3 ms·

I've been waiting to do this in the event of a power outage; that said, I'm really curious what the current draw of my furnace blower will wind up being. There

by jchw 4y ago

I've been waiting to do this in the event of a power outage; that said, I'm really curious what the current draw of my furnace blower will wind up being. There is a label that suggests it has a peak draw of 8.5A, but that seems like a lot. Still, it seems like a nice backup plan in the event of an outage, even if I only have a couple kWh worth of battery storage.

a9h74j 4y ago
Many furnaces use third-party controller boards with well-known suppliers. You might find in the furnace manual that there are different wire-to-board permutations to allow different fan (interior recirculating blower) speeds. From home testing a few years ago, I recall RMS currents 50%-80% of peak rated draw, but could be mistaken.
mikeyouse 4y ago
What's the model number of your furnace / blower? I've been doing a ton of research in this area as I'm remodeling my place and have several furnaces and duct loops to deal with. From what I've seen so far, unless your unit is super old, the 8.5A is almost certainly just a 'peak' startup current that's overstated as well. Your blower should say something like "1/3 HP" and from there it's straightforward conversion of 1HP = ~750W so a 1/3HP would be 250W. On a 120v service, that's 2 Amp. With maybe 15% efficiency loss, at peak speed, the blower motor wouldn't be drawing more than 2.5A. It could/will draw more than that to start, but literally just for a second or two.
- jchw 4y ago
  Thanks, that's useful! That was my thought w.r.t. startup current, since I assume it is an inductive load. The model is a Carrier 58STA090-14 and it is indeed 1/3 HP. Seems likely I could run that load off my car in a pinch, which is definitely a good option to have available.
  - mikeyouse 4y ago
    So per the spec sheet -- the "Full Load Amps" for that model could actually be up to 5.2A per the bottom portion of the 90-14 column second from the right on page 5: https://d1049ui2fjityy.cloudfront.net/userfiles/inriver/documents/dp/31/-t/product%20data%20-%2058st-13pd.pdf https://d1049ui2fjityy.cloudfront.net/userfiles/inriver/docu... That motor has a switch so it could be in 3 different settings -- As shipped, the blower motor speed is set to be faster for AC (more draw in the summer) to prevent the coils from icing over, and then it's shipped to be the slowest fan speed while in heating mode, but it is possible that your installer changed that. If indeed it is in the "slow" speed for heat, then 2.5A would be about right.
    - bombcar 4y ago
      Also anyone with a high efficiency furnace should be away there is a second smaller blower on the exhaust.

by jchw 4y ago

a9h74j 4y ago

Many furnaces use third-party controller boards with well-known suppliers. You might find in the furnace manual that there are different wire-to-board permutations to allow different fan (interior recirculating blower) speeds. From home testing a few years ago, I recall RMS currents 50%-80% of peak rated draw, but could be mistaken.

mikeyouse 4y ago

What's the model number of your furnace / blower? I've been doing a ton of research in this area as I'm remodeling my place and have several furnaces and duct loops to deal with. From what I've seen so far, unless your unit is super old, the 8.5A is almost certainly just a 'peak' startup current that's overstated as well. Your blower should say something like "1/3 HP" and from there it's straightforward conversion of 1HP = ~750W so a 1/3HP would be 250W. On a 120v service, that's 2 Amp. With maybe 15% efficiency loss, at peak speed, the blower motor wouldn't be drawing more than 2.5A. It could/will draw more than that to start, but literally just for a second or two.

jchw 4y ago
Thanks, that's useful! That was my thought w.r.t. startup current, since I assume it is an inductive load. The model is a Carrier 58STA090-14 and it is indeed 1/3 HP. Seems likely I could run that load off my car in a pinch, which is definitely a good option to have available.
- mikeyouse 4y ago
  So per the spec sheet -- the "Full Load Amps" for that model could actually be up to 5.2A per the bottom portion of the 90-14 column second from the right on page 5: https://d1049ui2fjityy.cloudfront.net/userfiles/inriver/documents/dp/31/-t/product%20data%20-%2058st-13pd.pdf https://d1049ui2fjityy.cloudfront.net/userfiles/inriver/docu... That motor has a switch so it could be in 3 different settings -- As shipped, the blower motor speed is set to be faster for AC (more draw in the summer) to prevent the coils from icing over, and then it's shipped to be the slowest fan speed while in heating mode, but it is possible that your installer changed that. If indeed it is in the "slow" speed for heat, then 2.5A would be about right.
  - bombcar 4y ago
    Also anyone with a high efficiency furnace should be away there is a second smaller blower on the exhaust.