MVHR vent locations. Opinions...

[AE-NMidlands]

Surely if it was 100% efficient with an inside temp of 20℃ and outside temp of 0℃ then the air from the inlet would be 10℃?

I agree, assuming the supply volume flow rate is the same as the extract.

It depends on the heat exchanger...
https://en.wikipedia.org/wiki/Countercurrent_exchange is how things like robins send blood to their legs but don't get frozen solid by the returning chilled fluid.
However I guess most MVHRs will just be one heat exchange matrix and you would be lucky to recover 50%.

I have been mulling over MVHR and probably wouldn't bother with a comprehensive whole-house set-up. I might have a flow and return in our main lounge coming out of the 2 sides of the top of the chimney breast plus another pair in the main bedroom, i.e. the 2 rooms where we spend most time and exhale most moisture...
Then maybe one extract in the bathroom and a supply to the bedroom next to it, possibly another pair split between the 2 front bedrooms.
Not bothering with the kitchen as a) it would foul up the filters and heat exchanger b) it would be difficult to run the duct to it!
A

[Stinsy]

Surely if it was 100% efficient with an inside temp of 20℃ and outside temp of 0℃ then the air from the inlet would be 10℃?

I agree, assuming the supply volume flow rate is the same as the extract.

It depends on the heat exchanger...
https://en.wikipedia.org/wiki/Countercurrent_exchange is how things like robins send blood to their legs but don't get frozen solid by the returning chilled fluid.
However I guess most MVHRs will just be one heat exchange matrix and you would be lucky to recover 50%.

I have been mulling over MVHR and probably wouldn't bother with a comprehensive whole-house set-up. I might have a flow and return in our main lounge coming out of the 2 sides of the top of the chimney breast plus another pair in the main bedroom, i.e. the 2 rooms where we spend most time and exhale most moisture...
Then maybe one extract in the bathroom and a supply to the bedroom next to it, possibly another pair split between the 2 front bedrooms.
Not bothering with the kitchen as a) it would foul up the filters and heat exchanger b) it would be difficult to run the duct to it!
A

I too have been looking at it. I'd like a simple upstairs solution. One supply vent in the hallway and 2x extract vents in the bathroom and en suit. All the gubbins can go in the loft. However, buying such a system isn't as simple as I imagined!

[Countrypaul]

Thus is one example of an explanation of the HE efficiency and explains the different ways of measuring it: https://www.heatspaceandlight.com/cheap ... fficiency/

For reference ours is a Vallox DV145 (or Airflow DV145) but the older model than currently being sold - and yes it was bloody expensive even when i bought it

[Gareth J]

Thanks all. Yep, I'm on board. Supply it is.

Regarding efficiency, the theoretical 100% maximum is temp in = temp out - it's a counterflow exchanger. Reality is of course very much not 100% efficient.

I chose to do it as was redoing the bathroom so had to put in some ventilation, had cupboard space and I know the house is under ventilated in winter. Windows are cheap to run but they don't get managed effectively and we're pretty exposed so "trickle" vents or windows cracked are huge over ventilation when it's windy. That and I put an mvhr in a porch/utility and it's been drying all our washing very effectively without any moisture issues so have felt quite positive about the idea. All pipework is in the loft apart from where I snuck downstairs via bathroom and now, bedroom cupboards. Waited around and found a s/h but never fitted one for about half price of new on eBay. They are a bit dear.

This one has really freshened bedrooms. Cooler too though so I've really dialed back the flow rates to just keep the room to acceptable measured CO2 and humidity levels by the morning. Flow rates make a massive difference to the former.

[Sim_C]

Surely if it was 100% efficient with an inside temp of 20℃ and outside temp of 0℃ then the air from the inlet would be 10℃?

I agree, assuming the supply volume flow rate is the same as the extract.

It depends on the heat exchanger...
https://en.wikipedia.org/wiki/Countercurrent_exchange is how things like robins send blood to their legs but don't get frozen solid by the returning chilled fluid.
However I guess most MVHRs will just be one heat exchange matrix and you would be lucky to recover 50%.

I have been mulling over MVHR and probably wouldn't bother with a comprehensive whole-house set-up. I might have a flow and return in our main lounge coming out of the 2 sides of the top of the chimney breast plus another pair in the main bedroom, i.e. the 2 rooms where we spend most time and exhale most moisture...
Then maybe one extract in the bathroom and a supply to the bedroom next to it, possibly another pair split between the 2 front bedrooms.
Not bothering with the kitchen as a) it would foul up the filters and heat exchanger b) it would be difficult to run the duct to it!
A

Thanks for the pointer on countercurrent heat exchange. I was not aware of that feature.

As the heat exchanger is more effective than I thought, the MVHR discharged air to outside is cooled down to almost as low as the fresh air intake giving the bonus of the supplied air temperature in to the building is almost up to the extracted air temperature.
Are there any measurable effects due to the relative moisture content of the fresh air and extract air, or does that require a specific type of MVHR?

[Stig]

Are there any measurable effects due to the relative moisture content of the fresh air and extract air, or does that require a specific type of MVHR?

You mean extracting the latent heat from the moisture in the extracted air? I think any MVHR will do that to some extent as water will condense out as the air is cooled, whether the effect is measurable I have no idea! It does mean that internal humidity drops when it's colder outside but in the UK that's seldom a problem and often useful for drying washing indoors (in my limited experience). In somewhere like Canada with colder, drier winters it would likely make the indoor humidity too low for comfort without some sort of humidifier.

[AE-NMidlands]

AIUI all MVHR systems need a condensate drain as the moisture in the outgoing air will inevitably condense on the cold incoming end of the heat exchanger, and yes, this is latent heat recovered.
However the machine needs a decent power supply because (to prevent the condensate actually freezing up on the HE) they have an electric pre-heat element to keep it working! (So no running it off the lighting circuit in the loft...)
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[Countrypaul]

Ours has a condensate drain which rund for longer periods in winter and often not at all in summer. There are no heaters in ours (it was an option but I didn't bother with) we see no ice on the HE. I strongly believe that unless you are in much colder temperatures (probably below -20C) or have very low internal temperatures the heat capacity of the warmer moist air (including the heat of condensation of the water vapour) would avoid ice on the HE.

[AE-NMidlands]

Ours has a condensate drain which rund for longer periods in winter and often not at all in summer. There are no heaters in ours (it was an option but I didn't bother with) we see no ice on the HE. I strongly believe that unless you are in much colder temperatures (probably below -20C) or have very low internal temperatures the heat capacity of the warmer moist air (including the heat of condensation of the water vapour) would avoid ice on the HE.

Thanks, that's useful to know. I'll bear it in mind when progressing mine...
A