We are getting ready to build a new house for a customer in North Central Florida. They have requested that we install the Mitsubishi Mini Split Air systems in 3 zones in this new house. We normally would use the traditional heat pump HVAC system with an attached ERV unit for the exchange of fresh air. Does anyone have any experience with the split air systems and whether it will still be necessary to have the ERV unit?

Posted By DavidBrownEnterprises on 20 Jul 2016 11:58 AM
We are getting ready to build a new house for a customer in North Central Florida. They have requested that we install the Mitsubishi Split Air systems in 3 zones in this new house. We normally would use the traditional heat pump HVAC system with an attached ERV unit for the exchange of fresh air. Does anyone have any experience with the split air systems and whether it will still be necessary to have the ERV unit?

I think you mean Mini split
For the past 2 years In our area they have been requiring fresh air, doesn't have to be an ERV, but does need to be introduced into the conditioned space with a motorized damper of some sort

My understanding of the two systems suggest that an ERV would still be advisable. The ERV's primary function is to introduce fresh air. It also will remove some of the latent heat of that fresh air, thus helping to reduce the load on the mini-splits. I think if it were a terribly leaky house the ERV may not be needed (no point, leaks are enough), but a relatively tight house should have a mechanism for bringing in fresh air.

The mini splits will do the heavy lifting on the latent heat/cooling chore but I do not believe are designed to satisfy fresh air needs.

Ventilation needs are independent of cooling or heating needs, and as a rule a completely separate ERV system with it's own ducts (tiny compared to cooling ducts) is a better ventilation solution than trying to tie it into a heat pump, heating or AC.system.

Mitsubishi has both ductless and ducted heat pump solutions (both mini-duct cassettes and variable speed not-so-mini air handlers), but none are really designed to be an integrated ventilation system as well. Ductless coils/cassettes have no means of accessing or distributing ventilation air, though one could (if they put a gun to your head) use mini-duct cassettes for distributing ventilation air, even though it's the wrong way to go about it.

The air leakiness of the house has no bearing on whether an ERV make sense or not, though the benefits of an ERV might be masked by the high "natural ventilation" (= air infiltration of unknown air quantity & quality) in a leaky house. In N-FL there is a benefit for using an ERV rather than a heat recovery only balanced ventilation system, especially if high ventilation rates are desired. ERVs do not provide latent cooling (none at all!), but they do exchange moisture between the incoming and outgoing air streams, in both directions. When the outdoor air is drier than the indoor air, the ventilation air entering the house will have more moisture than the outdoor air, since some of the moisture from the exhausting air stream is transfered to the incoming air stream. When the indoors is drier than the outdoors, some of the incoming moisture moves to the outgoing air stream. This transfer is only via vapor diffusion, there is no moisture removal being accomplished the ERV core, only a moisture balancing.

The moisture transfer efficiency of an ERV is typically in the 50% range, whereas the sensible temperature recovery efficiency can be north of 90%.

The advantage of a completely separate ventilation ducts is that they can be hooked to just a simple fan/filter, an ERV/HRV or a whole house dehumidifier. One strategy is to exhaust bathrooms and storage areas while supplying fresh to bedrooms. The problem with an ERV in this situation is that they could be transferring humidity from the bathrooms into the incoming air stream. An advantage of the inverter mini splits is they can run in a low mode that helps control humidity.