Determining CFM rating and BTU/hr for heat pump replacement
I have a Lennox SLP98UH090XV60C-04 98% furnace that is about 13 years old. I am trying to measure BTU/hr using CFM airflow and heat rise so I know how big of a heat pump to replace it with. But before I can calculate it I need to measure the static pressure to find the airflow so I got my manual out but I could not find the typical airflow/static pressure table for this model.
However within my Lennox IComfort display for the furnace I found this screen, as you can see that at 35% heating (lowest) it shows a default of 600CFM. Not trusting the screen, I ran a quick test and I watched the RPM of the motor at 35% heating change when I removed my air filter, so my thought was this blower motor will maintain that 600CFM as long as the static pressure is not above 1.0 (what the manual says, in heating mode my total static pressure is well below that.)
Can I trust this 600CFM number for my heat rise formula?
600CFM X 1.08 factor X 56.7F heat rise = 36,742 BTU/hr at 0F outside (design temp in Boulder)
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Oh, if you are curious how I tested the RPM when removing the filter, see this image.
My guess is the CFM number the thermostat displays is approximation based in the frequency voltage and current the motor is using. Not sure I would select new equipment based on that number.
I hate to see working equipment sent to the landfill simply because it is a few years old and someone has become afraid it might fail at inconvenient moment. The green thing to do is keep it in service if possible and delay the mining, manufacturing and transportation of the new equipment as long as possible.
Walta
There two seperate questions you need to answer.
-what is the heat load of the house
-how many CFM the ducts can carry
For the first one, it doesn't matter what your existing furnace is. You want to run a calculation based on your fuel use:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
For the 2nd, you need a decent differential pressure gauge like a Series 2000 Magnehelic rated at 1" WG and take a couple of measurements with the airh handler on max. The CFM reading there is close enough, so you can use that.
You need:
-pressure drop at the intake from ambient to just after air filter
-pressure drop across your air handler (from after the filter to after supply plenum above the AC coil)
-pressure drop from your supply duct (from plenum to ambient)
The fuel use calculation will tell you the size of the heat pump you need. From there you can look at the fan curves of the unit that will work and see if your ducting will support it.
Akos, thanks for your input.
I don't own that pressure gauge but I do own a UEI EM151. Here are the measurements:
600 CFM 35% heat (maintains temp at 5°F, but will drop 0.2F per hour at 0°F which is my design temp):
0.08 across the MERV16 filter
0.14 across the furnace
0.07 across A coil
1750CFM max Cooling: (blower is rated for 2000CFM but my ducts/filter can only handle 1750CFM from my testing):
0.41 across the MERV16 filter (this matches the Final static pressure from the MERV16 filter spec)
1.01 across the furnace (my furnace is rated at 1.0 static pressure)
0.23 across A coil
I have run a calculation from your link you provided and I get 32,828 BTUh. This is close to the 36,742 BTUh heat rise calculation I did, and the 39,855 BTUh gas meter reading calculation I also did. My furnace rated output is 87,000 BTUh, 35% of that is 30,450 BTUh.
Walta100: one option would be to keep the furnace and just add a heat pump to it, I would keep using the blower until it dies. It is 13 years old, running at a 1.01 static pressure in summer not sure how much it will last?
I have to think that Lennox spent millions are R&D on this furnace, since they know the exact RPM and the exact power going to the blower they know the exact airflow running through their cabinet which they can display the CFM on the display screen with confidence.
Sounds like you are at about 3ton of heat loss. Something like this would be in the ballpark:
https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006/M_SUBMITTAL_SVZ-KP36NA_SUZ-KA36NAHZ_en.pdf
or a bit oversized:
https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006/M_SUBMITTAL_PVA-A42AA7_PUZ-HA42NKA1_en.pdf
Other companies like LG/Midea/Samsung make comparable units, some with even better 0F performance.
The unit is around 900CFM max which is well under what your ducting can handle. The PUZ is a bit more flow and it might be a bit tight on high speed, should not be an issue as you would not have to run at max speed.
If you are worried about those polar vortex days, the unit can take a heat strip. The smallest Mitsubishi one is 5kW or 17000BTU, which should be plenty. If you have the electrical service capacity for 8kW, it would probably work as backup to keep your place from freezing in case of HP issues.
Akos,
Thanks for the 2nd recommendation, that looks like a good fit for my house.
As for heat strips, I just installed a direct vent gas fireplace with a blower fan. It's rated at 19,098BTU input at 72% efficient, so it does about 13,750BTU on its highest flame setting. I ran a test for a few nights where I turned off the furnace and the fireplace could hold temperature when outside got down to 22F at night. So I think I could add in the fireplace while the heat pump is running instead of adding heat strips to the unit when it gets down to -14F for a few nights out of the year. The additional ton of heating from the fireplace should do the trick?
Also I just ran 10AWG wire to the basement for new heat pump water heater a few years ago and dry-walled after I was done, I would hate to tear out the new drywall for another wire run for the heat strips.
Thanks again for your help!
Dave