Do low energy houses really need heating controls?

[ToughButterCup]

37 minutes ago, JSHarris said:

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The problem with trying to move heat around with air is that you need to move very large volumes of it in order to have any appreciable effect, simply because air has such a low volumetric heat capacity.

[...]

 

You'll make a scientist out of me yet.

 

....And I can even explain (at the right time to the right audience)  how that trope 'thermal mass'  benefits from a small tweak.

Who's a clever boy then?  I am.

 

Even my esteemed architect trots (used to) it out  occasionally .

[pdf27]

11 hours ago, JSHarris said:

 

The problem with trying to move heat around with air is that you need to move very large volumes of it in order to have any appreciable effect, simply because air has such a low volumetric heat capacity.

 

Say you wanted to shift an unwanted 500 W of heat out of the house, and that the temperature differential you were trying to correct was 2 deg C (the house is too warm by 2 deg C).  The volumetric heat capacity of air at 25 deg C is about 1210 J.m^-3.K^-1, which is around 0.3361 Wh.m^-3.K^-1, so if you wanted to move 500 W of heat in one hour you would need to shift just under 3,000m² of air to lose 2 deg C.  That is about 10 times more air than our Genvex Premium 1L MVHR can shift on full boost.  Because the Genvex has an in-built air to air heat pump, we can deliver fresh air on a hot day at around 12 deg C or so, and that does make a slight difference.  It's nowhere near enough to compensate for significant overheating, but is OK for a tiny bit of comfort cooling if we can restrict the incidental/solar heat gain a lot.

I think looking at it in hour terms is a bit unhelpful - 500W and 1210 J/m³K means that you need to exchange 5/12 of a m³K per second - at a 10°C temperature difference that's only 40 litres/second, which isn't ridiculous for a whole house.

Can I just check your numbers actually - something doesn't make sense. I agree with the 0.3361 Wh/m3K value, but to remove 500 Wh that means to me you need 500/0.3361 = 1487 m³K per hour to provide the cooling (same as I've got below). Your figure only works at 0.5K, i.e. the outside temperature is the same as the target temperature and the inside temperature is 0.5°C warmer than the outside - the 1487 m³K figure is air flow rate multiplied by temperature difference between the room temperature and that of the supply air.

 

500W x 3600 seconds is 1.8 MJ, dividing this by 1210 J/m³K means you need 1483 m³K per hour to provide that much cooling. A Paul Novus 300 can provide up to 300 m³/hr, so provided the outside temperature is more than 5°C below your target temperature you should be able to provide sufficient cooling that way. That's a small enough difference that you're unlikely to be in heating mode - at which point a water to air heat exchanger plumbed into the same circuit as the floor slab would give you precooled air in the same way your Genvex does.

[Jeremy Harris]

sorry, looks like I made an arithmetic error, comes from working in a hurry over a glass of wine!.

 

Our Genvex also has a max boost of 300 m³/h, but will the outside temperature be 5 deg lower than the target room temp when you need cooling?

 

My experience is that it doesn't, and that just having the MVHR on bypass, on (noisy) full boost doesn't make a great deal of difference during the day.  It works OK for night purging, but for day time cooling the active air to air heat pump tends to work a bit better, as that can deliver air at around 12 deg C (pulling it through the bypass first, then cooling it with a nominal cooling capacity of around 1200 to 1400 w).

 

We've discussed adding duct heat exchangers here several times, and they do make a lot of sense.  If I were doing in again I'd probably have fitted a heating/cooling duct heat exchanger, with a condensate tray and drain (there are some links in other threads here where we've discussed this).

 

The bottom  line is that, in practice, the Genvex Premium 1L is nowhere near enough to make a rapid difference, even on full boost.  It's oversized for our house, too.  The floor cooling, on the other hand, is massively more effective at cooling the house.  In cooling mode I run it with a flow temperature of 12 deg C and that gives a coolest floor surface temperature of around 18 deg C, so there is no realistic condensation risk for any probably house air relative humidity level.  Floor cooling works better than I expected and is our primary cooling system in very hot weather.

[pdf27]

11 minutes ago, JSHarris said:

sorry, looks like I made an arithmetic error, comes from working in a hurry over a glass of wine!.

 

Our Genvex also has a max boost of 300 m³/h, but will the outside temperature be 5 deg lower than the target room temp when you need cooling?

 

My experience is that it doesn't, and that just having the MVHR on bypass, on (noisy) full boost doesn't make a great deal of difference during the day.  It works OK for night purging, but for day time cooling the active air to air heat pump tends to work a bit better, as that can deliver air at around 12 deg C (pulling it through the bypass first, then cooling it with a nominal cooling capacity of around 1200 to 1400 w).

 

We've discussed adding duct heat exchangers here several times, and they do make a lot of sense.  If I were doing in again I'd probably have fitted a heating/cooling duct heat exchanger, with a condensate tray and drain (there are some links in other threads here where we've discussed this).

 

The bottom  line is that, in practice, the Genvex Premium 1L is nowhere near enough to make a rapid difference, even on full boost.  It's oversized for our house, too.  The floor cooling, on the other hand, is massively more effective at cooling the house.  In cooling mode I run it with a flow temperature of 12 deg C and that gives a coolest floor surface temperature of around 18 deg C, so there is no realistic condensation risk for any probably house air relative humidity level.  Floor cooling works better than I expected and is our primary cooling system in very hot weather.

• The obvious answer there is that either your duct pressure loss is relatively high (it'll do 300 m³/hr at 150 Pa but only 200 m³/hr at 300 Pa) of your cooling demand is significantly greater than 500W. Unless you've paid a lot of attention to avoiding direct sunlight except in winter, my money would be on the latter.
• Another potential benefit of a duct heat exchanger is that it gives you a guarantee that you won't get condensation on the floor - the heat exchanger will be at the water temperature, cooling the air flowing over it and guaranteeing that the dew point of the air in the house is warmer than the floor temperature. In reality it won't be an issue anyway unless you have a huge amount of solar gain to deal with - the heat exchanger in the MVHR will do the same thing since the air leaving will be pretty cool - but it does avoid an extra heating/cooling device.

[Jeremy Harris]

Nothing to do with duct loss at all, and everything to do with the positioning of MVHR fresh air terminals and the way the air circulates.  If you could reconfigure the ducting, to switch the fresh air feeds selectively to the specific rooms that have the incidental/solar gain issue, then it might have a greater impact, but the evenly distributed nature of MVHR means that cool air is distributed from several terminals in rooms that don't actually need much cooling.

 

This is all discussed in depth in older threads here, but to reiterate, before we fitted the long wavelength IR reflective external film to the front glazing, we had typical cold, but sunny, late autumn, early spring, days when we would get well over 2 kW of solar gain in the hall way, just from that glazing alone.  It wasn't a problem in summer as the shading from the extended roof overhang worked OK.  Also, and again discussed at length in other threads, we had a surprising amount of solar gain from the two east facing windows, as I'd overlooked the effect of clear morning air and the duration of the sun from the east.  Having around 300 to 400 W/m² coming in through around 3m² of glazing added a significant heat input too (and, as discussed in other threads, we fitted external long wavelength IR reflective film to the east-facing bedroom window to mitigate this).  The east windows were in a bedroom (with a single fresh air terminal) and in the kitchen (with no fresh air terminals).

 

At 18 deg C surface temperature there is a near zero risk of floor surface condensation, so much so that we've never had (AFAIK) any room air conditions where condensation would occur on a surface at 18 deg C (and is why I selected that temperature, as mentioned in the other threads on this).  The RH would have to be way, way higher than we have ever seen, even when cooking, for ground floor condensation to be an issue at this temperature.

 

The duct heat exchangers have a capacity that is similar to the Genvex air to air heat pump, as the limit is really set by the air flow rate, but they do have the advantage of being cheaper.  coupled with the floor cooling and run from the ASHP, I would expect that the performance would be around the same.  One option might be to separate out the duct coolers from the MVHR and have a separate system that recirculated air from warm rooms through local fan coil units (with condensate trays).  This would address the issue where incidental heating is localised to just two or three spaces in the house (which was certainly the case for us).

 

The bottom line is that shading is a better option.  Had we had the option to fit external blinds (planning constraints meant this wasn't an option) then they would have been my first choice.  I stayed in an apartment in Portugal with electrically operated external blinds and they were extremely effective, and had the added bonus of increasing security.

[TerryE]

5 hours ago, pdf27 said:

Another potential benefit of a duct heat exchanger is that it gives you a guarantee that you won't get condensation on the floor

 

As Jeremy says, this just isn't a real risk in a passive house.  The heat imbalance is typically at most 1kW or so and for a typical slab this might need it's surface to be 2-3°C cooler than room temperature to do this.  This is way above condensation temp for the RHs you find inside a passive house, thanks largely to the continued MVHR circulation.