ERV and heatloss calcs

Eugene Silberstein 3

What program are you using for your calculations?

The reason I ask is this... For Manual J calculations, infiltration is defined as the natural introduction of outside air into the home. The air introduced to the home via the ERV does not fall into this category. The air introduced to the home via the ERV falls under vetilation and the air removed from the home via the ERV falls under exhasust.

Since the amount (volume) of air introduced to the home is the same as the amount (volume) of air removed from the home, the net is zero. This is so because the ERV is designed to neither increase nor decrease the pressure int he structure so the "CFM in" must equal the "CFM out". Since the CFM imb (imbalance) is zero, there will be no effect on the rate of infiltration and your air change rate will remain the same.

Hope this helps

Simply Rad_2

Heatloss calcs and ERVs

My next project is a very tight home and we are using an ERV for infiltration. Just wondering about how this can effect my heatloss. I normally use around .4 exchanges for infiltration, but what if we are using an ERV which is preheating the fresh air. Just wondering what to input for an infiltration number.
Thanks Jeffrey

Simply Rad_2

Wirsbo 6.0

Eugene
right now I am using Wirsbo 6.0. It does not have an option for different types of infiltration(natural VS mechaical). I understand your neutral pressure explaination but what about BTUs saved from preheating the fresh air supply. I am sure this will lessen the heatloss of the building.
Jeffrey

Constantin

Prof. Silberstein,

I agree that the infiltration number will stay the same, but presumably the OP is after the ΔBTU that the introduction of a ERV produces. Here is my hamfisted attempt, one which can be improved, undoubtedly.

As Geoff McDonnell noted earlier, the basic formula for calculating the BTU content of an air flow across a coil is:
CFM x ΔT_air x 1.08 = BTU/hr

I will simply add one more tidbit to this equation, which is the efficiency of the heat exchanger. Thus, the above becomes:

CFM x ΔT_air x 1.08 x (1-HX Efficiency) = BTU/hr

• We know CFM from the data that the mfgr provides for a given pressure. I'll assume 150CFM for our Lifebreath TRV unit when it's loafing at low speed. I imagine that the best way to test though is to measure the air speed and infer CFM from there.
  
• We also know ΔT... the outgoing air is at 70°F, while the incoming air is at -10°F. Ergo the ΔT is = 80°F.
  
• The Efficiency is given in percent and can also be looked up in the manufacturer literature. I'll assume 87%, considering that's what a DCS unit achieves at it's lowest speed setting.
  
• Multiplying the above, I get about 1700BTU/hr of heat loss on a design day by running the HRV at its lowest speed continuously.

Professor, does the above make sense?

Eugene Silberstein 3

Correct Info

Constantin, the information you provided is correct, but Jeffrey's original post was asking about an ACH (air changes per hour)factor, not the resulting affect on the heat gain/loss. He has software that is crunching the numbers for him. Jeffrey was asking about the ACH factor he should input into the software.

Keep smiling

Eugene Silberstein 3

Indeed it does...

Heating outside air before it enters the occupied space will definitely reduce the rate of heat loss.

I am not familiar with using the Wirsbo 6.0 software, but would assume that there are separate infiltration losses/gains that do not rely on the ACH factor. Typically the ACH factor just relates to the CFM infiltration, the volume of the structure and the hour to minute conversion of 60.

ACH = 60 divided by (volume of house/infiltration cfm)

You would need to check the specs on the ERV to determine the heat transfer rate for your specific outdoor heating design temperature. The rate of heat transfer between the air in the occupied space and the outside ambient air is related to the difference between the temperatures of these two locations. If the outside ambient temperature is very low, the rate of heat trasfer will be greater than when the outside air temperature is higher.

Constantin

You're right of course

The only reason I injected the BTU calculation is that the OP was trying to figure out the heat loss...

When I was using HVAC-Calc for our home, I noted the same issue, i.e. that the program didn't have any provisions for HRVs/ERVs. Hence my back-of-the-envelope calculation, which indicated that the impact of the HRV/ERV would be minimal, as I had hoped.

Coming back to the topic of ERVs, what number do you aim for re: ACH? ASHRAE calls for 0.35ACH or 15CFM/person, whichever is greater, other countries like Sweden call for 0.5ACH... which should one use?

ALH_4

Design load

I would not run the ERV at heat-load design conditions. That way it really doesn't figure into the equation for maximum heat loss.

-Andrew

jerry scharf_3

looking for a SWAG

Jeffery,

It looks like you're looking for s SWAG (scientific wild a$$ guess) of what to put in for ACH to correctly model the heat loss.

I regular old house will have .3 to .4 ACH of heat loss through infiltration. I tight house may get this down .2 and a really tight house may get this down to .1.

You need to add any time you are running exhausts, as you need to replace that air with cold air, one way or another. Let's say you have 30,000 cuft of air in the house (~3300 sqft with a couple high ceilings.) This means every 500 CFM is one ACH. So your 500 CFM range hood is doing 1 ACH when it's running, your bath exhaust around .25, etc. Most people ignore these completely because it's small, but the is a SWAG so we include all the factors.

Next you want to figure in your ERV. You decide on the normal operating CFM, multiple it by the loss (1-eff in Constantin's post.) This gets you the effective infiltration added. A 60% ERV running at 150CFM would produce 60 CFM effective and .12 ACH for our example house. We can round this up to .15 for a little exhaust load.

So for our super tight .1 ACH example house, the vaule is more like .25 ACH with the ERV and ventilation added. With the ventilation and no ERV, it's back to .4 ACH. With a 90% ERV, it drops to .15 ACH...

That would be my swag.

jerry

Bob Eh?_2

Ummmmmmmmm.......... I think there is a fundamental disconnect here. If the delta T is 80* and the unit is 100% efficient then the return air would STILL have a 40* delta T according to the laws of thermodynamics....

This means you have an effective 50% of the CFM to deal with (at 80* delta T) plus (1 - efficiency)* CFM

This would give you significantly different numbers than indicated in the discussion thus far....

I would like to think I am wrong and have long contemplated that the whole rating scheme for HRV/ERV devices is to be charitably described as misleading....

Comments welcome!

Bob

Constantin

Hmmm...

Perhaps the solution lies in how one thinks of the HX.

Instead of assuming that the HX in the HRV/ERV has one temperature that is the result of the two air flows mixing, I would expect a thermal gradient that mimics the mixing characteristics of the two air flows passing, but not touching, each other. The end of the HX cube that is hit first by the -10°F air will be colder than the other end that is closer to 70°F being warmed by the outgoing air.
Would it be possible for the incoming air to exit the HRV at a temperature above 40°F under those circumstances?

jerry scharf_3

I am certain

You are indeed wrong. I have an ERV that is performing at higher efficiency than you pose as possible. I don't even see how you come up with the 50% idea. It's a heat exchanger just like any shell and tube or flat plate. Are you also saying that those are only 50% max efficiency as well?

The in and out air streams pass some form of heat exchanger. If that HX is 100% efficient (both sensible and latent,) then all the heat would be pulled from the exhaust air to the incoming air. This is not possible, and you have to account for the fans as well. Even accounting for these, my unit runs at much higher than 50% energy recovery.

jerry

Mike T., Swampeast MO

My Simplistic Answer

1) Heat loss programs don't calculate for ERVs/HRVs because they have zero net effect on infiltration. The assumption is that you handle the calculations for such otherwise...

2) Without a confirmed infiltration value, e.g. blower door test in a finished structure, you are, at best, making guestimates. If the home is "very tight" your "typical" infiltration value is likely too high--thus the need for the ERV.

3) ERVs are generally recognized as providing 75% efficiency of heat transfer--with both sensible and latent heat.

4) If the ERV provides reheat to counter its' own loss, the net effect both in loss and infiltration is zero.

5) In a "perfect" situation, you know the infiltration value of the structure, the flow velocity of the ERV/HRV and its' efficiency of transfer. In this case you use your best heat loss calculation, known infiltration rate and then add the assumed heat loss of the ERV/HRV itself (easily calculable) to your structure heat loss.

6) In the "real" world you must make assumptions. For a "tight" (by your measure) structure, reduce your infiltration based on your experience--ignore the HRV/ERV. Then re-calculate by adding the actual air flow of the HRV/ERV unit to your infiltration (all decent heat loss programs allow an absolute infiltration input and you can find the value of your percentage infiltration input by looking closely). For an ERV, add 25% of the difference between "without ERV/HRV" and "with ERV/HVR" to your heat loss.

7) If the ERV/HRV provides reheat just forget that it exists!

8) In the "real-real" world... The differences you'll find here are likely overwhelmed by environmental and occupancy differences. To quote Dan the Man, "The desire to oversize exceeds the sex drive on most days." To bad that my desire to fight to save my comrades has destroyed my sex drive...

Eugene Silberstein 3

Awesome

Mike T. ...

You da man!!!

Very well stated...

I knew I liked you, and now I know why... (snicker)

Mike T., Swampeast MO

Am Both Flattered and Humbled, Sir

Whether a blessing or a curse I see heat and accept people.