Hey all-
This may be asked all the time, so forgive me if its a repeat question, but here goes:
I am having hardwood floors refinished in my 1930’s 1-1/2 story house. My first floor is about 900 sq ft, and has 3 return air vents (with 1 more upstairs). The floor refinisher suggested I could remove one of these vents (in the hallway) and patch in new flooring, stating that return vents were often ‘overdone’ in old houses.
This makes sense to me, as the blower in the 1930’s furnace probably would have been less powerful, and need more return vents to lower static pressure. (Just my reasoning here, could be wrong).
I’d like to get rid of the return vent, but only if it is truly unnecessary and won’t affect heating/cooling. I tried using a mercury manometer on the return plenum of the furnace, but it wouldn’t register (i.e., too low of pressure to register).
I know there are probably a ton more variables involved, but can someone help me out with some guidelines as to how one would calculate if this particular return vent is superfluous?
My current test is empirical – I put a sheet of plywood over the return vent, and things seem to be going ok.
Thanks in advance,
Eric.
Replies
Why do you want to remove it? Why not relocate the return to a nearby stud cavity?
Tom
Tommy-
Not sure what you mean exacty by 'nearby stud cavity'...do you mean in a wall as opposed to on the floor? If so, I assume you mean a non-load bearing wall, and just cut out the bottom plate? That is an interesting idea, is that routinely done?
Or do you mean somewhere else in the floor? If so, that is not so appealing, because of room layout there's really nowhere else for it to go.
Thanks,
Eric.
"The floor refinisher suggested I could remove one of these vents (in the hallway) and patch in new flooring, stating that return vents were often 'overdone' in old houses."
And I suppose you're going to get your HVAC technician's advice on the best finish to use on the floors?
Get an HVAC person's advice on this one, not some flooring guy's "often overdone" (Or get a writen guarantee from your fl;ooring guy that following his advice won't screw up your furnace!)
Dear Eric,
Let me echo Bobs advice and try to make this a little scarier for you. Don't block the returns under any circumstances.
The return side of the ductwork is almost always undersized, screwed up, leaky, filthy, disconnected, etc. But it is exactly half of the system. If you block a significant portion of the return, the air in the entire system will move more slowly and deliver less heat.
But that's just the beginning. The fire in the heat exchanger doesn't know that the return is blocked. It just keeps burning and the heat exchanger get hotter. That will stress it and cause premature failure. Or it might get so hot that the high limit switch will trip and shut down the furnace (if you're lucky).
Now you've got no heat and you call the furnace tech. He diagnoses the problem immediately, b/c it's 2am, and has the perfect solution. He opens up the return plenum so that lots and lots of cellar air can get through the system. Nice cool heat exchanger, back to bed for everybody. Except now, the powerful blower on the furnace is sucking air directly from the same place that the flue pipe is drawing.
Chances are good that the air handler will win the battle for your combustion gases, draw them back into the furnace and deliver them through the supplies to the bedrooms. Pleasant dreams.
Regards, Fred
[email protected]
While we are at the subject of return vents, on the second floor of our house, each bedroom has two return vents on the same stud bay, one high close to the ceiling and the other low. They both have a lever to adjust the amount of air flow. Any idea why it is set up this way? Are they for adjustments to the seasons?
"each bedroom has two return vents on the same stud bay, one high close to the ceiling and the other low. "
Close off the high one in heating season (pull the cooler air at the bottom of the room into the return ducts) and close off the low one's during cooling season (& and pull the warmer air at ceiling level out.)
Eric,
Your floor guy is wrong. Seldom, especially in older houses, are returns treated properly, and never "overdone". Since you said there is a single return for the enitre 2nd floor is not indicative of good practice, unless the 2nd floor is a single, open room. I speculate that by removing the floor grill, the floor finisher's work will be easier. And he'll discount his price accordingly? Yea, right.
You have two reasonable options: 1) Leave the vent in place, or 2) have an HVAC professional review the specifics of your house and system, and recommend any changes that can be made.
Thanks guys,
You echoed my concerns. It sure is tempting to follow the refinisher's advice and have a nice, open hallway with no big hole in the floor, but something told me it was put there for a reason originally. Thank you all.
While you echoed my concerns, I was hoping to get some 'rules of thumb' that I could use as a basis for judgement as opposed to the (still valuable) anectdotal advice. Don't get me wrong, I don't expect to be taught everything about HVAC calculations through a newsgroup (as good as this one is...). But are there any quick calcs that, say for example, a home inspector would do during a walk thru that might enlighten me on the subject?
Thanks again in advance,
Eric.
One rule of thumb for return air registers is to limit the air velocity to around 400 feet per minute (FPM). So say the return register is 12"x24". Assume that its half blocked off by grillwork and you have a 1 sqare foot openning. 400 FPM would give you a volume flow rate of 400 cubic feet per minute (CFM). This gets us to the attached return duct size. Use ducts that produce velocities between 400 and 600 FPM. The next question is how much air does the furnace need. Assume you have air conditioning. You want 400CFM per ton of airconditioning (12000btu of ac=1 ton) You need less air flow for heating and that information is given on the manufacturers data placard located inside the front cover of newer furnaces. The supply side is usually operated at a slightly higher velocity. 700fpm max. These are just rules of thumb but they will get you good performance without allot of air noise.
As to your situation. It's true that return air always seems to be cut short on forced air system. About the only cases where the supply and or return duct systems are oversized are on turn of the century homes that used a gravity airflow system with no fans. In these cases the ductwork could be 50% larger than needed when converted to forced air. But, like everyone else has said, you should have a professional look at it befor making any alterations.
Tim,
In response to your comment about the upstairs being a single room- yes it is, or at least was when originally built. A walk-in closet has been added to one side, other than that, it is a single room up there.
As far as I can tell, the house is well built in all aspects, and in general I agree with all comments: I don't assume that the builders overdid the HVAC, nor do I think we are necessarily any smarter today than the builders in the 30's.
My current line of thinking is that the return air vent will stay put.
Eric.
Having an upper and lower return air intake with dampers makes me think that the second floor ductwork is not 1930's vintage. It's just that the upper return intake is used in more modern application durring air conditioning season. Maybe they put it in just to keep the air moving around and cool the second floor.
Gorouser-
Thanks for the info, this is what I was looking for. I appreciate it.
As far as the high & low vent upstairs question - that was posed by someone else. My house was definitely built in '32, and it definitely does not have a high/low return vent.
Thanks again. I've decided, the return vent is staying.
Eric.