Heating Control Systems

[SteamyTea]

2 hours ago, JSHarris said:

I still maintain that if you have a house with a low heat loss rate, even under worst case conditions, then a dead simple, but low hysteresis, thermostat should do the job. 

Yes, it will, as it is a small problem.

But take another scenario, say a refurbishment to a lower standard, and with the original boiler, then it is a totally different kettle of fish.

Or using E7 and regular storage heaters, which are still one of the cheapest overall methods of installing and running a heating system.

Variable time of day (TOD) energy pricing may make this a more important issue in the future.

Edited November 19, 2017 by SteamyTea

[joe90]

Just now, SteamyTea said:

Yes, it will, as it is a small problem.

But take another scenario, say a refurbishment to a lower standard, and with the original boiler, then it is a totally different kettle of fish.

Or using E7 and regular storage heaters, which are still one of the cheapest overall methods of installing and running a heating system.

 

Mine is a new build with good insulation and air tightness ( I hope?) and I have yet to work out whether E7 is worth it or not. I was thinking that with lots of heavy weight materials heating and DHW gained overnight would be sufficient, I am yet to work this out. My plumber thinks running both 24/7 at a low temp would be best. I also think (hope) that a single thermostat will be sufficient like JSH says ( and proved in his own build).

[SteamyTea]

4 minutes ago, joe90 said:

My plumber thinks running both 24/7

This is really down to the price of energy in my opinion.

The easy thing to work out is the capital cost of different systems, and the relative complexity of them.

If you want the whole house heated, then yes, a 'single' system is probably the best.

But, if you live a 'disjointed' life, i.e. different people living to different timetables, then a more controllable zoned system may be better.

Also, as you have probably found out, there is a big difference when the sun comes out, if only briefly.  That happens a lot where you are (usually, this last summer was very odd).

 

[Stones]

22 hours ago, PeterStarck said:

Do you suffer from the effects of decrement delay or is it always windy enough so that heat build up on the surface of the building fabric is lost to the atmosphere.

 

Both, I think, although to what extent I'm not sure.  We have a mix of insulation types, not the best in terms of decrement delay, but certainly not the poorest.  What I can say is the house seems to hold heat very well.

 

13 hours ago, TerryE said:

If you assume that the external house surface is at external ambient and the external skin is an essentially impermeable barrier, then wind will have three separate effects:

1. if the skin surface is wet and the air humidity is less than saturated, then you will get evaporative cooling which will reduce the skin surface temperature below ambient and there increase heat losses.
2. If the house is inherently leaky then small pressure differentials across the house will severely increase such air losses which bypass heat recovery.
3. Particularly if the MVHR inlet and outlet are on different walls then any pressure imbalance will degrade the effectiveness of the heat recovery.

I've put both our MVHR inlet and outlet on the same gable to  minimise the third, but where we are we rarely have strong winds.    In Jason's case, I would have though a good place to put the MVHR inlet and outlet would be in his sheltered "alleys" between the two wings of his house which would shelter them from the worst of air wind effects.

 

Yes, this is exactly what we have done, both intake and exhaust roof vents on the leeward side of sheltered alley.  Generally this does mean the ventilation is pretty smooth, although there are exceptions when wind direction changes, and wind speeds are high.

 

6 hours ago, JSHarris said:

The house external surface will be above ambient, though, as even with the best insulation there has to be some heat flow through the structure.  Also, there will usually be some external surface solar gain, even on a dull day, that will tend to make any external surface that is impacted by even diffuse sunlight a bit warmer than  ambient.

 

In the standard U value calculation for any external element there are two assumed surface heat transfer factors included (or there should be, if the calcs are being done properly), one for the internal surfaces and one for the external surfaces.  These factors are usually simplified to assume constants for surface emissivity (which determines the radiative heat loss) and still air surface convection loss rate, which depends on surface roughness (which effects both surface area and still air convective flow) and the angle of the surface (vertical surfaces have a lower convective heat loss factor than horizontal or angled surfaces).

 

What changes with wind is that the assumed convective surface heat loss rates in the U value calcs change, quite substantially in the case of a rough surface.  Add in a bit of moisture (driving rain or mist) with some wind and you also get a fair bit of evaporative heat loss, as well as increased loss because of the higher heat capacity of water vapour.

 

If the airtightness is good, and there is MVHR, then I doubt that there will be that much change in ventilation heat loss.  The resistance to flow of the air leakage paths will be high, whereas the resistance to flow of the MVHR, ducts and terminals will be a lot lower, so most wind-induced increased ventilation will probably flow via the MVHR, pushing the ventilation rate up, but still recovering some heat, albeit at a lower efficiency.

 

I can't really add anything to your explanation of such heat loss other than to say given the climatic conditions, wet windy weather, we are going to see a higher level of heat loss compared with your own house. 

 

The point about airtightness is interesting, as I think we do experience ventilation loss when wind speeds are at gale force (40mph+), as you can hear what is normally a steady fan speed of the MVHR, being thrown out in high gusting wind.  

 

3 hours ago, JSHarris said:

I still maintain that if you have a house with a low heat loss rate, even under worst case conditions, then a dead simple, but low hysteresis, thermostat should do the job. 

 

Say the house takes 10 hours to lose 0.5 deg C in winter (ours is a bit longer than that, more like 24 hours or so).  As soon as the temperature in the room drops by 0.1 deg C below the set point, the heating comes on.  The heating system then has several hours to pump heat gently in to the house before the temperature drops much below the set point.  Likewise, as soon as it has gone 0.1 deg C above the set point the heating will turn off, but the house tends to carry on warming up a bit over the next couple of hours, from the residual heat in the UFH pipes and the time taken for heat to travel from the warmer core of the slab to the surface.  It's this latter issue that makes the house comfort level so dependent on keeping the UFH flow temperature as low as possible - the more heat there is sitting in the core of the slab the greater the temperature overshoot when the heating turns off.

 

Overall this system seems to be able to control the house to around -0.2 deg C, +0.7 deg C normally, a fair bit better than the hysteresis on some of the pretty crappy thermostats that have been around for decades.  We do occasionally see an overshoot to around 1 deg C over the set point, but that's usually because of a bit of solar gain.  We never experience temperatures dropping more than about 0.2 deg C below the set point, no matter what. 

 

Personally, I can live with that.  That range of temperature variation around the set point seems perfectly acceptable to me, so I really don't see why there needs to be any more complication.  Best of all, it uses off the shelf stuff, so can be fixed quickly if anything fails.

 

Our control system seems to be performing in a comparable way, despite the fact that the master controller hysteresis uses 1C increments.