Pros and cons of HRV vs Fresh Air Ventilation in a dry heating/cooling climate.

[garya505]

I'm considering a HRV or Fresh Air Ventilation to improve my indoor air quality. The house is recently-built and fairly tight. Our summer design temp is 95 and winter is 16 (as I recall). Outdoor humidity is very low most of the time, with only short periods of maybe 60% on a few summer days. I keep the house at about 30% in the winter with a Aprilaire bypass humidifier. Summer usually runs about 30-50% with the AC running. I have dual-fuel (gas aux) with balance point set to 40F due to the current low price of natural gas here.

I'm looking for opinions on these two options. The Fresh Air Ventilation would use something like the Aprilaire 8126 or equivalent (6" duct with damper). Of course the HRV is going to have a higher up-front cost, and the Fresh Air Ventilation would cost a little more over time due to the outside air being brought in. The air here is normally very dry, so dew point really isn't an issue.

I would have this controlled by my VisionPro IAQ thermostat, either using ASHRAE or timed mode. My HVAC guy is very flexible and is willing to put in whatever I decide.

Thoughts and opinions?

[Chuck]

How about another option

Have you considered an ERV? This would recover not only most of the heat but also some of the moisture in the winter that it is exhausting out of your house. An ERV would put less of a load on your humidifier than the HRV or fresh air options. ERVs typically have fewer moving parts to potentially fail than HRVs.

In Albuquerque your winters are mild enough (I think) that the extra installed cost of HRV/ERV option may not pay off. You would have to calculate the amount of heat you would recover in a years time taking into account the total cfm of air exchanged and the efficiency of the HRV/ERV.

[garya505]

Yes, HRV or ERV would be options. If I was in some other area of the country I probably wouldn't even consider simple Fresh Air Ventilation, but I think it could work well here. I have a friend who does energy surveys (LEED stuff) and he said I should consider the HRV.

[garya505]

I was just wondering about the fresh air ventilation. Without a HRV or ERV, I would have mixed air from outside air that may be a lot cooler than inside in the winter. I run my blower (VS ECM) on low all the time. Would this cool the air enough to notice, coming out of the supply vents when the furnace isn't running?

[teddy bear]

A home needs an air change in 4-5 hours when occupied to purge indoor pollutants and renew oxygen. When the wind blows and its cold outside, expect an increase in natural infiltration. When the wind is calm and temps outside are warmer, supplemental fres air ventilation is usually required.
Therefore very little fresh air is needed during cold weather. Investigating the rise of the inside air dew point verses the outside dew point will indicate the amount of natural fresh entering the home. Currently your outdoor dew point is 24 ^F. What is you inside temp/%RH/dew point? If your current indoor dew point is 43^F and you have 2 occupants, you are getting 70 cfm of fresh air moving through your home. This may be enough fresh air infiltration to indicate no need for more fresh air during this type of weather.
Two occpants add 1 lb. of moisture per hour.
Give us your inside air conditions to help determine the amount of fresh air naturally entering your home.
Regards TB

[garya505]

Indoor coditions:
Indoor: 72F, 30%, 2 adults and 2 children
We heat to 72F and cool to 76F.
When cooling we generally have 30-50% RH indoors.

[teddy bear]

t

Indoor coditions:
Indoor: 72F, 30%, 2 adults and 2 children
We heat to 72F and cool to 76F.
When cooling we generally have 30-50% RH indoors.

Thanks for the interest.
72^F, 30%RH, = 39^F dew point
Outdoor for your area is 23^F currently. The air passing through your home enters at 23^F and leaves at 39^F. Your family typically adds an average of .5 lbs of moisture per hour respiration, presperiation, and activities. If your doors were wide open with high air flow, the air entering and leaving would be the same. If your home was a plastic bag with no air moving through it, expect 100%RH or saturation.
Assuming you were typical and adding 1.5 lbs. of moisture per hour, how much air moving through your home? Using the I phone app "Ultra-Aire" estimates that you have 110 cfm of fresh air passing through your home. The infiltration varies with wind and stack effect. Also clothes drier, bath fans, fireplace, and kitchen hood will change the air flow through your home. During calm weather with moderate temps, expect much less infiltration.
110 cfm is adequate fresh air for 3,666 sqft of home at .2 ach. I would size a make-up air duct fan for the fresh air you want, connect it to your return duct. Operating the make-up air when you are in the home and the winds are calm/moderate temps. An ERV/HRV will not have pay back because you do not need additional fresh air during cold weather. In fact you may benefit eliminating from additional house tightening during very cold, windy conditions.
Also you may want to add a humidifier if you indoor %RH is <30%RH for extended periods.
Regards TB

[garya505]

Teddy Bear, thanks for all the info.

We have a humidifier that manages to hold 30% RH indoors.

I understand what you are saying about infiltration and air moving through the house. A simple ASHRAE calculator says I need about 48 CFM (continuous), so if I'm getting 110 already during all but moderate conditions, it seems like ventilating during cold, hot, or windy condtions is a waste of energy.

Why couldn't the need for ventilation be based on CO2 in the home? Then, under conditions were you have enough "natural" ventilation, it would be OFF, and it would open the vent under moderate condtions when the CO2 goes up. Somebody must have thought of this before.

[teddy bear]

Teddy Bear, thanks for all the info.

We have a humidifier that manages to hold 30% RH indoors.

I understand what you are saying about infiltration and air moving through the house. A simple ASHRAE calculator says I need about 48 CFM (continuous), so if I'm getting 110 already during all but moderate conditions, it seems like ventilating during cold, hot, or windy condtions is a waste of energy.

Why couldn't the need for ventilation be based on CO2 in the home? Then, under conditions were you have enough "natural" ventilation, it would be OFF, and it would open the vent under moderate condtions when the CO2 goes up. Somebody must have thought of this before.

I am impressed! You are absolutely right. A CO2 control is ideal to activate ventilation when the home is occupied and natural ventilation is short. I have been making that argument for a couple years. The biggest problem is that there is not much of a martket for that kind of a device for retail.
Eventually, enough will understand the need for an ideal control to provide fresh air when needed.
Keep us posted on how you provide fresh air during the calm weather.
Regards TB

[garya505]

I am impressed! You are absolutely right. A CO2 control is ideal to activate ventilation when the home is occupied and natural ventilation is short. I have been making that argument for a couple years. The biggest problem is that there is not much of a martket for that kind of a device for retail.
Eventually, enough will understand the need for an ideal control to provide fresh air when needed.
Keep us posted on how you provide fresh air during the calm weather.
Regards TB

I think maybe one of the problems is that the controllers are not cheap. I see the Honeywell C7232A1008 that would do such a thing, but requires a 4-hour unoccupied period for "Automatic Background Calibration". I would imagine that this works well in commercial applications where the building is vacant every night, but for a residence maybe not.

[Chuck]

I think maybe one of the problems is that the controllers are not cheap. I see the Honeywell C7232A1008 that would do such a thing, but requires a 4-hour unoccupied period for "Automatic Background Calibration". I would imagine that this works well in commercial applications where the building is vacant every night, but for a residence maybe not.

Most commercial co2 sensors require that 4 hour period, it is needed to correct for sensor drift. It could be during the day too. Is there occasionally a time when your house is empty with everyone at work? It doesn't have to be every day.

[garya505]

Most commercial co2 sensors require that 4 hour period, it is needed to correct for sensor drift. It could be during the day too. Is there occasionally a time when your house is empty with everyone at work? It doesn't have to be every day.

I was thinking about that. There are times when the house is empty for 4 hours, but maybe only once or twice a week.
According to the manual for the honeywell C7232A sensor, it only needs one unoccupied 4-hour period per week.
"Sensor must be mounted in a location which sees at lease one 4-hour unoccupied period per week so that
the CO2 level drops to outdoor levels. Automatic Background Calibration will not work properly in locations
without four hours of unoccupied time per week, or where there are sources of CO2 other than people
(breweries, mushroom farms, etc)"

So, I think it would work in our house, but I guess it would rule out having a home brewery.

Using CO2-based ventilation seems like such an obvious improvement over a timed or ASHRAE control algoritm, I wonder why I can't find more information about people actually using a system like this. The controllers aren't cheap, but they're not hideously expensive either.

[BadgerBoiler MN]

We specify a lot of foam and ERV is the answer. I personally use one in my renovated 1921 farm house and it is needed (wind blowing or no). We also specify and install ERVs as bathroom exhaust fans with master timer and individual 20 minutes push timers...RenewAire most of the time.

I lived in Albuquerque for three years and now reside in Minneapolis. However you are "conditioning" the air, you certainly don't want to waste what you paid to condition.

[garya505]

We specify a lot of foam and ERV is the answer. I personally use one in my renovated 1921 farm house and it is needed (wind blowing or no). We also specify and install ERVs as bathroom exhaust fans with master timer and individual 20 minutes push timers...RenewAire most of the time.

I lived in Albuquerque for three years and now reside in Minneapolis. However you are "conditioning" the air, you certainly don't want to waste what you paid to condition.

Seems like if you don't want to under-ventilate or over-ventilate, you either need to know your exact infiltration, or you would need to use some other parameter (like C02) to decide how much extra ventilation to provide. And wouldn't using a CO2 sensor work with a HRV, ERV, as well as a simple fresh air intake? Yes, I know the ERV would be the most efficient, I would think but over-ventilation would be very inefficient too.


.

[BadgerBoiler MN]

Maybe if you live a space bubble. You are over-thinking this. In most cases for residential applications, simply using an ERV for bath and perhaps supplemental kitchen exhaust is sufficient. The rate of ventilation (after an ERV has been properly specified using long accepted standards) is a matter of user controlled personal preference. Why wife's nose is better than any CO2 sensor I have ever encountered.

[teddy bear]

Seems like if you don't want to under-ventilate or over-ventilate, you either need to know your exact infiltration, or you would need to use some other parameter (like C02) to decide how much extra ventilation to provide. And wouldn't using a CO2 sensor work with a HRV, ERV, as well as a simple fresh air intake? Yes, I know the ERV would be the most efficient, I would think but over-ventilation would be very inefficient too.


.

Here some data on a WI foam insutlated home. The data was from a windy week in WI. This week the home was operated with 200 cfm of exhaust ventilation. The ventilation was operated 23% of the time. This is part of series of test with various ventilation types.
Homes like this get much more natural fresh air during the cold windy weather and very little during calm weather.
You are about right the ideal method of control of fresh air ventilation. Also you see what we are up against in trying to move the industry along to more effective methods of handling the mechanics of the home.
"You are over-thinking this. In most cases for residential applications, simply using an ERV for bath and perhaps supplemental kitchen exhaust is sufficient. The rate of ventilation (after an ERV has been properly specified using long accepted standards)"
Also exhaust devices need make-up air to function as designed. In many homes, a make-up fresh air ventilation is more practical. Controlling the operation any ventilation with CO2 controller is ideal to determine that the home is occupied and the natural ventilation needs to be supplemented.
Operating ventilation when it is not needed cost energy and is uncomfortable winter or summer. Most in the trades avoid ventilation during the moderate seasons because of the a/cs inability to maintain <50%RH during low/no cooling loads.
In green grass climates, a whole house dehumidifier is importat. In cold dry climates, a humidifier is also important for comfort.
Thanks for the interest in new concepts.
Regards TB

[BadgerBoiler MN]

You are about right the ideal method of control of fresh air ventilation. Also you see what we are up against in trying to move the industry along to more effective methods of handling the mechanics of the home.
"You are over-thinking this. In most cases for residential applications, simply using an ERV for bath and perhaps supplemental kitchen exhaust is sufficient. The rate of ventilation (after an ERV has been properly specified using long accepted standards)"
Also exhaust devices need make-up air to function as designed. In many homes, a make-up fresh air ventilation is more practical. Controlling the operation any ventilation with CO2 controller is ideal to determine that the home is occupied and the natural ventilation needs to be supplemented.
Operating ventilation when it is not needed cost energy and is uncomfortable winter or summer. Most in the trades avoid ventilation during the moderate seasons because of the a/cs inability to maintain <50%RH during low/no cooling loads.
In green grass climates, a whole house dehumidifier is importat. In cold dry climates, a humidifier is also important for comfort.
Thanks for the interest in new concepts.
Regards TB

We are "up against" the cost of relevant mechanical systems and their practical value. I live and work in a "green grass climate" and one in which severe cold is common. In my own home, here in Minneapolis, the humidity and fresh air are both balanced with the relatively new technology of the ERV. Getting the "trades off the relatively primitive HRV would be the first step in conserving energy while still maintaining IAQ. We might also consider the cost of operation of the typical ERV and weigh that against the theoretical ideal. But if a CO2 driven ventilation system is what they want and can afford, we are all about design/build.

[BadgerBoiler MN]

I forgot to thank you for the chart. What I see in it is a predictable daily spike, followed by a certain ventilator run time resulting in a 22% operating cycle. I suppose it is just coincidence that my own ERV is set to 20% operation and would wager the CO2 spikes apparent in your chart are not occurring in my own envelope. With the addition of push button terminals in the bathrooms (not often used while showering in winter) our rH is also nearly always between 40 & 50 with the use of a mini-split and basement dehumidifier in summer. CO2 control? Not a bad idea perhaps, but certainly not perfect and cost/benefit is still in question.

PS. Since we use superior space heating provided by radiant floor panels, humidity is easier to manage and set-back unnecessary as infiltration is reduced and ambient temperature lowered with greater comfort and lower fuel consumption than forced air systems. We setback the design water temperature at night and use weather responsive reset to control the water temperature delivered to the radiant floors.

I tend to over-think ways to get people to invest in any reasonable mechanical system instead of "investing" in fancy roof lines, landscaping and floor coverings.

[garya505]

TB, nice chart! I have a question though. In my reading it looks like outside air is normally about 400PPM CO2, and some commercial DCV CO2 controllers bring in outside air when indoor CO2 gets to 800PPM. However, on your chart it looks like your controller is opening the OA vent at about 575PPM, and you never get over 650PPM inside.

ETA: Ah I see, now that I've looked at the DEH 3000R specs. This is a dehumidifer/ventilator controller that does not use a CO2 sensor. So, the lowering of the CO2 is just due to dehumidification using some outside air ventilation? So can inside home in a home ever get to 800PPM, even without ventilation?

[teddy bear]

TB, nice chart! I have a question though. In my reading it looks like outside air is normally about 400PPM CO2, and some commercial DCV CO2 controllers bring in outside air when indoor CO2 gets to 800PPM. However, on your chart it looks like your controller is opening the OA vent at about 575PPM, and you never get over 650PPM inside.

ETA: Ah I see, now that I've looked at the DEH 3000R specs. This is a dehumidifer/ventilator controller that does not use a CO2 sensor. So, the lowering of the CO2 is just due to dehumidification using some outside air ventilation? So can inside home in a home ever get to 800PPM, even without ventilation?

this is a custom designed Ultra-Aire CO2 controller. In this case we are looking for an air change in the home in 5 hours when the home is occpuied. That is why the controller activates the ventilation much soooner. The outside CO2 is approx 475 ppm. This allows agressive ventilation when needed and no ventilation when the home unoccupied and/or is windy. It appears to be very effective at minimizing the running hours of properly sized ventilator. Most are using on/off timers which cycle when not needed. The summer cycle is much different with more ventilation required.
Regards TB