Thoughts on negative pressure after balancing a Venmar X24 ERV?

[dgoodyear]

I have a x24ERV from venmar installed in a house where the blower door test measured about 0.3 ACH50. The house is really air tight.

I know that the fan tables with the unit provide a way to measure air flow across the core to facilitate balancing fresh and stale air flows.
my installer and I have both independently attempted balancing using a digital differential manometer with funny results. we can easily balance the machine air flow according to the manufacturer's specification within a couple of percent.
I have an anemometer and a hood and have measured the air flows from the rooms and verified that they are close to the air flows measured across the core.
I have also used the anemometer to do an 50 point average across the width and depth of the vent hood outside on the fresh and stale supply and have verified that they are practically the same as using the static pressure measurements accross the ERV core.

The issue is, with the machine's air flow balanced, the house is actually under negative pressure. To verify this, on a still day I have opened a window and taped the gap to creat an air tight diaphram around the window. I puncture a hole in the tape and insert one of the manometer tubes through the hole so it is hanging outside.
The pressure reads negative. If i put the machine in recirculate, the negative pressure dissappears as expected. As soon as I turn back to continious ventilation, the pressure becomes negative despite the air flows being “balanced”.
I have not been able to find any leaks on the exhaust or supply from the unit to the outside.

I was unaware that there was a balancing issue until smoke from the chimney trickled into the living space while lighting the stove. I realized what was going on quite quickly and opened the windows and the smoke stopped trickling through the joints. When the stove is not lit and the air flows of the unit are balanced
I can feel cool air trickling into the living space through the chimney joints. Venmar says that their tables are for a closed system. I am thinking that the ventilator and house aren’t really a
closed system since the house is really like a plenum with air flowing through it. any plenum, like a house for example has leaks. There is no way to prevent air from being drawn or expelled through those holes since they are part of the
plenum and hence a part of the static pressure associated with the duct system.

Anyway, when I unbalance the system(on a still day, with a slightly open window taped around like a diaphram and one manometer tube placed outside through a hole in the tape) I can balance the negative pressure of the house and I no longer feel air coming from around the chimney joints. The issue is that I have to unbalance the machine significantly (ie 100 cfm Supply, 70 cfm exhaust), in order to achieve the pressure balance. Because the unbalancing is so extreme it is affecting the moisture exchange dramatically and the house humidity is always above 55% and that is mostly maintained by keeping a dehumidifier going now that we are in the cold season. So the stove issue has been fixed but unbalancing the unit has affected the rate of moisture recapture through the core.

Does anybody have any thoughts as to what may be happening? If the machine is balanced, why is the house experienceing negative pressure? Is it because of the duct runs, air moving and differenty speeds through them and therefore the static pressure conditions of the supply and return not being matched? I thought this may be the case but I decided to switch the exhaust and intake at the hoods as well ast the supply and return on the unit. Each time I balance I see the same result. Based on this, I am convinced that the problem has some fundamental physics involved. My experience with air handling and ducts is fairly rudimentary although I have quite a bit of physics knowledge.

I am thinking that it could be explained with a simple model where a supply fan in a duct connected to the outside is blowing air through a duct into a leaky plenum (the house) and another independent fan pulls exhausts air out of the plenum to the outside. I know in real life it is more complicated. I am also thinking that the effect we are seeing is somewhat due to the air tightness of the house and in most houses the pressure would equilibrate quickly since tightness would be 10 time worse than this house.

DJG.

 

[miningman]

Interesting problem. I'm certainly no HVAC specialist and cant claim specific knowledge of ERV units. However in my mine ventilation experience, I have seen one or two examples of unexpected airflow behaviours that were ultimately traced back to minor temperature differentials.

Since the delta T between indoors and outdoors is obviously highly variable over short periods of times, if it were my house I would do whatever it takes to prevent chimney back drafting. I believe that there is a difference between balancing an ERV and balancing the house itself. Perhaps a deliberate introduction of exterior air into the basement to eliminate uncontrolled leakage thru minor leaks in a well sealed house is necessary.

 

[dgoodyear]

Thanks, I have thought about this. THe other option I have thought about is to use a zone damper to control the amount of air the ERV supplies. I would balance the ERV with a zone damper on the supply when it is partially closed. I would use a switch to open the zone damper to allow more air flow when the stove is being operated. THis way the only time I am operating out of balance is when the wood stove is going. The rest of the time the switch would be off and the system would be balanced. Operating in the balanced condition would allow for proper operation and would lead to better moisture control. Thanks for the post!

 

[berkshire]

dgoodyear,
Where does the air for the woodstove come from? Does it have a dedicated outside air vent, or does it draw air from infiltration into the house ? If your Venmar has a furnace option setting, you can use this to compensate for the negative pressure the wood stove generates. If the stove is unpowered you may have to add a thermal or airflow sensing switch. Also with the Venmar check all of your kitchen hood and bathroom fan settings and make sure they are wired into the unit so that the unit re-balances when they are running.
It is not uncommon to have to require more intake air to compensate for leakage in the house, and you may have to run the machine in a slightly unbalanced state to do this However 30cfm is a bit much unless you have a 10,000sqft house.
If that does not work you may have to hire a TAB specialist to balance the whole house.
B.E.

You are judged not by what you know, but by what you can do.

 

[dgoodyear]

The stove is a walltherm gasification boiler . It has a dedicated 5” air intake. It is a natural draft stove so it has no power source. The Venmar unit has no way to dynamically balance unfortunately. But I have seen that the zhender hrvs do have an auto balance feature.

 

[willard3]

Have you measured and taken into account pressures created by wind outside the house?

 

[dgoodyear]

Yes, I have taken that into account. I have balanced under both windy and ambient conditions with the same effect. I have tried flipping the supply and return and the outside hoods just to entertain myself. All scenarios the house balances negative and I can feel cold air leaking out of the stove pipe joins.

 

[ChasBean1]

dgoodyear, sorry my friend. I've never seen a worsely articulated post. Ever. Totally descriptive, but requiring 3 or 4 reads to decipher. You were very descriptive about many things except for the issues at hand. If you can better (and more concisely) define the problem, us nerds in here might be better able to help.

 

[dgoodyear]

ChasBean1,

I have described the observations to the best of my abilities. When I balance the ERV, the house depressurizes. My obversation of this was backed up with the fact that smoke from my stove pipe trickles into the living space unless I unbalance the ERV to pressurize the house. In order to do this I have had to supply the house with about 110 cfm while only exhausting 80 cfm. I have tried to flip the stale and fresh intakes as well as the supply and return ducts and once balanced I see the same problem. WIth respect to your post....you seem to indicate that you need more information because my definition of the problem lacks conciseness. If you could provide a better description of the information you require I would be more than happy to post it as I am looking for possible solutions.

 

[lilliput1]

looks like the ERV is not sized to provide enough air to overcome the building negative pressurization caused by wind, stack effect and stove chimney draft. From experience in hospital isolation rooms it needs about 400 CFM transfer air per door to provide the minimum 0.01"wg negative pressurization.

 

[Compositepro]

The most sensitive and accurate measurement you can make of air flow balance is the inside pressure of the house. This indicates that there is something wrong with your "air flow balance". You may have leaks or just too much pressure drop in your ducting. The whole point of balancing flow is to achieve a certain positive pressure. Just forget about airflow measurement if you can adjust your system to keep a steady pressure.

 

[ChasBean1]

Dgoodyear, the “best of your abilities” needs to improve. Sorry to be blunt.

“I know that the fan tables with the unit provide a way to measure air flow across the core to facilitate balancing fresh and stale air flows.”

CB: What?

“I have also used the anemometer to do an 50 point average across the width and depth of the vent hood outside on the fresh and stale supply and have verified that they are practically the same as using the static pressure measurements accross the ERV core.”

CB: Semi-copy that. But is there any separate exhaust, e.g., from lavatories?

“If i put the machine in recirculate, the negative pressure dissappears as expected. As soon as I turn back to continious ventilation, the pressure becomes negative despite the air flows being “balanced”.”

CB: Having a “machine” completely recirculating the air versus taking in outdoor air would tend to negatively pressurize a building rather than the opposite… That point is strange.

“The issue is that I have to unbalance the machine significantly (ie 100 cfm Supply, 70 cfm exhaust), in order to achieve the pressure balance.”

CB: A 30 cfm difference is not a significant imbalance between supply and exhaust. In fact if this were my lair, I’d shoot for 50 or 100 cfm. You can find stack (created by temperature differences) producing much stronger differentials than a 30 cfm airflow difference might yield.

“Because the unbalancing is so extreme it is affecting the moisture exchange dramatically and the house humidity is always above 55% and that is mostly maintained by keeping a dehumidifier going now that we are in the cold season.”

CB: No, on more levels than I can count. A 30 cfm difference is not an “unbalancing” that is SO extreme by any measure. Moisture exchange? Not understood. When it’s hot and humid outside, cool any incoming outdoor air to 55F or so. When it’s cold out you do NOT need to keep “dehumidifier going now that we are in cold season.” See me after class. (Sorry to be annoying on that, just a jab!)

“So the stove issue has been fixed but unbalancing the unit has affected the rate of moisture recapture through the core.” Ummm

Sorry, all this might be a language barrier but I’m having a heck of a time getting by it.