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Condensing Furnace and Humidity
Old 03-16-2019, 10:59 AM   #1
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Condensing Furnace and Humidity

The humidity has been lower in our house since installing the new 98% efficient furnace, but I think this is just coincidence. That is, I don't think the furnace is removing water from the household air.

Right?
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Old 03-16-2019, 11:16 AM   #2
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Maybe not removing water, but its probably not contributing as much as the old furnace. If they added a new outside air feed for the high efficiency unit, its pulling the cooler dry air into the house when heating.
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Old 03-16-2019, 02:06 PM   #3
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Maybe not removing water, but its probably not contributing as much as the old furnace. If they added a new outside air feed for the high efficiency unit, its pulling the cooler dry air into the house when heating.


Yes. I believe outside air for combustion is required to get up to 98%.

Op, how are you determining the RH is lower and not due to ambient?
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Old 03-16-2019, 02:37 PM   #4
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Our high efficiency furnace is a sealed closed loop; it draws combustion air in from outside and exhausts to the outside, both through PVC pipe. It has no connection to the air in the house.

For some code reason the furnace installer ran a fresh air vent from a side of the house away from the furnace intake/exhaust to just behind the new furnace. The last thing I need is a deliberate cold air leak, so I have stuffed the vent hose end with insulation.

Our bad old furnace used house air for combustion so cold dry outside air was constantly drawn into the house through cracks, weak door seals, etc.

Either way we have an AprilAire humidifier in the plenum above the furnace, so that is what controls our humidity. It has an outside air temp probe so it can reduce humidity to prevent condensation on windows when the temperature is low. Maybe your installer accidentally or deliberately disabled a humidifier?
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Old 03-16-2019, 03:09 PM   #5
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Yes. I believe outside air for combustion is required to get up to 98%.

Op, how are you determining the RH is lower and not due to ambient?
Two pipes go outside. One for water and one for exhaust. So, it must be getting combustion air from the garage.

Not sure about your other questions, but I'm basing the relative humidity on a number of digital hygrometers around the house. They've usually been between 60 and 70 percent, but recently around 50%.* The weather has been colder, so I think that's it.

I haven't calibrated them for a while:

https://youtu.be/B2MmCRZ4cD0?t=111
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Old 03-16-2019, 05:51 PM   #6
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The humidity has been lower in our house since installing the new 98% efficient furnace, but I think this is just coincidence. That is, I don't think the furnace is removing water from the household air.

Right?
Yes, probably coincidence.

Potential situations:

1) If your old furnace used inside air for combustion and so does you new furnace, then there would be no change in the indoor humidity.
2) If the old furnace used indoor air but your new furnace used outside air, then your indoor humidity would now be higher. (Why? Because using indoor air for combustion depressurizes the house, encourages outside air to come in. In winter, the cold outside air will contain less water than the air that is leaving your home).
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Old 03-17-2019, 04:08 PM   #7
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Most +90% furnaces have sealed combustion with a (2) PVC pipe vent system - (1) intake and (1) exhaust. The intake and exhaust terminate at the same place outside of the house. Furnace does not use indoor air for combustion. +90% furnaces also have a condensate (liquid) drain as the furnace cools the combustion air below where normal condensation occurs. It is a small diameter PVC pipe, and the condensate is somewhat acidic.

Odds are you still have a natural gas hot water heater connected to your main chimney, so chimney still naturally vents to the outside. Newer codes will usually require adequate combustion (fresh) air to be brought into the home's mechanical room/area for proper combustion and ventilation. Houses without adequate fresh air usually pull in required outside air in winter through improperly sealed areas of the home. This results in window, door and wall plate drafts, and oddly colder rooms than the rest of the house. It can also result in poor combustion and ventilation.

Humidity seeks its own level throughout a house. The colder it is outside, the more the furnace runs and the dryer the indoor air gets over time. You can see RH in the 30% ranges in the dead of Winter, and the normal 50% range in Summer (higher without A/C). You can have a furnace mounted humidifier or a self-contained console model - humidity will equalize throughout the house.
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Old 03-17-2019, 04:16 PM   #8
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All of these "high efficiency" units are starting to let the tail wag the dog.
When doing final inspection on a home built in 2016, there was general confusion around the function of the "fresh air intake" fan that drew in outside air whenever the HVAC fan ran and dumped it directly into the air ducts... bypassing both heating and cooling functions.
The control of this fan is behind a service personnel only password in the thermostat locking the homeowner out from changing the speed/duration/frequency of operation.

The story was that the new construction house is so air tight that they now have to pipe in outside air.
So when its 105 out, we get 105 air mixed in with our electrical AC cooled air, and when its 30F outside, we get 30F degree air mixed in with our nat gas heated air.
Yup. Real efficient.
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Old 03-17-2019, 05:22 PM   #9
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The story was that the new construction house is so air tight that they now have to pipe in outside air.
So when its 105 out, we get 105 air mixed in with our electrical AC cooled air, and when its 30F outside, we get 30F degree air mixed in with our nat gas heated air.
Yup. Real efficient.
If you are drawing in outside air like you describe you really should be using a heat exchanger. Some units do this and also make sure the humidity of the incoming is adjusted properly.
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Old 03-17-2019, 05:50 PM   #10
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Here's the piping:



I think it takes in garage air from the upper PVC port. The big and little PVC pipes go to the outside. We only have heating—no AC.

I don't think this region has seen humidity under 50% since the Mesazoic era, when the ocean was farther away.
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Old 03-18-2019, 08:22 AM   #11
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If the garage is not heated, that setup would be equivalent to using outside air for combustion.
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Old 03-18-2019, 08:32 AM   #12
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Originally Posted by TromboneAl View Post
The humidity has been lower in our house since installing the new 98% efficient furnace, but I think this is just coincidence. That is, I don't think the furnace is removing water from the household air.

Right?
Correct.

On your wood stove , heat an open pot of water. I am not a fan of any humidifiers that are installed in a forced air furnace.

Heating water on the kitchen range adds humidity, but it goes out thru the range hood , if you have one.
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Old 03-18-2019, 08:35 AM   #13
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Our high efficiency furnace is a sealed closed loop; it draws combustion air in from outside and exhausts to the outside, both through PVC pipe. It has no connection to the air in the house.

For some code reason the furnace installer ran a fresh air vent from a side of the house away from the furnace intake/exhaust to just behind the new furnace. The last thing I need is a deliberate cold air leak, so I have stuffed the vent hose end with insulation.

Our bad old furnace used house air for combustion so cold dry outside air was constantly drawn into the house through cracks, weak door seals, etc.

Either way we have an AprilAire humidifier in the plenum above the furnace, so that is what controls our humidity. It has an outside air temp probe so it can reduce humidity to prevent condensation on windows when the temperature is low. Maybe your installer accidentally or deliberately disabled a humidifier?
For some reason? I think I’d find out exactly what that reason was before I closed it off.
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Old 03-18-2019, 08:43 AM   #14
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Originally Posted by TromboneAl View Post
Two pipes go outside. One for water and one for exhaust. So, it must be getting combustion air from the garage.

Not sure about your other questions, but I'm basing the relative humidity on a number of digital hygrometers around the house. They've usually been between 60 and 70 percent, but recently around 50%.* The weather has been colder, so I think that's it.

I haven't calibrated them for a while:

https://youtu.be/B2MmCRZ4cD0?t=111
If you are above 20 % be happy and leave it be unless sinus problems from low humidity occur. This way you will not be fighting mold spots on your walls.
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Old 03-18-2019, 08:53 AM   #15
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What is proper humidity for your Home ??
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Old 03-18-2019, 09:01 AM   #16
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Is 50% instead of 70% an issue?
How long have you had the new furnace? Much longer than your current cold snap? As you know, cold air doesn't hold as much humidity as warm air.
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Old 03-18-2019, 09:15 AM   #17
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Here's the piping: ... I think it takes in garage air from the upper PVC port. The big and little PVC pipes go to the outside. We only have heating—no AC. ...
Al, I think the large pipe is the furnace exhaust and the small one is the condensate drain. As you say, the open port is the intake air.

I am 99% certain that this installation would not be permitted in our state -- they would insist that the intake air be taken from the outside. I suggest that you call your local building inspection department, describe the installation and see if they think it's OK. It may be. I don't know.

#Jerry1, I am not at all worried about plugging the outside air vent. We engineers know everything of course, and I know that our 50 year old house is not so tight that we need an artificial air leak. Also, the furnace (and the vent) has been in for maybe 10 years now and I have seen no ill effects. An ounce of data is worth the pound of theory that put that vent into the code "just in case."
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Old 03-18-2019, 09:35 AM   #18
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What is proper humidity for your Home ??


I think 25-50% is considered normal. I bought several of these units to monitor various areas. I’m not sure about accuracy but they seem to run within 2% RH and 1’F of each other which is good enough for my application.
AcuRite 00613 Humidity Monitor... https://www.amazon.com/dp/B0013BKDO8...p_mob_ap_share
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Old 03-18-2019, 09:48 AM   #19
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Here's the piping:



I think it takes in garage air from the upper PVC port. The big and little PVC pipes go to the outside. We only have heating—no AC.
I'm not really crazy about this installation, but I'm not ready to say it's wrong.


1) Combustion air source: A garage is not a great place from which to draw combustion air. It is a location that can have combustible vapors from spilled fuel (from a sloppy refill of a lawnmower, a slow drip from a car fuel line, etc), which is why gas water heaters must generally be located some distance off the floor (18"?)-- to give at least a little margin if heavier fuel vapors are present, they at least need to fill the room for a bit before they reach the ignition source and explode. Also, garages can be more damp than the outside air (a wet car pulled in to the garage brings a lot of water with it). That moisture isn't good for the furnace, and bringing more moisture into the inside of the heat exchanger 24/7 year round will not improve its expected life.


There's already a PVC flue with a route to the outside, IMO not running a parallel PVC pipe for intake air is shortsighted when it could eliminate a fire risk and extend the life of the equipment.



2) Gas connection: The connection as shown is legal and common in a lot of places. Other places would require use of black iron and a drip leg/sediment trap between the gas shutoff and the appliance. If I were doing it, I'd put a sediment trap right at the furnace, required by local code or not. But, people differ on this, some think it is a waste of time.


2012 IFGC code:

Quote:
2012 CODE:408.4 Sediment Trap. Where a sediment trap is not incorporated as part of the appliance, a sediment trap shall be installed downstream of the appliance shutoff valve as close to the inlet of the appliance as practical. The sediment trap shall be either a tee fi tting having a capped nipple of any length installed vertically in the bottommost opening of the tee as illustrated in Figure 408.4 or other device approved as an effective sediment trap. Illuminating appliances, ranges, clothes dryers, decorative vented appliances for installation in vented fi replaces, gas fi replaces and outdoor grills need not be so equipped.
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Old 03-18-2019, 09:55 AM   #20
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... The connection as shown is legal and common in a lot of places. ...
I missed that. Those fragile flex hoses are not legal in our state either. Another question to ask the building inspectors. In fact, I think that exposed PVC electrical cord would also not be permitted. Conduit or Greenfield (armored cable) would be required here.
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