Running ERV Unbalanced
My house is reasonably airtight (at least for one built in 1960): 1.8 ACH50, 300 CFM50.
I was curious how many Pascals my house was under normal conditions so I used a manometer to measure about -0.6 Pa. So it’s slightly depressurized. I had no fans running for the test, it wasn’t windy, but it was about 25F cooler outside so I assume this is mostly due to the stack effect.
I plugged this into Red Calc’s depressurization tool (https://basc.pnnl.gov/redcalc/tool/depressurization-analysis) and it estimated that having -0.6 Pa exerted on the house is equivalent to 15 CFM.
If I take my 300 CFM50 and do a very rough ACHnatural conversion (300/20), I get 15 CFM again. Not bad.
I also have a Broan AI Series ERV running at 50 CFM supply and 50 CFM exhaust.
Would it not make sense to change the ERV to 50 CFM supply and 35 CFM exhaust? This would eliminate any infiltration, theoretically (I think). My ERV would be less efficient… but I’m pretty sure the house overall would be more efficient since I currently have 15 CFM of natural infiltration which is coming in at zero percent efficient. Also worth noting that the bathrooms are not connected to the ERV. They have their own exhaust fans.
Is all this making sense or have I missed something?
Perhaps one reason to not do this is because my Pa reading is constantly changing. Still, I’d assume that providing slightly more supply CFMs to the house than exhaust would be a net benefit for the majority of the year.
Thoughts?
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I don't see any benefit to what you're proposing. When an ERV is unbalanced, it doesn't do energy recovery.
And 0.6 Pa is so miniscule, it's hard to believe that instruments can measure it. It's literally the difference in pressure from an altitude change of 2 inches.
The ERV would still be recovering energy, just at reduced rate. If the exhaust rate was 0 CFM and the supply was 50 CFM, then yes - the recovery would be 0%. But we are still exhausting and supplying air, so recovery is still happening.
The recovery rate curve probably isn't a linear relationship , but for simplicity, assuming it is... 35 CFM of exhaust and 50 CFM of supply would be a recovery efficiency of 70% of the ERV's normal recovery efficiency.
Also 0.6 Pa can definitely be measured. I'm using a DG700 which has an accuracy of +/- 0.15 Pa. The manometer responds as you would expect to very small changes (up'ing a few CFMs, cracking windows ever so slightly, etc.)
I think you are missing his points.
Point 1: your calculation of your erv working at its 70% of its rated efficiency is basically saying that the 15 cfm offset is working at 0 percent efficiency while the other 35 is working at 100 percent. This is the same as having the make up air coming from somewhere else. You can’t recover energy from nothing.
Point 2: your equipment might be fancy enough to measure it, but it’s essentially pointless as if you walk up a couple of stairs you will also see the value change, or at least you should as the air pressure decreases with altitude. Dc point is not that the testing equipment is not up for the task, but that the test is inherently flawed by other variables.
Right. In the original post he said, "I’m pretty sure the house overall would be more efficient since I currently have 15 CFM of natural infiltration which is coming in at zero percent efficient. " As you point out, unbalancing the ERV doesn't make that more efficient, at best you still have 15 CFM that is zero percent efficient. At worst the unbalanced ERV does nothing to counter the natural infiltration and you've doubled the amount of unrecovered ventilation.
"Depressurized" seems to be the latest buzzword, we seem to be getting a lot of people lately who are convinced that their houses are depressurized and this is a major problem that needs fixing. I wonder where it's coming from.
If you measure at ground level and at your eaves, you see a bigger difference than 0.6Pa due to stack effect in cold weather. A slight negative actually protects your wall assemblies in cold weather from indoor humidity getting pushed into the wall and condensing/freezing on cold surfaces.
On a two story building you'd get about 59 Pa difference between ground level and the eaves simply due to the difference in altitude (assuming 16 feet). So 100 times greater.
I'm trying to put the observed 0.6 Pa in context to show just how tiny it is. It literally is equivalent to a 2" difference in altitude. If you're measuring spot inside the building is 2" higher than the spot outside the building that's what you'd see. In the time it takes you to walk from inside to outside the weather could change that much.
Meanwhile, back at the ranch ...: I suspect typical ERV's stated ~70% efficiencies are due to cost-constrained limitations on installed heat transfer surface, i.e. with a much higher transfer area and counter flow, efficiencies would approach 100%. Extrapolating, if your exhaust side air flow decreases relative to intake at constant heat transfer area ... not an expert here but I would expect % recovery of energy from exhaust gas will go up. My point: I wouldn't think your postulated loss of heat recovery is as simple as strictly proportional to flow.
A while back I calculated the efficiency of an unbalanced ERV, don't have exact numbers handy but efficiency very quickly dropped bellow 50%. You definitely want the unit balanced.
As for the 15CFM. I would not worry about it unless you have smoke or allergy season. In that case, unbalance the unit to pressurize the house slightly, efficiency be dammed, but only when needed. This is what I do during ragweed season, have a high tech calibrated cardboard piece I jam into the exhaust port just for this purpose.
Thank you all for the responses! It sounds like this doesn't make sense to do aside from Akos' comment.
I did want to clarify something that is being missed (I think...maybe I'm still not getting it though). I'm hearing that this doesn't make sense because you are simply trading 15CFM of infiltration for 15 CFM of unbalanced ERV. But I think the situation is more nuanced than that. Here's an example:
Assume a deltaT of 25F and an ERV efficiency of 70%.
Option 1 - ERV balanced 50/50 supply/exhaust - resulting in 15 CFM of infiltration.
- Infiltration = 1.08 x CFM x deltaT = 1.08 x 15 x 25 = 405 Btu/h
- ERV ventilation = 1.08 x CFM x deltaT x (1- ERVefficiency) = 1.08 x 50 x 25 x (1-.7) = 405 Btu/h
- Total heat loss = Infiltration + ERV ventilation = 810 Btu/h
Option 2 - ERV unbalanced 50/35 supply/exhaust - resulting in zero infiltration
- Infiltration = 1.08 x CFM x deltaT = 1.08 x 0 x 25 = 0 Btu/h
- ERV ventilation = 1.08 x CFM x deltaT x (1- ERVefficiency) = 1.08 x 50 x 25 x (1-.51) = 689 Btu/h
(the ERV is optimistically(?) derated to 49%... assumes linear relationship for heat recovery curve).
- Total heat loss = Infiltration + ERV ventilation = 689 Btu/h
Therefore, you would be saving 121 Btu/h.
That said - I've been playing around with various CFM rates and you can very easily shoot yourself in the foot. For example, a 100 CFM rate in this example will not result in savings. And for all the other reasons laid out previously, this is probably not a good idea to do.
Still - great thought experiment. I appreciate everyone's time.
I don't see why you can assume that infiltration will be zero with the unbalanced ERV. Infiltration isn't driven by pressure differential, it's driven by temperature differential, and that will still exist.
If the stack effect was the only cause of pressure differences between inside and out, then the pressure difference will be negative at the bottom of the house (inside pressure less than outside pressure). As you move up, the pressure difference will gradually drop to zero and then turn positive (inside pressure greater than outside pressure). It will reach a maximum positive value at the top of the house.
Those pressure differences cause infiltration through holes in the lower part of the house and exfiltration through holes in the upper part. The actual amount of air flow also depends on the number and sizes of the holes, and where they are located. So, the single -0.6 Pa reading won't tell you how much air leakage there will be. Given your air tightness and the 25° delta though, 15 CFM seems quite high.
Suppose that you actually do have 15 CFM leaking in at the bottom part of the house, balanced by 15 CFM leaking out the top. Adjusting the ERV to run unbalanced with 50 CFM supply and 35 CFM exhaust will pressurize the house. At the top of the house, the positive pressure difference will become even greater, increasing exfiltration. At the bottom of the house, the negative pressure difference will become less, resulting in less infiltration.
You might end up with 7.5 CFM infiltration and 22.5 CFM exfiltration, or 6 CFM infiltration and 21 CFM exfiltration, or some other combination that equalizes the total air flows in and out. Again, the exact air flows will depend on number, sizes and locations of the holes.
So, you have reduced the amount of infiltration, but at the cost of more exfiltration, i.e. more heated air is leaving the house without going through the ERV. That's not a strategy that will lower your energy bills, even if the ERV is able to extract slightly more heat from the 35 CFM exhaust air still going through it.
You have also reduced the total (ERV supply + infiltration) ventilation rate. If 65 CFM was more than you wanted, it would be better to reduce both the ERV supply and exhaust rates by the same amount.
I think this is correct.
Two really good points here:
1. The pressure differential is not constant throughout the house, stack effect pressurizes at the top and depressurizes at the bottom. The whole house isn't pressurized or depressurized.
2. The two scenarios in post #9 don't result in the same levels of ventilation so aren't an apples-to-apples comparison.
DC,
I'd add:
1. No only is the stack effect not constant throughout the house, as you pointed out it also varies as the outside temperature does.
2. The other source of pressure differences and changes in ACH is wind, which again varies constantly.