heating to 15 degrees.  This is one heater downstairs and heat getting upstairs by convection up the stair well (no doors anywhere yet)

 

Last night the temperature was below freezing and just now it was up to the dizzy heights of 3 degrees outside.

 

The heat lost spreadsheet tells me for a 10 degree temperature difference, the whole house should need a heat input of 640W  so it looks like the 1250W I am pumping into the house is not only overcoming the heat loss, it is slowly warming the fabric of the house that has been cold for some time.

 

I feel a lot better now that the heat input requirements should indeed be low as predicted.

[SteamyTea]

If my memory is working right, thermal inertia is m².K^-1.s^-1 for a given thickness of material i.e. your walls.

So say you have 100m2 of wall and you have heated it up by 10°C in 24 hours, that is:

100 [m²] /(10 [°C] x 86400 )

100 / 86400

0.000166 m².K^-1.s^-1

 

Now if your wall is 200 mm thick, then

0.000166 / 0.2

0.0005787 m².K^-1.s^-1

 

The reciprocal of this is 0.48 h.K^-1, so about half an hour to raise the temp up by 1°C

 

I have made some assumptions there as I don't know the size of the wall area, or the thickness.

There is also the problem that the wall can only heat up (or cool down) once for every temperature change, it is not really like a proper battery as it only has the temperature differences to work with, not an excess of temperature.

Then there is the problem of heat loss to the outside, that probably accounts for the wall being at the median temperature point of inside and outside.

And I have not taken other losses, or gains, into account.

[SteamyTea]

And that really annoying thing that happens every now and again has happened.

No matter what I do with the formatting, it stays as strike though.

[TerryE]

10 hours ago, SteamyTea said:

No matter what I do with the formatting, it stays as strike though.

 

I agree that the lack of a toggle is a PITA, but if you cut then paste and select the "remove all formatting option", then this gets rid of any formatting including strike-through.