Combining AC and MVHR

[moldy]

Has anyone combined some AC units with a MVHR? I am wondering if I place a small AC unit in one part of the house would the MVHR "distribute" the coolth (assuming it is on summer bypass).

[jack]

MVHR isn't very good at moving around bulk heat. Cooling one room will have some impact, but isn't as effective as cooling a central air where the "coolth" will be distributed more widely as a result of air currents.

 

You don't want summer bypass on when you're trying to cool the house on a hot day. The effect of summer bypass is to take the heat exchanger out of the MVHR circuit. If you have an air conditioner running inside the house, you'll just be ejecting any cool air you generate.

 

Summer bypass is only useful when the air temperature outside is closer to your desired internal temperature than the current internal temperature. Think cool night following a hot day, when the inside of the house is 24 deg and it's 16 deg outside - in that situation, you want to encourage the (cool) outside air to replace the (warm) inside air. This is achieved by turning on summer bypass, which stops the incoming (cool) air being heated by the outgoing (warm) air via the heat exchanger.

 

During the day, however, if it's 28 deg outside and 21 deg inside (where due to aircon or insulation), you definitely don't want summer bypass on.

 

 

[SteamyTea]

Increasing the diameter of the MVHR ducting can greatly increase the ACH figures.  That may be enough to have some useful contribution from the AC or heating system.

 

Pi x r²

 

It is the ² bit that makes a huge difference.

[Conor]

If you have an ASHP, can be really effective at cooling the floor slab. Few here have done it and seems really effective. Alternative is an AC unit in a large space in the house (e.g. top of stairwell) and allow convection to distribute the cooling.

[jack]

19 minutes ago, SteamyTea said:

Increasing the diameter of the MVHR ducting can greatly increase the ACH figures.  That may be enough to have some useful contribution from the AC or heating system.

 

I think this is how US central heating often works - large bore ducts distributing lots of hot air.

 

One issue is that if you're using your MVHR at high rates, the efficiency drops dramatically (not to mention the increased energy consumed by fans to move that much air - not sure how much the reduced resistance of large ducts helps).

[edited to add:] I can't find it, but I'm sure we've dicussed a system where you have large-bore ducting internally, with a central connection to a smaller-bore MVHR system. The large-bore system distributes heated/cooled air, and small proportion of it is constantly exchanged by the MVHR component. Sounds like a recipe for complexity and extreme air drying, but it could work.

 

You also need to think about duct insulation and condensation if you're cooling using internal ducts.

 

7 minutes ago, Conor said:

If you have an ASHP, can be really effective at cooling the floor slab. Few here have done it and seems really effective. Alternative is an AC unit in a large space in the house (e.g. top of stairwell) and allow convection to distribute the cooling.

 

I cool our downstairs (polished concrete) floors like this and it's remarkably effective. It also doesn't dry the air like AC can. Upstairs isn't cooled though, and I do wish we'd planned for a way to keep the bedrooms, in particular, a bit cooler. 

 

I think if I were doing this again, I have tiled bedroom floors that are cooled in summer and covered with rugs in winter - not sure I'd bother with much or maybe any warmth in winter. Best of both worlds.