ERV Settings

[imakesawdust]

It is a very long and complicated story but the HVAC contractor for my new home turned out to be less than helpful beyond installing the equipment and walking away. The system consists of a Bosch Geothermal Heat pump, a Honeywell ERV150 ERV, a Honeywell 9R90 wholehouse dehumidifier and a Honeywell Prestige Thermostat (two zone).

During the process I was told that the Prestige would control both the ERV and the Dehumidifier. This turned out to be wrong. After the install started I was told I'd have to pick one to be controlled by the Prestige the other manually. We ended up with the dehumidifier on the thermostat and the ERV on a timer. The house is very tight. The conditioned space is about 53000 cubic ft. Blower door test was .05 ACHN and .82 ACH50. The installer set the ERV to run 4 times a day for 15 minutes each time. Is there any way to tell if this is correct or if I need to change it. The installer says to wait and see how the air "feels" and adjust accordingly. Any suggestions.

Thanks in advance

[LLigetfa]

It looks like you have all the numbers you need to do the math but it really boils down to lifestyle, occupants, and materials that out-gas.

On my HRV I can select one of two low speeds for it to run at and then I have a timer to kick it to high speed. It runs on the lowest speed all the time and we set the timer based on activity and how the air "feels". High humidity is usually not an issue. Lack of humidity is and our humidistat controls our whole house humidifier.

I'm not clear on what metric you expect a control to measure and regulate. If it is humidity, then monitor the levels and provided the outdoor humidity is such that the ERV is the best candidate to deal with it, adjust the timer or simply run the ERV on demand as needed.

[Dana]

The tighter the house, the more likely you are to run into higher than optimal wintertime moisture levels in a house. In a stick-framed wood sheathed construction in PA climate it's important to keep the interior humidity down to 35% @ 70F (or about a 40F dew point) or lower to limit moisture accumulation in the sheathing leading to a spring-time mold condition. Going much below 30% RH @ 70F has comfort and health consequences though. for a non-smoking household running the ERV under dehumidistat control during the winter works.

In summer when the outdoor dew points rise above 60F you won't be able to purge moisture with the ERV so you'd have to switch it over to duty-cycling. But duty-cycling sufficient for purging winter is usually fine for year-round use, and you can let the dehumidifier handle the higher latent loads of summer. Keeping it below 60% RH @ 75F in summer does a pretty good job of keeping mold well bounded and is reasonbly comfortable, but if you have dust-mite allergies holding the line at 50% is better, since the drier air keeps them from reproducing.

If you want to do the wintertime humidity control via duty cycling buy yourself a few $10 AccuRite humidity monitors and track a few places in the house (not the kicthen or bathroom, where peaks will be much higher than the house in general.) Controlling it to ~40% RH is fine in early December, but come January it's better if you get the average to come in between 30-35% for the duration of the winter.

[imakesawdust]

The tighter the house, the more likely you are to run into higher than optimal wintertime moisture levels in a house. In a stick-framed wood sheathed construction in PA climate it's important to keep the interior humidity down to 35% @ 70F (or about a 40F dew point) or lower to limit moisture accumulation in the sheathing leading to a spring-time mold condition. Going much below 30% RH @ 70F has comfort and health consequences though. for a non-smoking household running the ERV under dehumidistat control during the winter works.

In summer when the outdoor dew points rise above 60F you won't be able to purge moisture with the ERV so you'd have to switch it over to duty-cycling. But duty-cycling sufficient for purging winter is usually fine for year-round use, and you can let the dehumidifier handle the higher latent loads of summer. Keeping it below 60% RH @ 75F in summer does a pretty good job of keeping mold well bounded and is reasonbly comfortable, but if you have dust-mite allergies holding the line at 50% is better, since the drier air keeps them from reproducing.

If you want to do the wintertime humidity control via duty cycling buy yourself a few $10 AccuRite humidity monitors and track a few places in the house (not the kicthen or bathroom, where peaks will be much higher than the house in general.) Controlling it to ~40% RH is fine in early December, but come January it's better if you get the average to come in between 30-35% for the duration of the winter.

Dana - We used spray foam insulation, so I'm sure the "dew point" will never occur in the sheathing. Only moisture there should come from exterior sources.

Jim

[Dana]

You've had your house pressure tested and verified that it leaks less than 500cfm @ 50 pascals, and you used only 2lb closed cell foam to guarantee that the vapor permeance between the paint and the susceptible sheathing is under one perm, right?

And I'm sure you caulked between the doubled up stud plates, between the bottom plate & the subfloor etc, to guarantee zero infiltration & exfiltration, which is why it tested so tight, eh?

Half-pound foam in a standard dimension timber framed stud bay is NOT protective against vapor diffusion through the wall from accumulating in the sheathing in winter, even at 35% interior RH, let alone with a 50% interior RH.

While useful for moisture control, spray polyurethane foam is no more a panacea protecting against all evils than poly sheeting is. But it's somewhat more forgiving than poly at thicknesses of ~2" or less.

"Dew point" is just a temperature- the temperature at which it moisture will begin to condense. During the winter the sheathing will be below the dew point of the interior conditioned space air, but the outdoor air will be at a much lower dew point. Limiting both exfiltration & vapor diffusion of the assembly from the interior, and enhance drying capacity toward the exterior with highly permeable housewraps & back-ventilated siding systems to be fully protected in your climate zone. Unless the assembly has a wintertime drying capacity toward the exterior well in excess of any vapor diffusion or air-leakage transported it can accumulate moisture in the sheathing.

Raising the dew point of the interior air raises the level that the wall stackup/system needs to be able to tolerate. Keeping the interior dew point closer to the lower end of the human health & comfort range (above 30%, but well under 40%) during the coldest weather does not come with much of an energy use cost, nor does it reduce comfort.