Yep I know what you are thinking - well insulated homes don't need active cooling in the UK. But unfortunately my home isn't perfect and it is hot. I have actively cooled my ground slab in the evenings before V. Hot days. Here is my thinking and actual numbers/costs.

During the night, when it is coolest we choose to put the windows downstairs on night latch - on heatwave nights not enough air comes in to cool the walls and floor.

The MVHR can run on boost - the marginal energy usage litre of the extra air pumped is approx 1 watt per litre (edit - that's 1 JOULE per litre of air) - at normal ventilation rates the MVHR is much more efficient (and in any case is required). Boost isn't free - and if outside is 18 deg C and inside is 22 deg C we get around 4 watts for cooling for 1 watt of power. (1 litre of air takes 1 joule of energy per degree C of cooling/heating).

On a hot (30 deg C +), sunny day my house heats up by around 2 degrees during the hot part of the day when the windows are closed (say 12 hours). I estimate during this period the total gain is something like 5kWh solar gain, 12kWh conduction through the insulation, 7kWh electrical waste heat and DHW tank loss and 2kWh heat from us - a total of 26kWh - which can all be lost on a typical summer night/evening by leaving the windows open. But on nights like we are having at the moment I need more help - otherwise from day 2 or 3 the house gets too warm for 'perfect' - but still much better than almost any other house.

We have an air source heat pump and under floor heating and ceramic tiles throughout downstairs. If outside is the same temperature as inside then the cooling COP from the heat pump is around 6.0 (it delivers 9 litres/min of water with a delta T of 3.1 deg C - just shy of 2kW of cooling at 1.3 amps or 0.31kW). As the temperature drops outside the COP improves (but is counteracted by the slab getting colder) but overall I can get 16kWh of cooling for around 2 to 2.3kWh of electricity at most (costing 30 pence). By morning my slab is down to 19 deg C.

During the day the slab cools the house - today the slab has warmed up to 21.3 degrees C and has kept the temperature of the house to a maximum of 23.5 deg C.

This only works because the slab acts as a massive heat/cool store, the house insulation and thermal mass is adequate to allow the slab to retain enough cooleth for the whole day, which means I need only cool the slab at night which means the COP is no worse than 6 (during the day it costs twice as much).

This is a heatwave solution, when the night minimum temp is nearer 16 deg C - open windows upstairs and night latch downstairs cools the house ready for the next day with ease. So far it has been used for around 7 nights this year (and looking at the forecast for a couple of further nights) It is a little extravagant on energy - but not excessively so I think.

Hmm - of all people I thought you would be most critical (in a nice way). I suspect you are in a better position for passive cooling with your external shutters and excellent insulation. Out of interest what temp can you keep your house down to, with what additional expenditure on your MVHR?

yep - you are correct 1 joule per litre. I was being careful as well - even used the word energy - then went and said watts.

Interesting numbers, thanks!, and personally I don't see why active cooling is any 'worse' or better than active heating, and as your house is well insulated you will have minimised both.

Given the choice during design, it might even be good to maximize solar gains, so the house needs less heating in winter and more cooling in summer, as the CoP for cooling is better than heating, is better matched to solar generation days, and solar gain can be temporarily reduced by shading and shutters etc.

I think people in the UK maybe suspicious of active cooling, having seen countries where the aircon is wastefully set to shivering cold indoor temperature, but setting a sensible summer indoors temperature is no different thought process than setting a sensible winter temperature. Nobody blinks an eyelid at having aircon in a car which is much less energy efficient than an ashp.

Don't know much about your floor, but if there are say 10tonnes of screed/slab (50m2 area * 100mm of thickness actively exchanging heat with the room) it will take about 10MJ (3kWh) to heat/cool by 1degC. Does that match with your temperature swing of 19-21.3 = -2.3degC?

What is the main source of 7kWh of electrical waste heat? If it's cooking then the extractor fan will dump that straightaway. If it's not too noisy you could perhaps run it overnight to draw cool air through the upstairs windows and down to cool the downstairs, if you cannot safely leave the downstairs windows open.

The south facing surfaces of the roof and walls are likely to be much warmer than air temperature, mine get too hot to touch, so conduction will be more than might be expected.

Edit: where does the motor heat from the mhrv go, into or out of the house? If in, then each 1J of motor power causes 1J of heating, offset against the 4J of cooling you mentioned, so net 3J of cooling : CoP=3. So it's better to use the ashp for active cooling instead, CoP =7

Posted By: WillInAberdeenWhat is the main source of 7kWh of electrical waste heat? If it's cooking then the extractor fan will dump that straightaway.

I have no extractor fan in the kitchen - just MVHR - in heat wave conditions there is no way to loose the kitchen heat until the evening - the kitchen never gets more than a degree or two hotter then the rest of the house and during the heat of the day the summer bypass is off to keep the cool. In short I can't dump any heat because it's all cooler than outside. The main source of the electrical heat is fridges, freezers and computers - and we drink lots of tea. Cooking accounts for 1 to 3 KW depending on the day.

Posted By: WillInAberdeenThe south facing surfaces of the roof and walls are likely to be much warmer than air temperature, mine get too hot to touch, so conduction will be more than might be expected.

Walls are thin coat white and never seem to get hot - the roof on the other hand is dark grey concrete tiles - I've not felt it but I bet it's toasty. Heat getting through here is part of the solar gain of 5kWh. I calculated/guestimated the effects of solar gain in the early spring based on anecdotal evidence that sunny days can be 2 degrees colder than overcast days and require similar heating requirements over 24 hours. The summer sun effects may be radically different - but it is the best I can do at the moment.

Posted By: goodevansAnd for the record It looks like 1 m2 of ceramic floor will release around 2 to 3 watts of cool for each degree below room temperature in still air…

That's surprising. I'd expect the dominant resistance to be the boundary between the air and the ceramic so for it to be more like 5 W/m²·K or a little more. Just using Stefan-Boltzmann (i.e., just looking at heat transfer by radiation) implies that amount assuming usual household emissivities.

There'll also be some heat transfer by convection/conduction which are a bit harder to quantify. It's said BS EN ISO 6946:1997 gives an indoor downwards thermal resistance for an air/maternal interface of 0.17 m²·K/W so a conductivity of 5.88 W/m²·K whereas BRE Digest 108 is quoted as saying 0.14 m²·K/W so 7.14 W/m²·K.

So, does your ceramic have an emissivity significantly less than one (well, less than 0.9 or so)? Or is there another significant amount of resistance in the path? Am I wrong that the interface resistance is dominant and the time it takes for the heat to flow through the concrete matters more than I think?

Ed - I don't think this is the reason for the black body radiation being lower than the slab temp would indicate.

What happens here is - the body of the slab may be 2 degrees lower than ambient - but the surface of the slab is not - the energy gain (or loss in this case) occurs at the boundary and may only be around 1 degree colder or less. Convection doesn't help take away the cooleth here either - in the still air a thin layer of cold air sits over the floor.

For a warm floor - convection allows the layer of the warmer hair to be moved away from the surface so heat transfer is better for UFH in heating mode.

I imagine a race condition occurring here - cool escapes from the surface by radiation and a bit of conduction, and is replenished by more cool migrating from the slab body - that isn't instant and it depends on the conductivity of the floor material(s) - a thick solid wood floor would also have an emissivity of 1 but wouldn't release it's heat as fast because the conductivity of wood is lower - even if it's core body temp was kept constant.

And for those pedants out there - yes I know the actual way to think of the radiation transfer is that the room is radiating it's heat to the floor (or all surfaces radiate and absorb - its the balance that counts) and the floor is conducting that heat down into it's body - but I think are are all more familiar with heat flow coming 'from' the active element - in this case the UFH.

Posted By: WillInAberdeenCoolth transferred by radiation from the slab will not cool the air, but rather it will cool the walls and ceilings and furniture.

Good point, it's mostly not the air temperature which matters but the radiant temperature of the room and the interface resistance is competing not just with the resistance within the slab but also the resistances from the walls, ceiling, etc, to the air. A good reminder that the “ambient temperature” is not a simple thing when you're talking about relatively small temperature differences.

Posted By: goodevansWhat happens here is - the body of the slab may be 2 degrees lower than ambient - but the surface of the slab is not -

I think you're saying, in effect, that the thermal resistance of the slab is significant compared with the interface resistance.

For a warm floor - convection allows the layer of the warmer hair to be moved away from the surface so heat transfer is better for UFH in heating mode.

“hair” But more seriously, yes, that's why those sources (ISO and BRE) quote higher resistances for downwards heat movement than upwards (or upwards coolth movement rather than downwards).

And for those pedants out there - yes I know the actual way to think of the radiation transfer is that the room is radiating it's heat to the floor (or all surfaces radiate and absorb - its the balance that counts)

This pedant is entirely happy to think about moving coolth around but cringes to see “it's” and “its” swapped like that.

Posted By: Ed DaviesThis pedant is entirely happy to think about moving coolth around but cringes to see “it's” and “its” swapped like that.

The possessive "its" regularly catches me out - well spotted - it stays unedited as a lesson to me that I simply can't learn during the flow of typing. It did make me laugh given the "pedant" nature of the paragraph. That'll learn me.

Actually I was commenting on Ed's comment on Paul's observation and saying that I think thermal admittance may be more relevant than thermal resistance to the question at hand. And that question pretty much sums up the description of thermal admittance, I think. "Thermal admittance (Y) is a measure a material's ability to absorb heat from, and release it to, a space over time."

There's a pretty good description of the admittance method at https://www.cibsejournal.com/cpd/modules/2013-01/ that includes a link to a spreadsheet that allows the calculation of heat flows into a concrete slab including the effect of the air boundary, among more complicated scenarios.

Heat transfer is much slower than electrical currents. There are no real world materials that are pure 'resistances', nor any that are pure 'inductors'. Almost everything acts as a combination of the two when considering the analogies. It isn't possible to make a sensible model of heat transfer using just the resistance values, when the flows are not steady state for extended periods of time, especially for multi-layer structures.

I have an apartment that ideally needs some kind of active cooling, but with no possibility of installing external heat pumps / chillers I've been reviewing the research on traditional cooling techniques, particularly those linked to the cooling effect of evaporating water.

Evaporating 1 litre of water uses 2264.705 kJ of energy, or 0.629 kWh, which is a useful amount of cooling, and one which commercial aircon units sometimes use by spraying a water mist into the incoming or extract air. I haven't yet found anyone doing this for the domestic MVHR market, and legionella may be an issue, but it might be worth pursuing further.

Provided you're not in an area of high humidity, options such as indoor - or courtyard - fountains or green walls could also be worth considering. Maybe even a piezoelectric fogging system.

I hope to investigate the options further at some point (when I get time), but it would be interesting to hear from anyone who might already have considered these areas.

Posted By: Mike1: “Evaporating 1 litre of water uses 2264.705 kJ of energy…”

…at 100°C; more like 2450 kJ/kg around room temperature [¹].

As djh says, when you want cooling in the UK the humidity is likely to already be high. As well as adding to the discomfort this will also discourage evaporation of your water.

Looking at this psychrometric chart [²] imagine air at 30°C and 80% RH (follow the green 30°C line up to where it intersects the red 80% line) then look what happens if it's cooled till it's at 100% RH (move horizontally left from there to the 100% RH line) then read off the temperature by dropping down to the horizontal axis and we see that, at most, we can get down to about 26°C, i.e., 4°C of cooling.

In reality you won't get that much cooling because you're increasing the absolute humidity of the air as well as cooling it (so you move up and to the left on the chart) so you'll reach saturation at some higher temperature, around 27°C.

Evaporative coolers work well in hot dry climates (e.g, US south west), in wet places like the UK less so.

[¹] https://en.wikipedia.org/wiki/Latent_heat#Specific_latent_heat_for_condensation_of_water_in_clouds

[²] https://upload.wikimedia.org/wikipedia/commons/9/9d/PsychrometricChart.SeaLevel.SI.svg

Posted By: Ed DaviesLooking at this psychrometric chart [²] imagine air at 30°C and 80% RH

There's no chance you'd ever have that temperature and humidity combination in the UK. None. At 30C, if you're at 65% RH it feels like 40C. Our hottest days in Montreal were around 36C with 40% RH - this gives a humidex of 44C which feels pretty unbearable.

See https://memory.psych.mun.ca/tech/js/humidex/

Highest ever dewpoint I saw here in Montreal was around 24C - so high that windows were steaming up on the outside.

See http://bmcnoldy.rsmas.miami.edu/Humidity.html for a dewpoint and RH calculator.

Paul in Montreal.

Posted By: Ed DaviesAverage RH at Wick airport so far this month has been 89.7%. Temperatures have been a lot lower than 30°C, of course; highest was 21°C when the RH was 75%.

Averages aren't much use - RH max occurs at T min and vice-versa (approx). e.g. Wattisham today:

03:00 95.1% 19.9°C
19:00 32.9% 23.6°C

or on the 17th
03:00 99.3% 14.6°C
14:00 66.9% 21.9°C

but as you say, it's the interior conditions that are important here.