Another passive house with crazy tight .1 ACH50

[AC_Hacker]

Originally Posted by stevehull

HRVs and ERVs do nothing to remove humidity in a location like a New England summer. The only way to get the humidity out is to use a low capacity heat pump (AC).

How do you account for the water that drips (sometimes freezes) from an HRV?

When I said, "...and does it's own passive job to lower humidity."

... I was referring to this. Perhaps, "...lower humidity of incoming air" would suit you.

-AC

[stevehull]

Originally Posted by AC_Hacker

How do you account for the water that drips (sometimes freezes) from an HRV?

When I said, "...and does it's own passive job to lower humidity."

... I was referring to this. Perhaps, "...lower humidity of incoming air" would suit you.

-AC

First, an HRV or ERV will not drip condensation in the summer. The difference in latent humidity levels in the crossing airstreams are just not large enough.

In the winter, you certainly CAN have condensation and the infamous "freeze-up" if the unit does not have frost protection.

But I believe we were talking about a New England summer and humidity removal in that season.

HRVs and ERVs do differ in the way they handle moisture. Here is an excellent blog from the GBA (green building advisor to read, Some of the responses are rather astonishing (like advocating only exhaust air to ventilate homes).

http://www.greenbuildingadvisor.com/...ngs/hrv-or-erv


Steve

[AC_Hacker]

Originally Posted by stevehull

First, an HRV or ERV will not drip condensation in the summer. The difference in latent humidity levels in the crossing airstreams are just not large enough.

OK Steve, here is a psychrometric chart, used by designers in the HVAC industry.

Can you show me, using this psychrometric chart why an HRV will never drip condensation in the summer?


-AC

[stevehull]

AC,

Rather than go into enthalpy and such, simply think of ice water on a hot day. It condenses (like in winter). Room temp ice water does not sweat the glass (summer conditions.

Yes, it is that simple.

[AC_Hacker]

Originally Posted by stevehull

...Rather than go into enthalpy and such... Yes, it is that simple.

Steve,

Regarding your 'glass of ice water' thought experiment, simplicity is appealing, for sure, but I much prefer science.

I need to confess that I already have a nugget of knowledge that was the basis of my hunch that you are wrong. That nugget is knowing that there is a significant problem with certain air conditioned houses in Florida, in that hot humid air infiltrating through the walls can hit dew point inside the walls of houses, dropping their water load into the insulation, causing rot and mold growth inside the walls. This is not an imaginary scenario, it can and does happen with houses that are improperly designed.

Then I cheated and phoned around to several HRV manufacturers in the country, and got a very revealing array of answers.

An HRV maker in Sarasota Florida, told me that, "yes there will be condensation in the HRV", which I already knew I would hear.

An HRV maker in California told me, "yes it could".

An ERV maker in Wisconsin area told me, "no it wouldn't, but it would help humidify the house". Well now... that's a comfort, for sure.

He did describe to me how you calculate the incoming air dry bulb temp when you know the out side temp and the inside temp and the efficiency.

Temp_hrv_in = Tout - ((Tout - Tin) * efficiency)

So in a situation where outside was maybe 95 F (dry bulb) and the conditioned air inside was 70 F(DB), delta-T would be 25.

Therefore:

• If efficiency was 30%, the incoming air temp would be 87.5 F
• If efficiency was 40%, the incoming air temp would be 85 F
• If efficiency was 50%, the incoming air temp would be 82.5 F
• If efficiency was 60%, the incoming air temp would be 80 F
• If efficiency was 70%, the incoming air temp would be 77.5 F
• If efficiency was 80%, the incoming air temp would be 75 F
• If efficiency was 90%, the incoming air temp would be 72.5 F

So I went back to the psychrometric chart to illustrate some examples:

In the first example (30% efficient HRV), the outside temperature is 96 F (red line) and the humidity is 70%. The psychrometric chart tells us that there are 26 grains of water per pound of air.

When we cool the air to 82 F (blue line), it still has 26 grains of water, but it isn't cooled to the point that the water hits the dew point. So there is no condensation.

In the second example (70% efficient HRV), the outside temperature is 96 F (red line) and the humidity is 70%. The psychrometric chart tells us that there are 26 grains of water per pound of air.

When we cool the air to 78 (blueline), it still has 26 grains of water, and it is cooled past the point that the water hits the dew point, and condensation will occur in the HRV.

In the third example(90% efficient HRV), the outside temperature is 96 F (red line) and the humidity is 70%. The psychrometric chart tells us that there are 26 grains of water per pound of air.

When we cool the air (blueline), it still has 26 grains of water, but it is cooled to 72 F, past the point that the water hits the dew point, and condensation will occur in the HRV to an even greater extent.


Recalling your statement, "...an ... ERV will not drip condensation in the summer." As you have said before, "it just doesn't cut it."


Best,

-AC

[MN Renovator]

I'm hoping nobody has a dew point of 72 degrees inside a house, especially if it has a basement. I'd have a mold garden in mine if the dew point was above 55 degrees. When I pull back the insulation covering the concrete foundation and measure the temperature it is consistently just above 55 degrees throughout the summer. The wood framing isn't much different in temperature to this and then there is drywall in contact with that framing. I keep the dew point below 55 degrees to prevent mold and there are articles online that discuss dew point in relation to mold and finished basements.

An HRV needs to reach the dew point to produce condensation. In my case, the condensation would be coming from the outdoor air which in the summer almost always has a higher dew point the outside touching the cooler surfaces towards the indoor portion of the exchanger. We have a few days where the dew point hits mid 70's and if the outdoor air coming into the exchanger gets cooler than that there is going to be some condensation in the exchanger but most of the time it should be dry.

In the winter the outgoing indoor air will have a higher dew point and would be condensing on the way out fairly heavily, especially in a climate like mine.

No matter how you cut it, you'll still be adding moisture to your indoor air if the dew point outside is higher than the dew point inside with an HRV. There is no way around the physics. An ERV cuts that down a little bit but the best course of action is to reduce the air exchange as much as possible while still exchanging enough to have healthy and comfortable indoor air quality.

In Minnesota, it is common for HVAC pros to duct outside air into the return air during the summer, putting a positive pressure on the house, reducing the outdoor air coming through from the outside and dehumidifying that outdoor air on the way in. This backfires for people who want to leave the furnace blower on all the time, especially in the winter because then they are sucking unconditioned air into the house constantly. It's also a wasteful process and only exchanging air when the air conditioning is running. IMO this is a horrible practice in the winter because it brings in super cold air and forces indoor high dew point air to exfiltrate through the walls.

Of course with .1 ACH50 you aren't going to be doing much of anything except for putting a high positive pressure on your house by opening the outside to your HVAC return.

If you have no cooling demand for any length of time, as is common with good overhangs and generous insulation there will be a need for dehumidification. With little infiltration, most of the dehumidification needs will source from outdoor air exchange through ventilation, improper/wasteful cooking methods involving boiling water, cooking with combustion, and showers. I like to cook with natural gas but the combustion products and added moisture to the air become an added concern when things get tightened up, especially when you want to minimize ventilation to keep indoor moisture levels low. This is less of a problem with general cooking on the stove and becomes a larger issue when trying to bake a lazagne, cake, brownies, or a roast and the oven is running for an extended period of time.

In Minnesota and Wisconsin, every Passivehaus I've visited has skipped the natural gas connection entirely and they have been using either Fujitsu or Mitsubishi 9k mini-split heat pumps for the small amount of supplemental heat and cooling that is required on the worst days. ..except for one house where the owner added wall mount electric resistance heaters to every room after the fact when she found out that even with a Passivhaus she still needed supplementary heat and the builder of the Passivhaus told her not to use the Fujitsu mini-splits below 0f which was about the only time she actually needed to add heat. This was actually the Minnesota Habitat for Humanity Passivehaus too. :shudder: