On a 2-story home with a HRV system, where should one mount the outside air intake location?

I was thinking up high on the 2nd story wall, as that would be the area with the freshest charge of air, instead of down low near the ground which would be more prone to having dust and stagnant air.

Although in the winter the air lower to the ground would be warmer than the air up higher at another 25 feet. The opposite should be true in the summer, with the cooler air down low and warmer air up high. Or am I over analyzing this?

You're over analyzing it.

You do want to watch for putting the intake or exhaust opening in a place that has localized effects from the shape of the building or the prevailing wind.

Once you account for the efficiency of the HRV, slight differences in the temperature of the outside air make very little difference. Pressure might - if you put the HRV up high and facing the prevailing wind, you should see a higher house pressure than a more neutral location.

Do duct runs make a significant difference? I know static pressure is a problem in ducted AC systems but does the same apply to HRV ducts?

Yes. Even though it is relatively low pressure and moves much less air than typical HVAC ducting, you want the runs to be as short as possible with as few corners as possible. I used adjustable dampers at the outlets so as to be able to make sure that too much air wasn't passing outlets with short runs at the expense of outlets with longer runs.

Do duct runs make a significant difference?

How much difference depends on the duct size and air flow rate. Ie, with large enough ducts, length makes little difference.

Ideal is to have supplies and returns with adjustable dampers on both - so you can set the right flow rate and maintain neutral (or whatever is desired) pressure balance in closed door rooms/areas.

Much of this is pretty basic stuff, but there are pictures and you might even learn something new:

HRV/ERV Best Practices

sailawayrb - could you explain how an hrv take out more heat from income air then it gives up to the outgoing? first picture. Assuming air flow is balanced.

FBBP, I don't understand what you are exactly referring to? Do you really mean heat (i.e., psychrometrics involving the changing temp, humidity) or do you mean the delta temp?

The article Best Practices second slide. Stale air is 72º, fresh air is 32º, difference of 40º.
Somehow when it passes through the exchanger the fresh air gains 29º and the stale air losses 30º.

One would think that under natural flow (no compression and balanced flow) and 100% efficient exchanger, the stale stream would give one degree to the fresh stream till they hit the same temp so the best it could do is 40/2=20 degrees of exchange so the fresh in and stale out should be 52º.

How does the exchanger appear to be creating heat?

Let's ignore the issues of the fan motor adding heat and that a counter flow heat exchanger isn't limited to 1/2 of the temperature differential. For the HRV in slide 2, that leaves the difference in air density due to temperature - stale air going out is less dense (fewer btus/degree) than cool air coming in. Conservation of energy doesn't imply conservation of temperature. Use "mass flow" and it will come out right.

As you indicated FBBP, neither a HRV or an ERV can actually generate heat. So the best they can do (i.e., at 100% efficiency) is transfer the same amount of heat from one airstream to the other airstream. To understand heat transfer, you need to understand enthalpy. Enthalpy is a measure of the total energy of a thermodynamic system and includes both the internal energy (e.g., the energy absorbed or released when water changes from a liquid to solid ice or gaseous water vapor at a constant temperature) and the displacement energy (e.g., the change in air volume that would result with a given change in pressure or temperature).

HRVs can only transfer sensible heat. Sensible heat is the heat released or absorbed via solely a change in temperature (e.g., the way we typically calculate heat loss in buildings using the indoor/outdoor delta temperature and R-value of associated building envelope assembly). So sensible heat transfer and HRVs would only involve the transfer of temperature (i.e., only the temperature of one airstream is transferred to the other airstream). So the best HRVs can do is transfer the same delta temperature from one airstream to the other airstream.

ERVs are unique in that they can transfer BOTH sensible heat and latent heat. Latent heat is the heat released or absorbed via a constant temperature process (e.g., the phase transition of water from a liquid to solid ice or gaseous water vapor). So latent heat transfer and ERVs involve the transfer of both temperature and moisture (i.e., both the temperature and the humidity of one airstream is transferred to the other airstream). So the best ERVs can do is transfer the same enthalpy from one airstream to the other airstream. However, in this case, the delta temperature transferred from one airstream to the other airstream could be vastly different because the humidity of each airstream is also being changed.

So the slide you referred to would not be correct or accurate for a HRV. However, I really can’t say whether it is correct or accurate for an ERV. We would need both the dry bulb temperatures and the wet bulb temperatures (or the relative humidity’s) of the airstreams to ascertain whether the enthalpy that is being transferred is within an expected and reasonable ERV efficiency range. We do have free DIY psychrometrics software on our website should you want play around with the numbers to assess the enthalpy changes:

Borst Psychrometrics Software

Thanks sailawayrb. Every time HRV/ERV come up there seems to be the thought that they magically produce heat. some of it comes from that slide!

If in this case the highest possible return could be 52º and as we know plastic cores are not near 100% effective exchangers, the actual return temps at 32º outside would be closer to 48º. This puts us below the requirement of tempering outside air to 12ºc or 54ºf (ABC 9.32.3.5) therefore they must flow through some tempering device. (Even at 54 most people will feel the draft if not located away from areas normally occupied.) Obviously this is not as big a deal in warmer climes but many areas do get to freezing. Those areas that have much lower design temps would need to give this much more thought.

While I'm sure an erv will liberate some heat from water vapours, we are not talking very high air volumes so not much vapour to deal with. Would it effect the return temps by more than 5% under normal operation?? Maybe a bit more when showers are exhausted but then the units normally go into defrost till the main humidity is exhausted.

Bear - sorry to hijack your thread. btw the ABC list 16 items with regard to intake and exhaust opening locations. Most common sense items like above the anticipated snow pack, away from contamination sites like chimneys, dryer vents, gas relief valves etc. Consider that if you perch the inlet high on a wall it will be effected by what happens to the wall below it. If say its a stucco wall with the sun beating on it the temps will be much higher near the top or trapped under a soffit then lower down. If it is located by a nice pine tree you will have automatic air freshener, next to the sewer vent, not so much. Make sure you are not near where cars idle and things like that.

I would probably try for about ten feet off the ground if all other things are equal but take time to review the location based on your neighbourhood.

Posted By FBBP on 09 Oct 2013 02:06 PM

Bear - sorry to hijack your thread. btw the ABC list 16 items with regard to intake and exhaust opening locations. Most common sense items like above the anticipated snow pack, away from contamination sites like chimneys, dryer vents, gas relief valves etc. Consider that if you perch the inlet high on a wall it will be effected by what happens to the wall below it. If say its a stucco wall with the sun beating on it the temps will be much higher near the top or trapped under a soffit then lower down. If it is located by a nice pine tree you will have automatic air freshener, next to the sewer vent, not so much. Make sure you are not near where cars idle and things like that.

I would probably try for about ten feet off the ground if all other things are equal but take time to review the location based on your neighbourhood.

No harm no foul. There is always something to learn when the topics morph into other topics.

Nearest neighbor is a 1/4 mile away so no issues there. I was thinking putting it on the north wall since the home will face due south and that is the approach side of the home. No chimneys or gas relief valves on the home. The north side will be the "cool" side in summer but of course the "cold" side in winter.

The HRV will not run 24/7 but will be set on a timer/need to run basis.

I guess it will depend also on whether I stick the HRV in the 1st floor mechanical room or 2nd floor conditioned attic. Which brings me to my next question: Since the HRV intake duct goes through the wall and draws in cold winter air/hot summer air, is the duct insulated to prevent thermal loss into the surrounding air?

Not only to prevent thermal loss but also to prevent condensation on the pipe.

Posted By FBBP on 08 Oct 2013 10:39 PM
The article Best Practices second slide. Stale air is 72º, fresh air is 32º, difference of 40º.
Somehow when it passes through the exchanger the fresh air gains 29º and the stale air losses 30º.

One would think that under natural flow (no compression and balanced flow) and 100% efficient exchanger, the stale stream would give one degree to the fresh stream till they hit the same temp so the best it could do is 40/2=20 degrees of exchange so the fresh in and stale out should be 52º.

How does the exchanger appear to be creating heat?

What you have stated would be true for a parallel-flow heat exchanger; a 100% efficient heat exchanger should have equal temperatures coming out for what were hot and cold inlet streams. On the other hand, with a counter-flow heat exchanger, a 100% efficient heat exchanger would cause the inlet temp on each side equal to the outlet temperature of the other side. A cross-flow heat exchanger appears to perform between these two extremes.

If you refer to page 1047 of http://www.rshanthini.com/tmp/PM3125/RM_ChapterOnHeatExchangersWithProblems.PDF, there is a section titled "Multipass and Cross-Flow Heat Exchangers: Use of a Correction Factor." It treats the deltaT for the cross-flow relative to the opposed-flow heat exchanger with a correction factor F, deltaT_cross-flow = F * deltaT_counter-flow.

On page 1048, figure 23-18 there are diagrams for various heat exchangers including a single-pass cross-flow with both fluids unmixed. I think this corresponds to the heat exchanger in my HRV. With a fresh air inlet of -4.5 C (23.9 F), and a stale air inlet of 17 C (62.5 F), the fresh air outlet is 10.2 C (50.3 F), and the stale air outlet is 2.8 C (37 F), so the temperatures "cross." Using these values, I compute P=0.66, R=1.035, and according to the diagram, that gives a correction factor F of 0.76. Thus, for a perfect heat exchanger, the cross-flow could swap 76% of the thermal energy between the streams. The actual temperature transfer efficiency was calculated to be 68%.

The values above came from measurements described in Table A-2 at http://www.residentialenergylaboratory.com/rel_energy_use_hrv.html. This was for a high flow rate condition with condensation unlikely. The temperature transfer efficiency was higher at the low flow condition shown in Table A-1, 72%, but some condensation might have occured at this condition.

So it appears from the measurements and the literature, that cross-flow heat exchangers can do "better" than parallel flow heat exchangers, and can transfer enough heat to cause the temperature curves to "cross."

Lee - thanks for posting that. I would not have expected that cross flow would have made that much difference on a short travel in a plastic core.

Never the less, we would still have to temper the incoming air.

On A-1 Room temp is 66 but entering temp is only 51.7. Similair on fresh air side. What is happening to the missing ºs?

Something doesn't seam right with the stale delta.

51.7 is a typo, it's 61.7. But they must have run the ducts somewhere cold to get the drop from the room temp (66.7) down to 61.7 at the HRV stale air inlet.

You both are absolutely correct. The temperatures were measured and all calculations were performed in metric units, so degrees Centigrade. The stale air inlet temperature was measured at 16.5 degC, which should have been converted to 61.7 degF. The deltaT on the stale air side was 16.5 C - (-3.0 C) = 19.5 C which is about 35.1 F. The deltaT was correct in degF, since it was converted from the degC results.

The difference between the room air temperature of 66.7 F and the stale air inlet to the HRV of 61.7 F was due to the fact that the HRV pulls air out of the conditioned crawl space that is cooler than the living space. Heat is only added into the living space, so the crawl space is cooler due to heat losses (including to the fresh air inlet as discussed below), and the outside temperature was 3.8 F at that time.

The temperature of the fresh air inlet to the HRV was 12.4 F, much warmer than the outside temperature of 3.8 F. There is a moderately long run of insulted ducting from the outside to the HRV, about 25', and at low flow HRV conditions there is significant heat exchange with the air in the conditioned crawl space. This long duct run was to place the fresh air inlet on the south side of the house in this heating-only climate. While the sun is shining, this southern placement is effective in getting fresh air inlet temperatures to the HRV significantly higher than the outdoor temperatures in the shade.

Concerning tempering of the fresh air inlet, you indeed do not want to sit right in this fresh air inlet stream which was 48 F at this condition. Once it mixes with the room air, it is OK. If an exhaust fan were used rather than an HRV, the air being pulled into the living space through doors and windows would be 4 F rather than 48 F, and that would be a more severe comfort issue.

Regarding the tempered fresh air outlet duct into the home. Would it be better to have this duct in a large open area of a home, preferably high at ceiling height?