MVHR and summer bypass in flats

[Rachel Mitchell]

I'm looking at the specification of MVHR units for small appartments – about 50- 60 m2 and the client is keen to have summerbypass to help with summer overheating. I've been speaking to Itho and their view is that the benefits of summer bypass are minimal for smaller volumes of air as the amount of heat taken out is very small – about 250 watts if there is a temperature difference of 10C

does anyone else have a view on this?

[Nick Grant]

Rachel

The maths is about right in terms of night cooling and 10°C is optimistic.

You will need secure window night vent for summer to deal with cooling.

I'm hoping Alan Clarke or Andrew Farr will chip in.

Nick

[Alan Clarke]

I'd agree with that – also summer bypass is a bit of a nuisance in terms of preventing it working by accident if the heating is turned up in winter, and it makes the MVHR too big to fit easily in a 50m2 flat.

And if you thought the MVHR would solve summer overheating you have to go back anyway and reduce the amount of heat getting in and improve facility for night ventilation. If through this you can keep the flats a good bit cooler than daytime peak temperatures you may want to run with MVHR heat recovery enabled anyway, to stop bringing hot air in.

[Rachel Mitchell]

thank you Alan and Nick for you comments- do you think that summer bypass should be specified in houses where is there more air volume and I assume more heat loss?

On another but similar subject, I have a scheme which has a planning restriction on opening windows- so overheating is a big problem, do you have any suggestions.
We can use passive stack, but this is not getting the air changes in SAP or we could over size the MVHR and use this, which works in theory, but I'm worried it will be noisy and drafty in practice, or we could use a centralised extract system but then lose the heat

Are there any other solutions?

[Nick Grant]

Was good to have a bit of a chat with you at at the conference.

As Alan says the solution can't be bolted on. If vent is really limited then need to really drive the gains down although some of those may be from adjacent flats. If this is standard Building Regs build then good luck!!

Ticking the SAP box is one thing but this needs to work.

Nick

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