[zyzhu2000]

I am trying to improve the ventilation of an existing house by adding an ERV unit. I realize that the best thing to do would be to build separate ducts for the ERV. However, that is not feasible as the home is not built. Therefore, the next best thing is to integrate the ERV unit with the existing furnace in the basement.

Carrier has a two-duct ERV designed to hook into the return duct. Whenever the furnace runs, it will be turned on, too. One problem is that this model will frost in winter, as I am in Chicago.

Does anyone have any experience adding an ERV to the existing HVAC system (that can also work in cold weather)?

 

[rundawg]

I moved this thread to the "AOP Residential HVAC" forum where homeowners should ask their questions.

Please take note of this new location as the "Moved" icon in the other forum will only last a day.

 

[teddy bear]

With typical homes in your area, during winter conditions, you need very little fresh air ventilation because of wind and the extreme stack effect on the home. During mild calm winds of the mild seasons, you need a mechanical filtered fresh air change in 3-4 hours. One more complication is that during the mild seasons, the outdoor dew points are high enough that you need supplemental dehumidification during evenings and rainy days to maintain <50%RH especially in the basement. The moisture from the fresh air and occupants is 2-6 lbs per hour. depending on the size of the home and number of occupants. The ERV will reduce the size of the dehumidifier by 2 lbs. per hour. If you have not purchased the ERV yet, I would use a small whole house dehumidifier with a fresh air ventilation option for ventilation. The pay back is not very good because of the lack of need for fresh air during cold weather. You have committed to the ERV use it as it will decrease your operating cost moderately. A small whole house dehumidifier like the Santa Fe Ultra 70, Broan 70, or the Trane 70 with fresh air ventilation option. The ERV and dehumidifier are connected to the a/c to circulate the fresh, filtered, and conditioned air throughout the home.

We can discuss the actual connections of the equipment to the a/c after you decide on the equipment.

Keep us posted on what you want to do.

Regards Teddy Bear

 

[zyzhu2000]

Thanks, @teddy bear. This is very helpful. I've not thought about the dew point and the dehumidification. Let me check out the dehumidifier with the ventilation option.

My motivations for ventilation are:
1. The radon level in the basement is slightly higher than normal (~4 -5 pCi/L).
2. there is always this bad smell in the house after closing the windows for hours.
3. Some family members are allergic to pollutants in the air.

So we are looking at a combination of ventilation and good air filters on top of the HVAC system. Trying to use one stone to kill all the birds.

PS. I think there is a way to measure the stack effect by monitoring the radon level. Right now, with the inside temperature being almost the same as the outside temperature, there should be little stack effect. This is when I find that the radon level in the basement is about 4 pCi/L. If there is big stack effect in the winter, we would expect to see much lower radon levels. It would be interesting to monitor it through the winter.

 

[zyzhu2000]

I did some research on the dehumidifier. In Chicago, it is most likely not necessary. Our winter here is usually too dry; it doesn't feel very humid inside in the summer either, mostly because of the AC. So I think I still want to install the ERV.

 

[teddy bear]

I did some research on the dehumidifier. In Chicago, it is most likely not necessary. Our winter here is usually too dry; it doesn't feel very humid inside in the summer either, mostly because of the AC. So I think I still want to install the ERV.

Here is what really happens with simple fresh air ventilation
Introducing a fresh air change of 3-4 hours of ventilation(100 cfm) would reduce the radon by pressurizing the home, purge indoor pollutants including radon, and renew oxygen. Within a couple of hours, the home will become the same dew point as outside plus the moisture the occupants add. Radon level infiltration declines because of pressurization and air change. Wind and stack effect will increase natural infiltration/exfiltration, more drying or wetting depending on outside weather.

Let us start with your home in mid day in summer heat 85^F, 51% RH, 65F dew point. with the a/c maintaining 75^F, 50%RH, a 55^F dew point. assuming the a/c is setup ideally. Your a/c is removing 3 lbs. of moisture per ton per hour of run. Your a/c will maintain <50%RH when it runs more than 50% of the hour. Evenings and rainy days the a/c will not maintain 50%RH. You need 2-5 lbs. of dehumidification per hour to maintain <50%RH with normal chicago mild season dew points. You need 2-3 lbs per hour of humidification during outdoor 10-35^F winter with natural stack/wind natural infiltration in typical homes.

The ERV does the following.
You activate the ERV and a fresh air change occurs in 3-4 hour, roughly 100 cfm of fresh air. The ERV exhaust and intakes 100 cfm. Exhausting and intakes pass each other through the heat/moisture exchanger. The two air stream transfers a small portion of heat/moisture from damp to dry/warm to coo air stream.
The ERV will not control or reduce the indoor %RH but only reduce the overall effect. During the many conditions, full humidification and dehumidification is needed to maintain indoor conditions.

Also your home needs make-up air for the clothes drier, gas water heater, kitchen exhaust, and bath fans to operate. ERVs good for applications in extreme heat/cold, wet/dry, large air flow conditions.

A small whole house dehumidifier with the fresh air ventilation (Merv 13) is the best solution to this problem in most green grass climates.

Your choice. Monitor your homes indoor %RH and CO2 levels to watch the effect of weather on your home to test my statements. I have sold thousands of HRVs/ERVs through the energy crunch of 80s. They good in the right application. But you will need supplemental dehumidification to maintain 50%RH with a good ERV on your home.

Keep us posted on whatever you do. I share your concern about your indoor air quality.

Regards Teddy Bear

 

[zyzhu2000]

Hi, @teddy bear. Thanks so much for the explanation. I think I get your point that even if an ERV is installed, dehumidification will still be required in some seasons.

I had some people checking out the mechanical room, and it turns out that it is really hard to fit any additional equipment into it. Adding more ducts from the mechanical room to the outside is also challenging without major construction. I guess ERV and whole-house dehumidifier are both out.

But maybe there is still a way. My main idea is to run a bathroom vent constantly at a low speed. This will create some negative pressure in the house, causing fresh air to be sucked into it. I think this is what is called exhaust-only ventilation. To have some control over the air that replaces what goes out from the bathroom vent, I think I may install a small vent in the basement to push air inside through a filter. The humidity will have to be controlled by portable dehumidifiers.

What do you think of this solution?

Thanks

 

[teddy bear]

Hi, @teddy bear. Thanks so much for the explanation. I think I get your point that even if an ERV is installed, dehumidification will still be required in some seasons.

I had some people checking out the mechanical room, and it turns out that it is really hard to fit any additional equipment into it. Adding more ducts from the mechanical room to the outside is also challenging without major construction. I guess ERV and whole-house dehumidifier are both out.

But maybe there is still a way. My main idea is to run a bathroom vent constantly at a low speed. This will create some negative pressure in the house, causing fresh air to be sucked into it. I think this is what is called exhaust-only ventilation. To have some control over the air that replaces what goes out from the bathroom vent, I think I may install a small vent in the basement to push air inside through a filter. The humidity will have to be controlled by portable dehumidifiers.

What do you think of this solution?

Thanks

Certainly better than nothing. The Santa Fe/Broan have a stout portable 70 pint per day unit. More money but worth it.

Most bath fans move less than 100 cfm. So operating at normal speed may be ok.
For the fall/winter try the bath fan concept.

Keep us posted on what you do and how it works.

Regards Teddy Bear

 

[zyzhu2000]

I left the bathroom fan in the basement on for about 24 hours, and it turned out the radon level went up instead of down. My theories are that 1) the fan in the basement has created negative pressure that sucks randon out, 2) the fan is not powerful enough to exhaust sufficient amount of air, and 3) it may be better to exhaust air from an upstairs bathroom to take advantage of teh stack effect. I am doing more experiments.

 

[teddy bear]

Causing negative pressure sucks in more radon and fresh air. Positive pressure slows radon entering the home and ventilates the home. Results are lower levels of radon in the home.
No surprise to me.

Keep on trying, you get to where we are.

Keep us posted.

Regards Teddy Bear