Home HVAC airflow weirdness

[LVSteve]

I would like the home HVAC experts to comment on what follows an if they think there is a fix or that this is how things are, now. Basic setup is two units, one for each floor, with the returns in the ceiling of the upstairs hallway.

There have been some changes in my home over the last year. A water leak was finally fixed and the mold dealt with, but while the cabinets were off the wall, I bought a replacement "over the stove" microwave with a built in extractor. Last May this unit was hooked up the same as the old one with a duct leading to an outside vent. The difference is that in the new equipment, the door to cutoff any reverse flow when the extractor is off functions correctly. On the old microwave, lots of cold air poured in during the winter until I covered 2/3 of the vent. That seemed to prevent it from "siphoning" the cold air inside to a great degree.

Thing is, during this last winter I noted that there was cold air pouring out of the downstairs decorative gas fireplace that we never use. This didn't happen to any noticeable degree when I had the outside air leak via the old microwave. Hmmm. In desperation I taped up the glass doors and the side vents of the fireplace as the damper in the chimney cannot be fully closed for obvious reasons. Thinking back, that same area also got rather warm last summer immediately after the new microwave went in. I assumed this was just the result of successive 110°F+ days just overwhelming the insulation, given that the fireplace area in question is on the South side of the house and gets mightily beat on by the Sun.

Second thing that happened was that my alarm panel was replaced last August. Trouble is the new one is bigger and the top edge is getting close to the downstairs thermostat. I meant to move the stat a bit higher up the wall but got busy, and now we are in full A/C season I 'm not about to mess with it. There is evidence to suggest that the stat is picking up heat from the alarm panel, but also it seems the reduced gap has compromised airflow around the stat. This has led to the stat sometimes turning the cool on when it's not needed, and refusing to call for heat in the winter when we could do with a touch more. However, at other times it sits there and does nothing while the family room gets too warm in the summer. This may have something to do with my next observation.

When EITHER HVAC system is cooling, I seem to be getting a lot of cool air tumbling down from the upstairs hallway. I fitted new filters around Memorial Day, they live behind the return grilles, so I know there cannot be a flow restriction at that point. Besides, the flow from the registers for both HVAC units seems as healthy as it ever was. One effect is that when the upstairs unit is running, there is some evidence to suggest that this falling air is getting to the downstairs stat, fooling it into thinking all is good in that part of the house. Recall what I said about stagnant air around the downstairs stat..

Clearly, I have to move the downstairs stat as best I can to get it away from the alarm panel. But, I'm wondering if these other effects I'm seeing are due to the kitchen air leak no longer being present. What say the experts, and is there any need to worry.

 

[chief38]

I am not an HVAC expert but do have plenty of experience. First off, I am questioning as to WHY your alarm panel is throwing off an appreciable amount of heat!? I don't recall ours exhibiting that. The only thing getting warm would be a back up battery when trickle charging if applicable and even that should not generate much if the charging unit is working properly. If for whatever reason all is well with the alarm panel you can easily move the thermostat - not a big job and sheet rock is easily repaired. YUP - cold air falls and when you have a two story home (had one for 40+ years) getting the upstair and downstair balanced is always a challenge. You should have a few flow regulators in the system to adjust how much air goes where. In my old house they were in the attack.

Cold air falling down a chimney is another issue I have faced. Part of the solution is to install a tight fitting, pull down, gasket sealing, stainless steel cap that has a chain going down the chimney to close and open it. Obviously before you start a fire, you release the chain and springs open up the cap. That helps but won't solve the issue 100%. You say you never use the fireplace so you could possibly put some sort of soft, formable insulation that seals up the cast iron damper. Cast iron dampers (if that is what you have) never really seal well as they are sort of crude. If you do use insulation, hang an obvious visible note inside the fireplace so anyone who might use it in the future knows to remove it first! Although these ideas will not solve your issues 100%, they should help quite a bit.

You also might try installing a very slow spinning ceiling fan upstairs above the airway that is spinning in reverse. This could help pick up the cold air falling. When it comes to issues like this I find you solve most of them in small increments as there is not really a one shot fix. Every little bit helps.

 

[jeffrefrig]

Mounting anything below the stat makes no sense at all, whether it emits heat or not. Start with a good thermometer before going to the trouble of moving it. If you aren't going to use the fireplace plug it up with some insulation. It seems strange the fireplace pulls cold air in.

 

[dockmurgw]

Thing is, during this last winter I noted that there was cold air pouring out of the downstairs decorative gas fireplace that we never use. This didn't happen to any noticeable degree when I had the outside air leak via the old microwave.

I'm no expert, but based on this it appears you have negative air pressure in the home, and the microwave air leak was equalizing the air pressure between the exterior and interior. They make systems that will solve this for you, they are commonly used in the northern climes in the winter in very tight homes. Just a guess, I'm no expert.

 

[LVSteve]

Mounting anything below the stat makes no sense at all, whether it emits heat or not. Start with a good thermometer before going to the trouble of moving it. If you aren't going to use the fireplace plug it up with some insulation. It seems strange the fireplace pulls cold air in.

I wondered about that when we bought this house. The old alarm panel was smaller, so I'm thinking the new one is interfering with air movement around the thermostat, which I have read is a bad thing.

 

[LVSteve]

I'm no expert, but based on this it appears you have negative air pressure in the home, and the microwave air leak was equalizing the air pressure between the exterior and interior. They make systems that will solve this for you, they are commonly used in the northern climes in the winter in very tight homes. Just a guess, I'm no expert.

It looks that way, doesn't it. Now the chimney is the only path, air is being drawn in that way, which is bad. My heating/plumbing buddy back in the UK used to say that a good chimney always draws, flowing air out of the house. Thing is the chimneys here appear to be a basic metal tube with a damper that cannot be closed due to carbon monoxide concerns. Blocking the chimney is not recommended because if you have any problem that involves the gas company and your supply have to be restarted, the gas company is required to check all gas appliances. If they find you have blocked a chimney, they will get major grumpy.

 

[Golphin]

I am no AC expert but have spent my 67 years in Florida and run one year round pretty much. If you are sucking air from the outside when the unit is running it sounds to me your return air vents are undersized or not n the right location. That is just my guess. Like I said I am no AC tech. My son is and I let him solve my issue when they arrive. If I talk to him I will ask him and see what he says. It may be hard to know without knowing more about your house and type of AC units you have.

 

[mtgianni]

A decorative gas fireplace is required to have no restrictions or damper in the flue. If you're getting a downdraw, then you are loosing air somewhere higher than the top of the fireplace. It might pay to have a temp sensing gun get a read around ceiling light fixtures, windows and wall joints to see where you're leaking at. With the microwave leak fixed the air is leaving somewhere and getting replaced/ It is costing you some money. Moving your T stat even 16" to the next stud might do wonders.

I used to explain to people that in the old days you needed two holes in a beer can, one for fluid and one for air. You are seeing and feeling the replacement air.

In a balanced heating/cooling system your return air ducts should be 10% larger than supply ducts. The end results of not doing that are generally efficiency loss and noise.

 

[smthbros]

Make-up air systems have been required in new houses here for over 25 years. Conventional fireplaces are not energy efficient.
To keep cool air upstairs, close the doors to the upstairs rooms. If each room has already a return, you should be good. If you have a common return, you will need to add transfer grilles to the rooms that are allowed to have them.

 

[LVSteve]

A decorative gas fireplace is required to have no restrictions or damper in the flue. If you're getting a downdraw, then you are loosing air somewhere higher than the top of the fireplace. It might pay to have a temp sensing gun get a read around ceiling light fixtures, windows and wall joints to see where you're leaking at. With the microwave leak fixed the air is leaving somewhere and getting replaced/ It is costing you some money. Moving your T stat even 16" to the next stud might do wonders.

I used to explain to people that in the old days you needed two holes in a beer can, one for fluid and one for air. You are seeing and feeling the replacement air.

In a balanced heating/cooling system your return air ducts should be 10% larger than supply ducts. The end results of not doing that are generally efficiency loss and noise.

I think you just described every home HVAC system I've ever seen in Vegas. I suspect you would cry if you saw the route taken by the ducts from the two return registers to the inputs of their respective HVAC units. Both require 180° turns.

Moving the stat would be a major job as it sits on a narrow section of wall between two closets under the stairs. As for the concept of a tight house in Vegas built in the 90s, that would raise a laugh with anyone who lives here.

 

[LVSteve]

Make-up air systems have been required in new houses here for over 25 years. Conventional fireplaces are not energy efficient.
To keep cool air upstairs, close the doors to the upstairs rooms. If each room has already a return, you should be good. If you have a common return, you will need to add transfer grilles to the rooms that are allowed to have them.

I have seen the odd home here with a transfer grille, but it's more common not to bother or just take 1/2 inch off the bottom of the doors. I wish I was making this up.

 

[SMSgt]

Anything that exhausts air outside has to be countered with air being drawn inside. There is a reason old homes were "drafty." A fireplace won't work if a house isn't "drafty." Every room needs to be drafty. The house needs to be drafty. That exhaust fan in the crapper needs to draw air from somewhere. Suction is suction. A vacuum will take air from whatever source is available.