How many air intake pipes do I need for a furnace in a sealed closet?
4airconcern
| Posted in Mechanicals on
I have a furnace in a bedroom closet. Its very noisy. It is an 80 percent efficient furnace. Tempstar DC80.
There is 1 3inch pvc air intake pipe and one 3 inch metal exhaust pipe. Also, there is an air return coming from the living room. There is a louvered door on the closet that the furnace is in.
So, i want to put a solid door on the closet with weatherstripping and a door sweep, cement board, plywood, drywall, and fire and soundproofing insulation, to really deaden the sound.
My question is, do i have enough air intake now to put a new door and seal up that closet?
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I couldn't find the installation manual online. What is the full model number?
If the 3" PVC is a combustion air intake (as opposed to a ventilation air intake in to the return plenum) it's probably OK to seal the closet. If the door louvers are the combustion air path you can't.
If the "-80" in "DC80" is 80,000 BTU/hr fuel input rate, the deeper problem might be ridiculously (way sub-optimally) oversized for your actual heating load, but it might refer to 80% AFUE (the clue being the metal exhaust). What is the BTU rating of this furnace?
Hi Dana,
Thanks for your reply. I have attached a photo of the model number and the whole sticker on the inside of the furnace. In case it is too small and you are not able to make the picture bigger by moving your fingers apart. It says something about input 75,000 btu/ hr.
Hi Dana,
Model # NTC7075BFA3. THANKS!
Hi Dana,
I will take a picture of the pvc pipe attached to furnace.
One less drastic swap is changing out the blower to ECM. ECM blowers are significantly quieter and consume about 1/2 the electricity.
There was an older thread on here:
https://www.greenbuildingadvisor.com/question/swapping-ecm-for-psc-fan-motor
It would still be nowhere near as quiet as a modern ducted mini split.
>"It would still be nowhere near as quiet as a modern ducted mini split."
Just the burners on a 75K furnace are louder than a beefier class 3 ton ducted mini-split capable of driving substantial static pressures.
Per the user manual that furnace has 4 possible blower speeds, and it might be that simply dropping the blower speed will bring the noise under control. See the air flow data chart on page 2:
http://icpindexing.toddsit.com/documents/086477/44121200105.pdf
Assuming it's something like 2-3x oversized for the actual heating load (typical) a 2 or 3 ton ducted mini-split that might fill the bill could be one of these:
https://hvacdirect.com/hvac/pdf/24RGLXD.pdf
https://ashp.neep.org/#!/product/25350
34 dbA @ max speed, 26 dbA at low, where it would run most of the time, delivers over 22,000 BTU/hr @ +5F.
https://hvacdirect.com/hvac/pdf/36RGLXD.pdf
https://ashp.neep.org/#!/product/25352
37 dbA @ high speed, 30 dbA at low, where it would run most of the time, delivers nearly 33,000 BTU/hr @ +5F.
There are others out there that may be more appropriate, with different outputs & turn-down ratios with mid-30s dbA at max speed.
It's an expensive solution to a "mere noise problem", but when right-sized for the heating load it can be more comfortable and even cheaper to operate than 80% efficiency gas (or even condensing gas), and you'll never hear it unless actively listening for it.
Does this look like the right manual?
https://www.utcccs-cdn.com/hvac/docs/1011/Public/03/44101200303_ICP_16077.pdf
If that's the manual, the short answer is that the second port can be hooked up or not, but even if it's hooked up you still need the louvered door, unless you make comparable sized holes to the outdoors to provide makeup air.
(If your house needs ventilation the 5" port to the return plenum would be a lousy way to provide it, so it's better to just leave that alone.)
The long answer:
At the bottom of p1, left column:
"This furnace is dual certified. The means that the INLET pipe is
optional. See Figure 1 for identification of INLET and OUTLET
pipe. Combustion air can be drawn from outside the structure or
inside the structure."
There are two air inlets, labeled T and Y in the Figure 1. Read the discussion on air inlets starting on p.6. If it's just the ~3" combustion air vent (labeled "Y") it may not be good enough, depending on local code regulations, but it's not clear that piping the the ~5" vent (labeled "T" ) to the outdoors would be satisfactory (or necessary). That port is merely a "ventilation air" input that mixes (potentially) outdoor air with the return air plenum when piped to the oudoors.
In the discussion on the right column of p.7 covering "Ventilation Air" the requirement that the mixed incoming air entering the heat exchanger must not drop below 60F or destructive condensation may occur in the heat exchanger.
The next section under Direct Vent Installations:
"In a closet or utility room installation, it will be necessary to provide
ventilation from an area of adequate air supply."
It then goes on to explain that it can be either openings to the interior of the building (as in a louvered door) or holes to the outdoors, but notably does NOT show it piped to the outdoors (indoors or anywhere else) from the 5" port. Connecting that 5" port seems purely optional, but there still has to be the "makeup air" to the enclosed space, most likely to provide sufficient dilution air to keep the exhaust stack operating properly. The port for ventilation air does NOT provide makeup air, only (optional) ventilation air.
On another note...
A ~75K-in 80% furnace delivers ~60,000 BTU/hr output, which is quite a LOT of heat. (It's enough heat to keep my 2x4 framed sub-code 2400' antique house + 1600' of insulated basement at 70F indoors down to temps below -40F. If it's more than 1.4x oversized for your 99th percentile heat load it's even less comfortable. Since it's noisy and bothering you it may be worth swapping it out for a right-sized modulating ducted heat pump solution, some of which are VERY quiet (under 35 dB-A, which is whisper quiet.) To get a handle on your design heat load and oversize factor, run a fuel use based heat load calculation using these methods:
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
Short of that, the size of the house, approximate insulation levels, and location/post-code (for design temperature purposes) would be enough to at least take a WAG at at.
Hi Dana,
Im not sure if that is the right manual. I dont have one around anywhere. I really hope its not the right one, bcuz it sounds like i wont be able to put a new door on there and seal it up.
Im not familiar with all those terms you spoke of such as, makeup air, ventilation air, 5 inch port, plenum. Can you explain those terms to me? The white pvc pipe in my photo i sent, was that for combustion air? Was it attached to 5 inch port on the plenum?
Can you be sure that was the right manual?
Thanks Dana
Ventilation air is outdoor air that gets put into the stream of air in your heating ducts. If there is a connection for a 5" diameter pipe on your furnace, it would match the specs in that manual for where to hook up outside ventilation air.
Makeup air is extra air that needs to be mixed with the exhaust for the exhaust to work properly. Almost all ~80% efficiency equipment needs makeup air. The louvered door is what is supplying the makeup air for your furnace.
The 3" PVC pipe is for "combustion air". It is internally piped to the chamber in the furnace with the burners. Using outdoor air for the combustion air is generally safer than using room air, limiting risks of air pollution or fires related to ignition startup. But it doesn't affect the exhaust operation on this model.
I can't be 100% sure if it's the right manual since I'm not there to measure and compare features on the furnace against specifications in the manual, such as the dimensions in the tables in Figure 1, but the ,model part numbering system is consistent makes me 99% sure it's the right manual. There may have been subtle year on year changes where it doesn't match perfectly, but the need for makeup air it almost certainly hasn't changed.
https://www.utcccs-cdn.com/hvac/docs/1011/Public/03/44101200303_ICP_16077.pdf
Other documentation for that series indicates there are 4 blower speeds, but it doesn't explain how to set the speed. If the noise is primarily blower noise rather than burner noise, setting it to the lowest speed will help at least a little. See the Airflow Data tables on page 2:
http://icpindexing.toddsit.com/documents/086477/44121200105.pdf
Hi Dana,
Thankyou for explaining all of those terms. The pvc pipe looks to be 3 and a half inch, and with connectors more like 4 inch, maybe it dont matter though. The exhaust pipe is 4 inch metal. I didnt see anywhere a 5 in. Port. So maybe its not the right manual?
Im wondering about the air return coming from the living room returning to the furnace. Does that not supply makeup air?
If it does not provide makeup air, how many pipes do i need to outside to provide makeup air? 1 or 2? And how big? 1 4in. Pvc pipe? Or maybe 2. 3in. Pipes?
Also, aproximately how much does a modern ducted mini split cost?
Would it fit in a small closet?
>"Im wondering about the air return coming from the living room returning to the furnace. Does that not supply makeup air?"
The return air is separated from the closet air by the duct & air handler walls, and cannot (and SHOULD not) be used for makeup air. The return side of the ducts is usually at negative pressure relative to the room air AND the outdoor air and would force a backdrafting condition.
>"Also, aproximately how much does a modern ducted mini split cost?
Would it fit in a small closet?"
A ducted mini-split is quite a bit smaller in size than a 75K furnace, and would definitely fit the space. They're not cheap though. Just the hardware for a 3/4 or 1-ton ducted Fujitsu would run about $1800-2000, a 1.5 tonner would run $2400-2600. With the ducts already installed would probably come in $4-5K , fully installed in competitive bidding, more if a duct system needed to be built. There are others that may be appropriate. But we'd need to get a better handle on what your actual BTU/hr needs are, and at what outdoor temperature. The existing furnace's fuel use can be used to roughly measure the BTU/hr requirements at any arbitrary out door temps using this napkin math:
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
Use a louvered door, purchase a quieter unit or install an adequate amount of blank duct plates cut through the drywall to an adjoining room to that closet
Thankyou all for your help. I guess my cheapest option by far is to have an hvac tech put in 2. 4 in pipes to the outside for makeup air. They will have to be run through another bedroom closet.
For the 75,000 BTU/hr version it requires a pair of 5" rounds, not 4". See Figure 5 on p.6, and Table 1 on p.7:
https://www.utcccs-cdn.com/hvac/docs/1011/Public/03/44101200303_ICP_16077.pdf
To avoid condensation on the makeup air ducts during colder weather they need to be insulated to at least R4. If the ducts are longer than a few feet they should be hard-piped, not flex. At any length flex duct needs to be stretched tight, and any ells need to be hard duct, not a bend in the flex. Also note instruction #3 under Figure 5- one has to be within a foot of the floor, the other within a foot of the ceiling.
For maximum soundproofing benefit the new door should be solid, and weatherstripped (air tight.)
Thanks so much Dana. You have been a great help to me. I truly appreciate you and all your help. I hope all is going well for you. I have the new solid door on the furnace closet, and trim. I still need to put on the weatherstripping and door sweep, and door handle.