I was referring to fresh air from a HRV/ERV. Combustion air should be coming into a sealed combustion gas-fired appliance.

I was referring to fresh air from a HRV/ERV. Combustion air should be coming into a sealed combustion gas-fired appliance.

I was referring to fresh air from a HRV/ERV. Combustion air should be coming into a sealed combustion gas-fired appliance.

I was referring to fresh air from a HRV/ERV. Combustion air should be coming into a sealed combustion gas-fired appliance.

Posted By jonr on 06 Aug 2015 06:13 PM

Fresh air is pumped from the HRV into the air ducts on the exit side of the hot-air furnace, so the fresh air goes into the living space.

I'd be interested in knowing what pressure your HRV produces and what pressure your furnace produces (when on). I expect they interfere with each other.

You might calculate the amount of additional negative pressure you are creating in the crawlspace and positive pressure above it and consider the effect on ACH.

The total annual energy used for heating is about 27,000,000 Btu, while the input rate for the hot-air furnace is 60,000 Btu, so at full power, the annual furnace usage is 450 hours, or 5% of the time averaged over a year. Thus, any pressure imbalance caused by the HRV and the furnace running simultaneously is insignificant averaged over the year. All of the other homes in this neighborhood use an exhaust fan in the conditioned crawl spaces, so there is a pressure imbalance 100% of the time.

The ducting is sized for an 80,000 Btu/hr furnace, while the modulating furnace that I have runs at a relatively low output much of the time that it does operate, so pressure drops through the ducting should be small. Further, the ducting from the HRV that enters the furnace ducting makes a turn and is pointed coaxially downstream, so the HRV flow operates like a air ejector, and should not cause any pressure problems in the ducting.

I think providing a separate ducting system for the HRV would tilt the economics toward leaving out the HRV entirely, and operating with a continuously running exhaust fan.

at full power..5%...while the modulating furnace that I have...

So the HRV and induced building imbalances and reduced/stopped ventilation occur for something much more than 5% of the year and an even larger portion of the heating season?

If the HRV output pressure isn't higher than the furnace duct pressure (unlikely during max furnace output or maybe if the HRV is on low) and there is no backdraft damper, then hot air gets diverted outside (pointing downstream doesn't matter). I'd be concerned if this occurred any percentage of the time, even if it was only when wind lowered pressure near the HRV intake.

If dedicated HRV supply and return ducts are too expensive, perhaps people should use fewer/shorter ducts and leave interior doors open. No balance problems this way.

Posted By jonr on 09 Aug 2015 08:24 PM
So the HRV and induced building imbalances and reduced/stopped ventilation occur for something much more than 5% of the year and an even larger portion of the heating season?

If the HRV output pressure isn't higher than the furnace duct pressure (unlikely during max furnace output or maybe if the HRV is on low) and there is no backdraft damper, then hot air gets diverted outside (pointing downstream doesn't matter). I'd be concerned if this occurred any percentage of the time, even if it was only when wind lowered pressure near the HRV intake.

If dedicated HRV supply and return ducts are too expensive, perhaps people should use fewer/shorter ducts and leave interior doors open. No balance problems this way.

The Venmar HRV that I have shows the fresh air inlet temperature during operation. That temperature reading verifies that no hot air is flowing from the furnace back through the HRV. The Venmar HRV uses ECM blower motors that have the characteristic that they draw whatever current is necessary to maintain a constant speed, independent of the pressure head. Most modern, high-efficiency HRVs uses ECM motors.

You seem a bit overly sensitive about pressure imbalances in a house. Any time that you run a bathroom or kitchen exhaust fan, they create a pressure imbalance in the house. I prefer the pressure imbalance to the smells that would result if the exhaust fans were not used.

If the HRV exhaust and fresh air are installed properly in the return plenum of a full-time central air furnace, the HRV will not be effected by the operation of the furnace, regardless of speed or mode of operation. It is essentially a bypass and differential pressure is constant between the central and the auxiliary fans.

Most homes will run 90 to 130 CFM ERV/HRVs and interior doors will present little resistance to this modest air flow. The central fan eliminates this altogether if designed properly.

Posted By BadgerBoilerMN on 10 Aug 2015 09:46 AM
If the HRV exhaust and fresh air are installed properly in the return plenum of a full-time central air furnace, the HRV will not be effected by the operation of the furnace, regardless of speed or mode of operation. It is essentially a bypass and differential pressure is constant between the central and the auxiliary fans.

Most homes will run 90 to 130 CFM ERV/HRVs and interior doors will present little resistance to this modest air flow. The central fan eliminates this altogether if designed properly.

One obvious disadvantage of ducting the HRV into the supply side of the furnace is that the HRV blower must work against the added pressure drop of both the HVAC blower and the furnace heat exchanger when the furnace is not operating, which is the majority of the time. Because of these added pressure drops, the HRV flow would be more likely to flow "backwards" through the HVAC system, with fresh air flowing out the furnace return air ducts. This would not be catastrophic, but would not provide the desired air circulation.

When the HVAC system is operating, then the suction caused by the pressure drop in the return air system would cause an imbalance in the HRV flows. Again, this is not catastrophic, but the imbalance is not a lot different in magnitude (but not sign) from ducting the HRV outlet to the furnace outlet.

In a well insulated home with some solar heat gains, commercially available sized hot-air furnaces do not run that much of the time. (In my case, about 5% over the year for the smallest available, high-efficiency, modulating furnace.) Thus, it is a fallacy to design the HRV system assuming that the furnace is constantly running at full load.

The ECM blower on a modern furnace in a modern house will be on full-time at a low velocity. This mode of operation is considered best practice as it is the only way to facilitate the various "conditioning" required in a tight house e.g humidity control, filtration and naturally heating and cooling.

A properly sized, installed and balance HRV will not effect or be effected by the central plant, drawing from and injected a predetermined volume of air as required.

Simple by-pass fluid dynamics.

AFAICT, ACH natural figures for a house are based on a yearly and building average of ~.7 pascal. So create an imbalance of 1 more pascal and you have created ~2x the normal leakage. That's likely to be significant for either energy use or wall moisture reasons - yet ACH50 seems to be the last building pressure/flow measurement anyone is concerned with.

Fan induced building pressure is always avoidable*, although I agree that for small rooms for small percentages of the time, it's not worth fixing.

* - avoiding shared ducts, avoiding closed rooms as plenums, using push/pull fans, larger vents between rooms, always balanced HRVs (constant speed motors don't do this), etc

Here is some info regarding the effects of small building pressures. It suggests that some of the emphasis on air sealing and wall designs that can tolerate moisture movement via in/ex-filtration should be directed towards pressure management.

The Florida Solar Energy Center (FSEC) has found that building pressures as low as +1 pascal (Pa) relative to outside conditions are sufficient to prevent outside air infiltration problems. On the other hand, even a slightly depressurized building (-1 Pa relative to normal outside conditions) in hot, humid climates can develop devastating moisture and microbial growth problems when the building envelope traps this moisture.

Can you have a proper unconditioned crawl space by sufficiently insulating the building from the foundation? For example... spray foam in the floor joist bay would you get you a pretty good insulation value.  You could even coat the entire joist.  Not sure if it's practical, but just curious if this is a valid way of doing it?

If the crawl space is damp, where does the moisture go? Conditioning the crawl space is part of conditioning the house.

An unconditioned and vented crawl space is common and can work fine. I'm sure you can find lots of discussions of the trade-offs.