How do I know if my return airs are adequate/sufficient? I have an old 1 1/2 story farmhouse that has been all renovated. air source heat pump/central air with 93% propane backup furnace. Unit is in basement with 1st floor lines through crawl space/floor joists. There are only two 6x12 returns on ground floor. A main trunk runs vertical to attic and runs/registers for 2nd floor are in ceiling. One return air in central hallway upstairs (6 supply runs). The system is basically all new in last ten years but honestly kind of a conglomeration of remodels..... Just curious what I can do to try to measure/improve overall system effectiveness/efficiency? Any input is appreciated.
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Return Air Sizing?
- Thread starter OldNeons
- Start date
sands35
Well-known member
With supplies in the rooms and the return in the hall, you will get drafts under the doors. It's up to you if that is a problem. If it is, then install returns in each room opposite from the supply.
With an old farm house, you probably loose more through windows, walls and doors than you will miss from an internal AC system. Just make sure that the supply lines are insulated in the attic and the system is approximately balanced.
Do you have exhaust fans in the bathroom? That will go a long way to control the ambient conditions in the house.
With an old farm house, you probably loose more through windows, walls and doors than you will miss from an internal AC system. Just make sure that the supply lines are insulated in the attic and the system is approximately balanced.
Do you have exhaust fans in the bathroom? That will go a long way to control the ambient conditions in the house.
Thanks Sands, I think my doors are pretty snug to carpet upstairs and kids are getting to age they want to close them so I may cut off doors a bit. Adding returns will be a major project so trying to avoid that? Maybe old farmhouse was a bit misleading. We have removed all lathe/plaster to wire and insulate and all new casement windows. Supplies in attic are insulated.
Balanced?? I have NO idea, this is more the focus of my question I guess. Please elaborate. Do have bath fans, what does this do for me and should I be running them more or is it just getting rid of heat/humidity from shower?
Balanced?? I have NO idea, this is more the focus of my question I guess. Please elaborate. Do have bath fans, what does this do for me and should I be running them more or is it just getting rid of heat/humidity from shower?
pseudorealityx
Well-known member
Instead of returns to each bedroom, provide an indirect transfer grille or undercut the doors more. You just want to make sure you have a path for the air to move.
Falcon67
Well-known member
You will want to seal all the ducts at the outlets, any splices, at the plenum and all the return chases (such as there are). If you imply that your return area is a total of 12"x12" then you need to have that thing looked at by a HVAC person that can figure out what it needs. Our 3 ton uses a 20x20 return and a return plenum area 4'x3'x18" tall. The retuning air volume will **** a towel up to the grill.
Having lived in a two story, I'd bet you are/will have problems. And doors that hug the rug will make it a lot worse. Air has to move to do it's job - no moving = no cooling or heating. The two story we had used a return chase in the middle of the house with return grills 16x20 on both floors. That was a 3.5 or 4 ton unit as I recall.
Having lived in a two story, I'd bet you are/will have problems. And doors that hug the rug will make it a lot worse. Air has to move to do it's job - no moving = no cooling or heating. The two story we had used a return chase in the middle of the house with return grills 16x20 on both floors. That was a 3.5 or 4 ton unit as I recall.
sands35
Well-known member
re: air balance. The "real" way is to rent or buy an anemometer to measure CFM out of the registers, then play with them to get each room supplied with the right amount of air so that the number of air changes is about the same (bigger rooms need more air).
The DIY way is to just hold your hand over the registers and play with them so each room "feels" the same. Of course that gets hard depending on time of day and where the sun is, etc. Over a few days, I'll play with them so that each room is about the same temp - then mostly not pay attention anymore.
re: bath fan. The recommendation is to size the fan so that the air is changed out every 10 minutes. CFM of the fan needs to be the same or bigger than the volume of the bathroom. As long as the mirror doesn't fog too much, everything is OK. The problem with no or under sized bath fans is that moisture needs to go someplace. With a central return it's going to go into the hall.
Yes to pseudo's suggestion. Air needs to move.
Basically, the return(s) needs to be as big as the supply.
The DIY way is to just hold your hand over the registers and play with them so each room "feels" the same. Of course that gets hard depending on time of day and where the sun is, etc. Over a few days, I'll play with them so that each room is about the same temp - then mostly not pay attention anymore.
re: bath fan. The recommendation is to size the fan so that the air is changed out every 10 minutes. CFM of the fan needs to be the same or bigger than the volume of the bathroom. As long as the mirror doesn't fog too much, everything is OK. The problem with no or under sized bath fans is that moisture needs to go someplace. With a central return it's going to go into the hall.
Yes to pseudo's suggestion. Air needs to move.
Basically, the return(s) needs to be as big as the supply.
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please provide your model number of air unit so I can get an output cfm, from there we can get an actual open sqin you need for returns
two 6x12 for downstairs and one for upstairs for return. Correct? can you provide a picture of grille used for return air, just to estimate clear space
If doors are being closed you will need a minimum of 1" clear under cut on doors if they are 36" wide. with smaller doors I would suggest a transfer grille over door.
two 6x12 for downstairs and one for upstairs for return. Correct? can you provide a picture of grille used for return air, just to estimate clear space
If doors are being closed you will need a minimum of 1" clear under cut on doors if they are 36" wide. with smaller doors I would suggest a transfer grille over door.
pseudorealityx
Well-known member
re: air balance. The "real" way is to rent or buy an anemometer to measure CFM out of the registers, then play with them to get each room supplied with the right amount of air so that the number of air changes is about the same (bigger rooms need more air).
The DIY way is to just hold your hand over the registers and play with them so each room "feels" the same. Of course that gets hard depending on time of day and where the sun is, etc. Over a few days, I'll play with them so that each room is about the same temp - then mostly not pay attention anymore.
re: bath fan. The recommendation is to size the fan so that the air is changed out every 10 minutes. CFM of the fan needs to be the same or bigger than the volume of the bathroom. As long as the mirror doesn't fog too much, everything is OK. The problem with no or under sized bath fans is that moisture needs to go someplace. With a central return it's going to go into the hall.
Yes to pseudo's suggestion. Air needs to move.
Basically, the return(s) needs to be as big as the supply.
Neither supply air or exhaust air should be sized based on air changes in a residential application.
The exhaust should be 50 cfm per code. The supply air to a room should be based upon the HVAC load calculations, which are based on construction, orientation, glazing, insulation, equipment load, occupant load, lighting load, weather data, etc.
For 95% of residential applications, the HVAC units do not bring in any outside air, and therefore running an exhaust fan for any length of time is just 'forcing' infiltration of outside air into the house, and is not ideal.
Also, the return should be BIGGER than the supply main. You want to run the return duct at a lower friction factor.
sands35
Well-known member
Sure, you need to do the math. Visit a house where rooms are not balanced and you will know imediately. Following code is fine, getting the rooms comfortable is another thing entirely. The house ca meet code, but not have even distribution between room.
If the OP's system is a retrofit, then i'll bet the math was not done.
Bath fans are not supposed to be run continuously. Not sure where that idea came from.
Returns. Good luck getting bigger returns in a retrofit. Again, this is a retrofit. Yes, the math needs to be done. I doubt it was. If you want to nit pick, i should have said at least as big. Get them too big and particulates won't stay suspended. Too small and you get system losses and too much noise.
If the OP's system is a retrofit, then i'll bet the math was not done.
Bath fans are not supposed to be run continuously. Not sure where that idea came from.
Returns. Good luck getting bigger returns in a retrofit. Again, this is a retrofit. Yes, the math needs to be done. I doubt it was. If you want to nit pick, i should have said at least as big. Get them too big and particulates won't stay suspended. Too small and you get system losses and too much noise.
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Falcon67
Well-known member
>getting the rooms comfortable is another thing entirely.
This in spades. We have an adequately sized unit but still have one room (bedroom, end of the longest run) that doesn't have enough return to really stay with the rest of the house. No amount of jacking with vents helps it, it's a return issue and maybe a volume issue. You can close the other ent on that run and it doesn't change the feel of the room. It's the farthest from the return and exposed on three sides to the south sun. The room is like the others, well insulated with a high end vinyl Low E window. It's fed by a 6" duct that maybe needs to be an 7 or 8". It could use a return by the door but with an upflow closet unit it'd be a weird deal to try and plumb a return into the plenum 30' away. It could be "done" but would it be worth "doing".
This in spades. We have an adequately sized unit but still have one room (bedroom, end of the longest run) that doesn't have enough return to really stay with the rest of the house. No amount of jacking with vents helps it, it's a return issue and maybe a volume issue. You can close the other ent on that run and it doesn't change the feel of the room. It's the farthest from the return and exposed on three sides to the south sun. The room is like the others, well insulated with a high end vinyl Low E window. It's fed by a 6" duct that maybe needs to be an 7 or 8". It could use a return by the door but with an upflow closet unit it'd be a weird deal to try and plumb a return into the plenum 30' away. It could be "done" but would it be worth "doing".
pseudorealityx
Well-known member
Any reason you cannot provide a transfer grille to aid return?
Thanks for all the input guys. I will get specs on unit and some basic dims on ductwork to guy give you an idea on what I have. It's a 100+ year old house - but it's mechanically all new (wiring, plumbing, hvac system including ducting, insulation, drywall, windows/doors, are all new in last 15 years). I think the quickest, easiest thing I can do for upstairs (warm in summers!) is to cut off bottoms of doors. I also wonder about the upstairs stairwell door - I normally try to keep it closed ALL the time (to keep all the warm air from going upstairs and making downstairs too cool in winter, and upstairs even hotter in summer). Is this what would be recommended? On another note, I've heard in a situation like mine it's best to run the system fan continuously to keep air moving. True?
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bazar01
Well-known member
On another note, I've heard in a situation like mine it's best to run the system fan continuously to keep air moving. True?
In a way it is good because it distributes and evens out the temperature to the whole house.
But it is also bad in another way because when in cooling mode and the compressor stops, the condenstate pan inside the air handler maybe full of condensate and when you leave the fan on, you will put the moisture back inside the rooms and raise the humidity. Check your air handler drip pan if it is holding water. If it is not, then run the fan continuosly. Some brands do not hold water if unit is pitched correctly.
pseudorealityx
Well-known member
I think the quickest, easiest thing I can do for upstairs (warm in summers!) is to cut off bottoms of doors.
Probably.
I also wonder about the upstairs stairwell door - I normally try to keep it closed ALL the time (to keep all the warm air from going upstairs and making downstairs too cool in winter, and upstairs even hotter in summer). Is this what would be recommended?
I would keep it open. Reduce the openings on the vents in the lower floor, and force more air to the 2nd floor. Keep that stairway door open, and you won't get the flow up the stairwell, but rather down, or at least static.
On another note, I've heard in a situation like mine it's best to run the system fan continuously to keep air moving. True?
I would also suggest this.
I would keep it open. Reduce the openings on the vents in the lower floor, and force more air to the 2nd floor. Keep that stairway door open, and you won't get the flow up the stairwell, but rather down, or at least static.
.[/QUOTE
Does the suggestion for the stair door apply equally for heating and cooling? Could you explain in more detail please? Thanks
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Falcon67
Well-known member
Any reason you cannot provide a transfer grille to aid return?
If in replay to my post - we're considering it. There is a space above the door that faces a short hallway into the main living space. However, we leave the door open all the time and when it's warm run a small floor fan under a hall table pointed at the room. We did have the room thermal imaged and didn't see anything unusual. LOL/ARRG - unlike the dining room where it was obvious the builder ran out of batt and left a 12" blank area inside the wall at the corner, top of the wall.
pseudorealityx
Well-known member
I would keep it open. Reduce the openings on the vents in the lower floor, and force more air to the 2nd floor. Keep that stairway door open, and you won't get the flow up the stairwell, but rather down, or at least static.
.
Does the suggestion for the stair door apply equally for heating and cooling? Could you explain in more detail please? Thanks
In cooling, you want the vents on the 2nd floor 'more open'.
In heating, you want the vents on the 1st floor 'more open'.
In both cases, I'd leave the stairway door open, as long as both floors are occupied and being used.