Post a reply to the thread: Replaced my A/C this weekend, results of downsizing by 1/3.
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I tried blocking off the closest 20x20 return, blower motor wattage dropped from 359W to 302W using my Kill-a-watt tester. Airflow slightly dropped out of th registers, return grill got real noisy.. Delta T went from 14 to 18 degrees as tested at the register/return with a hand-held IR thermometer. I don't have a thermometer to test delta T @ coil, but when I borrowed one for a previous test I found my IR test is about 3-4 degrees off when compared to the coil readings. Installing a damper for the closest return at the furnace may be the solution to my high SST, it would help balance the returns and be adjustable. Easy to open in winter if I need the airflow to keep furnace temp rise in check.
Originally Posted by Poodle Head Mikey What kind of furnace is it? PHM -------- 91-14
What kind of furnace is it? PHM -------- Originally Posted by 54regcab I'd love to hear more about how to properly reduce the BTU capacity of the furnace.
Hie , can anybody please help me with the esp and pump head calculations . was looking for a simplified and easy to learn way to get heat loads . i would be glad if u could help me with this .
Originally Posted by Poodle Head Mikey This is your own furnace, right? In which case I would reduce the furnace capacity to 45K BTU's and then adjust the airflow to get the correct rise across the heat exchanger. This will also get your cooling air flow into a more correct range. What motor speed are you using now for cooling? What happens to the AC performance numbers if you use LOW speed for cooling? It seems unlikely that you can't find a motor speed to match the cooling but if even LOW moves to much air you can then do this: Measure the side of the blower housing. Cut a piece of sheet metal about an inch smaller in diameter. Cut a 1" hole in the center of it and attach it to the open side of the blower housing - not over the motor - on the other side from the motor. Don't shoot screws where they will hit the blower wheel. You only need maybe three screws. Or use foil tape if the housing can be cleaned up for it to stick. Re-start the system and check the A/C performance. If the suction temperature trends woo low then make the 1" hole into a 2" hole with sheet metal shears and recheck the A/C performance. I'll come back to how to make the furnace smaller later. <g> PHM ------- I'd love to hear more about how to properly reduce the BTU capacity of the furnace.
Originally Posted by RoBoTeq Which is a problem, not a solution... But still not harmful to the blower motor.
Originally Posted by beenthere Would be no different then having an undersized return. Which is a problem, not a solution...
I really like the idea. <g> It's an innovative man's damper. PHM ------ Originally Posted by bigtime Hey, its a poor mans damper.
Originally Posted by SBKold Hey, its a poor mans damper.
Originally Posted by 54regcab Split is measured return grille to supply, not across the coil itself. Don't know how much of a difference it makes. it typically makes a difference as some r6 or less insulated ducts running through and attic pick up 2-4 degrees of heat..I always try and take my reading with astick thermometer right near the entering return plenum and the supply plenum..
Originally Posted by drife678 Id like to see a minimum of 18 degree split where at the return/supply plenum or at the supply duct and near return plenum opening in home? Ive always wondered how much delta T reallty mattered but do use it for at least a start on whether i should look for issues or in combo with sc/sh and pressure readings..
Originally Posted by socotech 14 degrees delta t could be on target. All acording on indoor dry bulb and wet bulb. There are charts for target delta t. where are these charts?
SQ Ft. Rule Originally Posted by ferd1942 It's quite refreshing to see people acknowledge 500 SF/Ton is not cast in stone. On average here in Texas, I'm using 670sq ft. per ton! But you have to remember that I strictly use heat pumps.
SQ Ft. Rule
Originally Posted by bigtime . Maybe cut an access door in the supply or return and start stacking bricks to reduce the size of the duct until you get the right flow.
I think Id either get a variable speed blower or replace the furnace with one for the size cond unit you now have. Dont mess around and start getting mold growing in the house. Gotta get that air cold. If you cant swing the new motor or furnace there are ways to restrict the air. More restrictive filters as have been suggested. Maybe cut an access door in the supply or return and start stacking bricks to reduce the size of the duct until you get the right flow.
If you dropped it to 400 CFM it would probably over heat the motor. Doubt you'll get it blow 290watts to move 800 CFM.
No need to buy expensive test equipment for wet bulb! Trance factory reps use thermometer with short wet shoe lace over the bulb, you can do same thing with k style temp sensors. Take reading when temp stops dropping.
Originally Posted by beenthere Block off part of your air filter. take a amp reading and see if it isn't drawing a lot less wattage. Any speculation what a motor amp draw is unloaded vs. loaded? Is there a risk to running a motor w/o enough of a load?
Originally Posted by Poodle Head Mikey This is your own furnace, right? In which case I would reduce the furnace capacity to 45K BTU's and then adjust the airflow to get the correct rise across the heat exchanger. This will also get your cooling air flow into a more correct range. What motor speed are you using now for cooling? What happens to the AC performance numbers if you use LOW speed for cooling? It seems unlikely that you can't find a motor speed to match the cooling but if even LOW moves to much air you can then do this: Measure the side of the blower housing. Cut a piece of sheet metal about an inch smaller in diameter. Cut a 1" hole in the center of it and attach it to the open side of the blower housing - not over the motor - on the other side from the motor. Don't shoot screws where they will hit the blower wheel. You only need maybe three screws. Or use foil tape if the housing can be cleaned up for it to stick. Re-start the system and check the A/C performance. If the suction temperature trends woo low then make the 1" hole into a 2" hole with sheet metal shears and recheck the A/C performance. I'll come back to how to make the furnace smaller later. <g> PHM ------- I am using low speed, still too much airflow. I may try the block one side trick this weekend. Its worth a shot and cheaper than a motor. Could use a peice of cardboard as a test jig before making something more permaent
Originally Posted by SBKold If I read correctly he is using the 4" filters that go in filter back return grille's. Blocking them off so far away from the unit could lead to increased infiltration . Would you agree beenthere? Id side with PHM and restrict the blower orifice. With filter grilles, yes it will tend to do so. But he can block it off to test wattage draw at reduced air flow. Since he seems to think it will still be high. then block off at the blower opening after confirming lower wattage.
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