Heat Pump HVAC unit odd smell

[escape]

My allergist recommended that I run the fan at all times. But since we've started doing that, a weird smell comes out of the vents when the AC kicks in. We've never noticed this smell before and if we don't run the fan outside of the AC, no smell. When i called the HVAC co. that installed it, they said they would come out and look at it but we don't want to get slammed with a charge for the service call if they decide there's nothing wrong with it. Any guesses or advice?

This is a new unit (Nov. 2021) - its a 2.5 Ton Trane Heat Pump.

Thank you.

 

[travelover]

My allergist recommended that I run the fan at all times. But since we've started doing that, a weird smell comes out of the vents when the AC kicks in. We've never noticed this smell before and if we don't run the fan outside of the AC, no smell. When i called the HVAC co. that installed it, they said they would come out and look at it but we don't want to get slammed with a charge for the service call if they decide there's nothing wrong with it. Any guesses or advice?

This is a new unit (Nov. 2021) - its a 2.5 Ton Trane Heat Pump.

Thank you.

What do you mean by "the fan outside of the AC"?

 

[Aerides]

A new unit should come with a 1yr labor warranty, so no service call charge.

Has the smell stayed for more than a day or two? Whenever we run the heat on our AC it always stinks for the first hour or so, since it's only one once per year.

Only other guess is it's something in your ducts.

 

[Joylush]

It’s never recommended to keep the fan on “on” when running the AC. Always use the auto setting. An allergist doesn’t know how an AC unit works. This will help explain why:
https://www.energyvanguard.com/blog/don-t-set-your-air-conditioner-thermostat-like-this/

 

[COcheesehead]

Have you had your air ducts blown out? You might be surprised what’s in there.

 

[Telly]

The first thing that comes to mind is mold. Mold spores are everywhere, they're just looking for a nice (for them) home environment.
Since you have the fan switch on "ON", rather than "AUTO", that leaves out the fan motor itself as a possibility of smelling, as if the fan motor had a problem, you would smell it all the time, not only when the compressor kicks on.
So the smell appears to go with the compressor in operation. With the compressor running, the evaporator gets cold, and moisture condenses on it. And that moisture could be the problem.

Since it is a very new installation, and this is the first A/C season for it, I wonder if there is an installation problem with respect to condensate drainage. Like air handler unit is tilted such that the drain pan cannot empty completely, leaving some standing water in it all the time. Or a drain line/trap not positioned properly doing the same. Or a partially plugged condensate drain line, etc. etc.

Others may disagree, but I think that running A/C with fan set to "ON" increases average humidity in an air-conditioned space as opposed to "AUTO", in which the fan runs with the compressor operation. Having a hygrometer, I can see that in houses I have lived in. The design of some two-story houses, where the main living area has a two-story tall ceiling, and a second-story balcony etc. overlooks the big space downstairs, often need to run the fan on "ON" to get air mixing. As opposed to a two-story house that has just a stairway opening to second floor, which minimizes warm air rising to mess up the conditions for a second-story A/C unit. In that case, if the second-story unit has air returns in each upstairs room, that's the best.

One inspection option (the company that installed the unit should really do this) is removing whatever panel to access the evaporator coil, and doing a careful inspection with a light, looking for black mold. Also to look into the supply plenum to see if mold has adhered onto the sides/top/bottom of the supply plenum. The supply plenum is the box attached to the evaporator/air handler on its output side. The supply ducts connect to the supply plenum. It may/may not be possible to see into the plenum from the coil area. It may require detaching the supply plenum from the air handler, and moving one side or the other enough to see into the plenum.

A word on "plenum" naming... the Supply plenum is on the output of whatever heats or cools in a forced-air system. The "Return plenum" is on the inlet side of the unit, it collects air from various return air sources in the house and presents it to the fan (blower motor).

The mold can be killed by spraying a water-soluble cleaner made for that purpose onto the evaporator coils/pan/ whatever, letting it sit, then flushing away with water. But its not a cure-all. If the friendly-to-mold condition still exists, it will come back eventually. If your A/C season will wrap up shortly, it might not be till next A/C season that the mold convention grows enough members to let you know.

 

[escape]

What do you mean by "the fan outside of the AC"?

i mean if the fan is on and the AC is not on.

In that case, when the AC kicks on, a weird smell comes out of the vents.

 

[escape]

A new unit should come with a 1yr labor warranty, so no service call charge.

Has the smell stayed for more than a day or two? Whenever we run the heat on our AC it always stinks for the first hour or so, since it's only one once per year.

Only other guess is it's something in your ducts.

Thank you. They will charge me if they decide there's nothing wrong with the unit.

The smell only lasts for about 30-60 minutes right after the AC kicks in ANY time the fan has been running.

And if it's something in the ducts, wouldn't the smell be there all the time?

 

[escape]

It’s never recommended to keep the fan on “on” when running the AC. Always use the auto setting. An allergist doesn’t know how an AC unit works. This will help explain why:
https://www.energyvanguard.com/blog/don-t-set-your-air-conditioner-thermostat-like-this/

thank you very helpful!

 

[escape]

The first thing that comes to mind is mold. Mold spores are everywhere, they're just looking for a nice (for them) home environment.
Since you have the fan switch on "ON", rather than "AUTO", that leaves out the fan motor itself as a possibility of smelling, as if the fan motor had a problem, you would smell it all the time, not only when the compressor kicks on.
So the smell appears to go with the compressor in operation. With the compressor running, the evaporator gets cold, and moisture condenses on it. And that moisture could be the problem.

Since it is a very new installation, and this is the first A/C season for it, I wonder if there is an installation problem with respect to condensate drainage. Like air handler unit is tilted such that the drain pan cannot empty completely, leaving some standing water in it all the time. Or a drain line/trap not positioned properly doing the same. Or a partially plugged condensate drain line, etc. etc.

Others may disagree, but I think that running A/C with fan set to "ON" increases average humidity in an air-conditioned space as opposed to "AUTO", in which the fan runs with the compressor operation. Having a hygrometer, I can see that in houses I have lived in. The design of some two-story houses, where the main living area has a two-story tall ceiling, and a second-story balcony etc. overlooks the big space downstairs, often need to run the fan on "ON" to get air mixing. As opposed to a two-story house that has just a stairway opening to second floor, which minimizes warm air rising to mess up the conditions for a second-story A/C unit. In that case, if the second-story unit has air returns in each upstairs room, that's the best.

One inspection option (the company that installed the unit should really do this) is removing whatever panel to access the evaporator coil, and doing a careful inspection with a light, looking for black mold. Also to look into the supply plenum to see if mold has adhered onto the sides/top/bottom of the supply plenum. The supply plenum is the box attached to the evaporator/air handler on its output side. The supply ducts connect to the supply plenum. It may/may not be possible to see into the plenum from the coil area. It may require detaching the supply plenum from the air handler, and moving one side or the other enough to see into the plenum.

A word on "plenum" naming... the Supply plenum is on the output of whatever heats or cools in a forced-air system. The "Return plenum" is on the inlet side of the unit, it collects air from various return air sources in the house and presents it to the fan (blower motor).

The mold can be killed by spraying a water-soluble cleaner made for that purpose onto the evaporator coils/pan/ whatever, letting it sit, then flushing away with water. But its not a cure-all. If the friendly-to-mold condition still exists, it will come back eventually. If your A/C season will wrap up shortly, it might not be till next A/C season that the mold convention grows enough members to let you know.

Thank you, this is very helpful!

 

[escape]

"The first thing that comes to mind is mold. Mold spores are everywhere, they're just looking for a nice (for them) home environment.
Since you have the fan switch on "ON", rather than "AUTO", that leaves out the fan motor itself as a possibility of smelling, as if the fan motor had a problem, you would smell it all the time, not only when the compressor kicks on.
So the smell appears to go with the compressor in operation. With the compressor running, the evaporator gets cold, and moisture condenses on it. And that moisture could be the problem.

Since it is a very new installation, and this is the first A/C season for it, I wonder if there is an installation problem with respect to condensate drainage. Like air handler unit is tilted such that the drain pan cannot empty completely, leaving some standing water in it all the time. Or a drain line/trap not positioned properly doing the same. Or a partially plugged condensate drain line, etc. etc.

Others may disagree, but I think that running A/C with fan set to "ON" increases average humidity in an air-conditioned space as opposed to "AUTO", in which the fan runs with the compressor operation. Having a hygrometer, I can see that in houses I have lived in. The design of some two-story houses, where the main living area has a two-story tall ceiling, and a second-story balcony etc. overlooks the big space downstairs, often need to run the fan on "ON" to get air mixing. As opposed to a two-story house that has just a stairway opening to second floor, which minimizes warm air rising to mess up the conditions for a second-story A/C unit. In that case, if the second-story unit has air returns in each upstairs room, that's the best.

One inspection option (the company that installed the unit should really do this) is removing whatever panel to access the evaporator coil, and doing a careful inspection with a light, looking for black mold. Also to look into the supply plenum to see if mold has adhered onto the sides/top/bottom of the supply plenum. The supply plenum is the box attached to the evaporator/air handler on its output side. The supply ducts connect to the supply plenum. It may/may not be possible to see into the plenum from the coil area. It may require detaching the supply plenum from the air handler, and moving one side or the other enough to see into the plenum.

A word on "plenum" naming... the Supply plenum is on the output of whatever heats or cools in a forced-air system. The "Return plenum" is on the inlet side of the unit, it collects air from various return air sources in the house and presents it to the fan (blower motor).

The mold can be killed by spraying a water-soluble cleaner made for that purpose onto the evaporator coils/pan/ whatever, letting it sit, then flushing away with water. But its not a cure-all. If the friendly-to-mold condition still exists, it will come back eventually. If your A/C season will wrap up shortly, it might not be till next A/C season that the mold convention grows enough members to let you know."

Another question: If the mold is caused by the condensate, why wouldn't we smell it anytime the AC kicks in. We only smell it when the AC kicks in after the fan has been on without the AC.

Thank you!

 

[JoeWras]

Another question: If the mold is caused by the condensate, why wouldn't we smell it anytime the AC kicks in. We only smell it when the AC kicks in after the fan has been on without the AC.

Thank you!

I'm not a mycologist, so take this for what it is worth. But here's my guess.

When you run the fan all the time, you are evaporating off the condensate sitting in the pan or coils. You are also pulling in new spores or dust substrates through the coil and onto the pan.

Mold seems to like a moist environment, but not a wet environment. So, by pulling in new spores and new food (dust, microbes, etc.), you are doing so on a perfectly just-moist environment.

When you don't run the fan all the time, the coils and pan stay wet longer, and even self-flush. Meanwhile, new food/spores are not coming into the box as everything drains to just a moist environment.

Just guessing here.

It doesn't really matter. There are more problems (mentioned above) than smell by running the fan all the time in A/C season. Don't do it.

One more thing: I'm going to guess that dust and spores should be low if your install was good. However, if you have bad seals on the return vent, or a bad seal around the filter, you can get blow-by and that's not good no matter what. The more unfiltered or poorly filtered air coming in, the better the chance for nastiness on the coil. I don't have a solution for you to check this since it is a new install except for you to check that your filter is sitting well, and perhaps call them in to check.

Is this a brand new install, new ducts and all? Or a replacement? If they are using the old return duct, perhaps they didn't seal something well, or your old return duct(s) was always leaky.

 

[escape]

I'm not a mycologist, so take this for what it is worth. But here's my guess.

When you run the fan all the time, you are evaporating off the condensate sitting in the pan or coils. You are also pulling in new spores or dust substrates through the coil and onto the pan.

Mold seems to like a moist environment, but not a wet environment. So, by pulling in new spores and new food (dust, microbes, etc.), you are doing so on a perfectly just-moist environment.

When you don't run the fan all the time, the coils and pan stay wet longer, and even self-flush. Meanwhile, new food/spores are not coming into the box as everything drains to just a moist environment.

Just guessing here.

It doesn't really matter. There are more problems (mentioned above) than smell by running the fan all the time in A/C season. Don't do it.

One more thing: I'm going to guess that dust and spores should be low if your install was good. However, if you have bad seals on the return vent, or a bad seal around the filter, you can get blow-by and that's not good no matter what. The more unfiltered or poorly filtered air coming in, the better the chance for nastiness on the coil. I don't have a solution for you to check this since it is a new install except for you to check that your filter is sitting well, and perhaps call them in to check.

Is this a brand new install, new ducts and all? Or a replacement? If they are using the old return duct, perhaps they didn't seal something well, or your old return duct(s) was always leaky.

I will most definitely keep the fan on "auto" mode. this was a replacement. I assume you mean the air filter that must be cleaned or replaced every so often, in which case it is sitting OK as far as I can tell. Is there a different filter.

I'll have to decide whether I want to hire a 3rd party to check on all this, but of course, the better we understand it, the more effectively our money will be spent.

I appreciate you sharing your knowledge!

 

[arbeadub]

Another thing to check is any drain that may have not have been used for a time, I have to flush all drains after only 2 weeks.