cold air returns

[iron]

we are planning to demo some interior walls to open things up in the house. i have investigate them all and most are non load bearing (one is, it'll be replaced by a beam).

one of the walls which is currently part of a coat closet contains 1 of 2 cold air returns for the main floor of our house. both returns are 10"x24". the returns are located about 10ft apart from each other, with the one we'd like to keep located directly below the thermostat. see attached pic for approx layout.

my question is: can i just abandon/seal off the air return in the wall we want to demo, and double the size of the other return to keep a roughly = air volume handling?

thanks

 

[johneh]

I had my youngest take a look at your drawing he is HVAC Teck
The shot answer is yes you can shut off the one in the closet and
increase the other .
But he also recommended you move the thermostat to the wall across
the hall so that is not in the return air flow and that the bed rooms
should have a cold air return so that you get better air flow in all rooms
I know just what you needed more work

 

[iron]

thanks. super helpful.

i don't think i'm going to do the returns in the bedrooms since they seem to heat just fine right now and that would be a serious amount of work for limited returns. if there's a problem, i will consider it.

did your youngest suggest that doubling the size of the single cold air return would be sufficient?

 

[johneh]

Yes you will have to increase the size of the return you will
need at least the same as the original amount .
Do move the stat so as not to be pulling cool air over it
You could get a cooler reading than what the real air temp is.

 

[iron]

bummer. really don't want to move the stat. seems like a lot of work there as well. we could always just raise/lower the stat's settings based on what we're feeling vs. what it says we should be feeling...

 

[semipro]

Hmm. Why are there 2 returns? Do they both connect to the same air handler?
In general, I don't think it does much good to have a return that is significantly larger than the inlet at your air handler regardless the size of the two original returns.

 

[johneh]

I may be wrong but I think he was talking about a furnace
Not air handlers and you don't want to restrict return air to a
Furnace

 

[burnham]

bummer. really don't want to move the stat. seems like a lot of work there as well. we could always just raise/lower the stat's settings based on what we're feeling vs. what it says we should be feeling...

Don't move the thermostat. The return air going back to the furnace shouldn't be much cooler than the desired temp anyway. I'm an electrician and wire a lot of new houses, the HVAC guys always have me put the thermostat near a return. The fact that the air is moving doesn't change the air temperature.

 

[moey]

If it were me Id figure out a way to add a return in your living room maybe a floor return although those are less desirable. Keep in mind if you only have one return you need a larger size all the way to the air handler not just a larger wall grill. It also may be quite loud pulling that much air. You may want to spend $50 for a nanometer if your messing with air returns.

 

[iron]

yes, it's an electric furnace with a heat pump. seems to work pretty well as the house heats up quickly and easily. we leave it off during the day in the winter, and the house will drop to 50* or so. after getting home, turn on the furnace and it heats up to 68* in about 15 minutes and runs vary sparingly - especially for a 1979 house.

if i were to test sealing off the return, what should i look for? i'm thinking i could just put a piece of plywood and caulk the current return (the one to be sealed off, ideally). that should simulate my intended final condition, no?

just doing a hand test (placing my hand in front of the returns), both of them seem to have very low air flow.

 

[begreen]

The diagram doesn't indicate the location of the furnace. Usually the return trunk duct will increase in size closer to the furnace as it picks up the addition return grille. Increasing the size of the one return to 20 x 24 will only work if it is closer to the furnace and on the larger part of the trunk duct. It should be closed off at the trunk duct, with sheet metal.

 

[iron]

the furnace is in the garage, almost directly below the cold air return i'd like to keep - maybe offset 5ft. it seems the trunk is rather large for a good portion of the basement, but there's a finished ceiling, so i am not 100% certain.

 

[Highbeam]

The fact that the air is moving doesn't change the air temperature.

I agree and it is a common mistake. Non-living things don't "feel" wind chill.

 

[DoubleB]

i don't think i'm going to do the returns in the bedrooms since they seem to heat just fine right now

I don't think the advantage of returns in the bedrooms is to get the heat to the rooms, rather I understand the benefit is to help prevent the rooms from getting pressurized when the doors are shut. Pressurized rooms push warm air outdoors from the bedrooms and suck cold air into the rest of the house to make up the difference. (And also in summer too with air conditioning.)

My bedrooms don't have return ducts, either, but we leave the doors open a couple inches to keep the warm air inside the house.

 

[iron]

so i went ahead and blocked off the old cold air return in the subfloor. then, we installed oak flooring over the subfloor, so it's pretty covered up now.

recently, when using the furnace, it seems to take way longer to get the house up to temp. and, it seems to blow out colder air, but this is subjective, not objective. some of this could be due to colder outside temperatures + using an old heat pump. one thing for sure is that the pressure difference between our bedroom and the hallway is significantly higher as the hallway cold air return is clearly working harder to make up for the vent we decommissioned.

so, my question is: did i screw up our furnace? what's a good way to measure this? any tips for troubleshooting?

if need be, i can always reopen up the wood floor and put a vent grille there, but that's low on my preference chart.

 

[begreen]

Possibly. One shouldn't block off a return or supply without knowing how closely balanced the system is. It's possible that the blocked off return will need to be reopened or relocated. Not doing so can cause the blower to race and burn out prematurely.

 

[chew72]

Did you increase the other return yet? Also it has to be increased cold air duct and all right back to the furnace.

 

[semipro]

This may help you understand some of the diagnostics. http://www.achrnews.com/articles/103710-interpreting-duct-static-pressure
In general, pressure and/or flow in the ductwork is used assess whether flow is overly restricted.
One symptom of an undersized return is excessive air flow noise.

 

[moey]

Spend some money and measure the static pressure. Buy something similar to this they are about $30.

http://www.ebay.com/sch/i.html?_fro...H0.TRS0&_nkw=Dwyer+Magnehelic+Gauge+&_sacat=0

Having a lower temp does not really make sense unless your return is running through a unconditioned space and the air is cooling more rapidly then before somehow. Your basically providing a lift in temp of 20-30F by the heat pump. If you notice a pressure difference doors slamming air movement youve got to add a return somewhere if nothing more then to be comfortable.

 

[iron]

ok, i haven't purchased the pressure tester yet. seems like there's a bit more to it than that.

i agree that it seems weird that the temp seems lower. now that outside temps have warmed up a bit, that seems to help (probably more of a heat pump issue on that matter). still, it takes a lot longer to reach the temps we want than it did before.

general question: if i were to open up the now-closed-off air return using the basement ceiling instead of the new wood floor, would that work or would i be short circuiting the system since it would be using warmed air (assuming we're heating the basement)?

also, i'm curious if the supply vents are part of the issue: the four vents that previously had carpet around them and were metal vents are now oak vents that almost certainly have less % of opening. i've also temporarily abandoned one of the supply vents (currently under the floor), but will daylight it when we know the final location of our island.

so yeah, lots of variables that i've introduced into the system. i should've thought through it more...

 

[moey]

Usually changing a vent style just causes the surface velocity to increase ( more noise ) . Unless you've seriously restricted them.

Usually basement are better off under slight positive pressure. So if your just sucking air from a return with no supplies anywhere thats not good. If its finished and there are already supplies there you can pull some air from there. If Radon is a problem in your area tread carefully on adding a return in the basement.

 

[iron]

thanks for the feedback.

while not objective, i feel like the system is slightly louder now than it was before. this could be a side-effect of not having carpet in the living room anymore to dampen the sound.

the basement currently has a large return at the bottom of the stairs which shares the same wall as the furnace (opposite site). were i to install the return in the ceiling, it'd be probably 6ft horizontal from that floor return. seems kinda close, though not much different than where it was before relative to the upstairs hallway return.

 

[iron]

so i had an HVAC tech come over last night to measure things. he had the following readings:

supply static: 0.2"
return static: 0.01"
delta T (temp differential between supply and cold air return): 32* F
supply air temp: 100* F

so, it seems like everything is within a good working range of where it should be (according to the tech). i'm still going to daylight a cold air return, but i will do so in the basement ceiling instead of the oak floor upstairs. supposedly, you can never have too much cold air.

i also need to clean my furnace coils and give it a general tune up. anyone have a good tutorial they could point me to for an electric furnace?

thanks for everyone's help!