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Old 11-15-2011, 08:11 AM   #1
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Closing off HVAC return and supply vents


Hello. I recently bought a split foyer home and there are TWO HVAC returns. a 12x14 grill downstairs (i'm unsure of duct size) that connects almost directly to the blower (on the other side of the wall from the blower) The other return is upstairs, about 6 feet from the blower straight up (that gril is 16x24 I believe. I would like to essentially shut off the vents downstairs (airtight) as well as the return down there and seal the entry door to downstairs to prevent airflow to conserve energy costs. Are there any problems with the IDEA? I've read about static air pressure loss as well as general air pressure inside the vents increasing (causing the air to escape somewhere else.) but all of these issues I believe arise from a difference in proportion to return / supply. If I close the vents downstairs as well as the supply can I not expect the HVAC to run half as hard (assuming the downstairs is the same size as the upstairs)?

btw, the HVAC is Carrier CD5BX though their site was... unhelpful.
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Old 11-15-2011, 08:27 AM   #2
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Closing off HVAC return and supply vents


You will use more energy than you save and it could damage your furnace with overheating.
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Old 11-15-2011, 08:31 AM   #3
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Closing off HVAC return and supply vents


not recommended, your system is designed as an all or nothing system
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Old 11-15-2011, 08:40 AM   #4
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Closing off HVAC return and supply vents


I appreciate you guys' responses thus far. But can you explain why it could potentially overheat the blower? Additionally, what is it that makes my system 'all-or-nothing'. Essentially I'm just trying to change the "All' to only upstairs instead of upstairs and downstairs?

Thanks again for your time.
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Old 11-15-2011, 08:53 AM   #5
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Closing off HVAC return and supply vents


Hello and welcome Kulingar, to the best darn DIY'r site on the web.

The important thing to point out here is your furnace has to maintain a minimum air flow across the heat exchanger in order for the heat exchanger to effectively raise the temperature of the air passing over it. Not enough air flow and the Delta T across the exchanger will increase beyond the manufacturers design recommendations.

Something else is heat rises, so by putting heat in the lower level especially if the outside walls are insulated is not wasted, the heat will rise through the floors, providing more comfortable and uniform temperature throughout the house.

Mark
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Old 11-15-2011, 09:13 AM   #6
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Closing off HVAC return and supply vents


Quote:
Originally Posted by Jackofall1 View Post
The important thing to point out here is your furnace has to maintain a minimum air flow across the heat exchanger in order for the heat exchanger to effectively raise the temperature of the air passing over it. Not enough air flow and the Delta T across the exchanger will increase beyond the manufacturers design recommendations.
Mark
With statements like this, I'm beginning to believe this might be the best DIY forum on the interwebs.

Is there anyway I can find out how much CFM is required at minimum to pass over the heat exchanger? Then, if the single vent won't supply return for it we can consider the option of rerouting the downstairs return to a secondary upstairs return or stopping there. However, if the single vent supplies enough CFM, THEN we can consider the heat rising factor.
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Old 11-15-2011, 09:22 AM   #7
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Closing off HVAC return and supply vents


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With statements like this, I'm beginning to believe this might be the best DIY forum on the interwebs.

Is there anyway I can find out how much CFM is required at minimum to pass over the heat exchanger? Then, if the single vent won't supply return for it we can consider the option of rerouting the downstairs return to a secondary upstairs return or stopping there. However, if the single vent supplies enough CFM, THEN we can consider the heat rising factor.
The CFM ratings will be stated on a plate mounted on/in furnace, this information will be with the model and serial #.

It will state, Delta T at xx CFM for a given fan size and rated HP. You will have to know what the HP rating of your fan motor is to make a qualified judgement as to what section of the information you have to follow.

With this information you could make adjustments to your system based on the Delta T over the heat exchanger.

Mark
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