MVHR and cooling

[flanagaj]

I appreciate that MVHR does not provide active cooling, but can anyone tell me whether it is adequate given the current warm weather.  We are renting a Persimmon 3 storey property and last night, the bedroom was 29c.  It's a timber framed house and as you climb each staircase, the temperature rises.  I really want to avoid building a house that becomes a giant heatsink.  With the revised insulation measures and glazing, I can see how it happens.

[jack]

MVHR doesn't have anything like the airflow needed to provide serious cooling.

 

Design in air conditioning (completely separate from the MVHR), and throw on some PV so you can run it for free on hot days.

 

MVHR will help keep the heat outside.

[Nick Laslett]

5 hours ago, flanagaj said:

I appreciate that MVHR does not provide active cooling, but can anyone tell me whether it is adequate given the current warm weather.  We are renting a Persimmon 3 storey property and last night, the bedroom was 29c.  It's a timber framed house and as you climb each staircase, the temperature rises.  I really want to avoid building a house that becomes a giant heatsink.  With the revised insulation measures and glazing, I can see how it happens.

Fancoils, A/C or A2A heat pump. 
 

My build isn’t finished, so MVHR not commissioned. But I did install the ComfoPost fed from UFH circuit to provide 2°C to 3°C degrees of cooling. Can’t wait to get it working. This is probably the worst way to address this issue. All the supply ducts need 13mm insulation, and the plenums, manifold etc. The MVHR moves so little air, that this cooling will not be noticeable. But if I hadn’t done this, I would be kicking myself later on. It would not be going to far to suggest that few on BuildHub would endorse this approach, you are just throwing away money. 
 

In any case, right now with just the UFH the house is still at 21.5°C, with it being 32°C outside today. 
 

My strategy for cooling our house. 
 

1. ASHP for UFH cooling

2. Insulated foundation with UFH

3. Insulated poured concrete 1st floor with UFH. (part of ICF build)   

4. Solar control glass on south facing windows

5. 50cm Roof Over hangs on upstairs south facing windows. (Not nearly enough overhang, possibly pointless?)
6. Comfopost for MVHR

 

Other things to consider:

1. Properly designed window shading

2. External Shutters

3. Fancoils

4. Smart glass

 

 

 

 

[Indy]

7 hours ago, jack said:

MVHR doesn't have anything like the airflow needed to provide serious cooling.

 

Design in air conditioning (completely separate from the MVHR), and throw on some PV so you can run it for free on hot days.

 

MVHR will help keep the heat outside.

 

After a lot of research into UFH cooling with ASHP and Fan coils replacing radiators, this is the approach I'm going with. 

 

UFH with standard MVHR. Separate multi-split Aircon system to cool down the key rooms (bedrooms and living area). 

 

Heating things keep house nice, warm and relatively airtght.

Cooling system blows nice cold air when I want it. 

Solar PV will mean you can run the air con on extremely hot days for very low costs. 

[Nick Laslett]

Good thread about the ComfoPost

 

 

[KineticBuser]

Can understand that piping the Air Con into the MVHR ceiling ducts is a bad idea, however never been able to understand why you cant intecept the MVHR exhaust pipe and connect into there somehow?

Surely at that point its post the MVHR system and therefore you are just using the ducting to vent to the outside?

[ProDave]

By far the simplest way to manage heat, is during the day keep all windows shut and curtains closed on sunny side of house.  Then at night throw all the windows open to cool the house from the cooler night time air.

 

Trying to tell someone NOT to open a window when "it's hot" just does not seem to sink in.

[JohnMo]

I spoke to a contractor last year, he had been south to hot world called England. Anyway driving down the motorway all windows open in his car, Aircon full cold, couldn't understand why the car was so hot inside. 30+ outside all windows open, 70mph blast of hot air!

[Bancroft]

On 01/07/2025 at 15:32, Indy said:

UFH with standard MVHR. Separate multi-split Aircon system to cool down the key rooms (bedrooms and living area). 

After going around this circle many times over the last year this is how I'm planning on solving the issue.

[JohnMo]

1 hour ago, KineticBuser said:

never been able to understand why you cant intecept the MVHR exhaust pipe and connect into there somehow

To do what? 

 

Read the post or pre cooling data sheets and related water temps, air flow rates and then kW output in cooling mode. Yes you can cool, but not very well at normal flue rates, even if you configure to run boost it's still a low kW output. Really doesn't make much difference if you install anywhere in the MVHR circuit. If you install a MVHR that was massive, and min flow rates suited your house design, set up a normal boost and super boost you may have something, but ducts would be oversized, every inside the house fully insulated.

[KineticBuser]

21 minutes ago, JohnMo said:

To do what? 

 

Read the post or pre cooling data sheets and related water temps, air flow rates and then kW output in cooling mode. Yes you can cool, but not very well at normal flue rates, even if you configure to run boost it's still a low kW output. Really doesn't make much difference if you install anywhere in the MVHR circuit. If you install a MVHR that was massive, and min flow rates suited your house design, set up a normal boost and super boost you may have something, but ducts would be oversized, every inside the house fully insulated.

Sorry that didnt make sense - I meant fit a New Aircon Unit and attach its exhaust pipe to the existing MVHR ducting (but into the exhaust outlet pipework so its doesnt impact existing MVHR)

[JohnMo]

1 minute ago, KineticBuser said:

Sorry that didnt make sense - I meant fit a New Aircon Unit and attach its exhaust pipe to the existing MVHR ducting (but into the exhaust outlet pipework so its doesnt impact existing MVHR)

Now lost? What is an Aircon exhaust pipe?

[Nickfromwales]

16 minutes ago, JohnMo said:

Now lost? What is an Aircon exhaust pipe?

Some units are ducted to atmosphere, like the one in my boys attic bedroom.

 

19 minutes ago, KineticBuser said:

Sorry that didnt make sense - I meant fit a New Aircon Unit and attach its exhaust pipe to the existing MVHR ducting (but into the exhaust outlet pipework so its doesnt impact existing MVHR)

Don’t do this. 
 

It’ll be a much higher force of airflow and will push backwards into the unit and unbalance all your MVHR. 

[KineticBuser]

1 minute ago, Nickfromwales said:

Some units are ducted to atmosphere, like the one in my boys attic bedroom.

 

Don’t do this. 
 

It’ll be a much higher force of airflow and will push backwards into the unit and unbalance all your MVHR. 

 
Ok kinda makes sense, AirCon exhaust is too powerful for the ducting and therefore the air will be forced backwards into the unit - Thanks for explanation 

 

[jack]

On 01/07/2025 at 13:34, Nick Laslett said:

3. Insulated poured concrete 1st floor with UFH. (part of ICF build)   

 

Very interesting. You're the first person I can recall who's done this. I've said before that if were doing another build, we'd have some sort of screed on the first floor, with tiles on top. In summer, the floors would be cooled as needed. For winter, they'd be warmed just enough to keep the rooms at a comfortable sleeping temperature, with temporary rugs around beds and along halls.

 

On 01/07/2025 at 13:34, Nick Laslett said:

5. 50cm Roof Over hangs on upstairs south facing windows. (Not nearly enough overhang, possibly pointless?)

 

Depends on how tall the windows are. We have 400 mm overhangs on all south-facing windows. Some of the windows are very wide but only maybe 500 mm tall, so they're completely shaded when the sun is high and to the south in mid-summer. This is very effective. The south-facing sliding doors don't get much benefit from the same overhang.

 

On 01/07/2025 at 13:34, Nick Laslett said:

2. External Shutters

 

I can give a concrete example of how important external shading can be. Our bedroom is at the east end of the house. For cost reasons, we idiotically decided not to include external blinds on the large east-facing window/door that looks out onto our balcony. The balcony isn't very deep, but it's covered, so there's a 1000+ mm overhang over the east-facing window/door. One of the other bedrooms is west-facing. It only has a 600 mm overhang. It has a big window too, but has external venetians that are only ever partly open during summer. There's a permanent temperature difference between these two rooms throughout summer. In very hot periods, the difference can be several degrees. The difference in comfort between 22 and 27 degrees is marked, especially when you're trying to sleep! 

 

On 01/07/2025 at 13:34, Nick Laslett said:

3. Fancoils

I wish we'd provided for these, as they're not really practical to retrofit! If there's doubt about whether they'll be needed/wanted, it wouldn't cost too much to add the required (insulated) runs at first fix, but only install the fancoil units if needed. I think you also need to consider condensation drainage, depending on the temperature you plan to run them at.

 

On 01/07/2025 at 13:34, Nick Laslett said:

4. Smart glass

 

I've heard this can be effective, but it seems like an expensive and complicated option with a high risk of failure over a long enough period of time. External shutters or blinds feel like they'd be as effective.

 

More generally, I'd also consider careful selection of plants. For example, a deciduous tree in the right place can provide a lot of useful shade in the summer. Same with deciduous vines on a trellis or pergola.

[SteamyTea]

Not read, in detail, all the above.

 

There is nothing to stop you designing a forced air heating and cooling system, and then adding heat recovery to it.

It is done already, but it is expensive.

 

But why do you need active cooling, you can design temperature extremes out at the beginning.

It will mainly mean you will not have huge windows, but will probably make the design cheaper to build.

 

I am about as far south as one can get in the UK (not quite the Lizard Peninsula, but close). Only only in 20 years have I had to temporarily mitigate high OAT. 

If it really does get too hot to sleep upstairs, sleep downstairs for a few nights. It is a lot cheaper. Slightly more expensive option is to book into a Travelodge, but check they have Aircon.