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Main Question

I've already visited this similar, but different question and saw a commenter mention that about 500W seems appropriate for blower fan of an HVAC system. The past few months we've had high electricity bills compared to the same period last year (lived here 2.5 years), but it's also been a hot summer. I've plotted the kWh that have appeared on my statements (for science!); the increase begins in March.

Historical Usage

To try and investigate, I purchased "The Energy Detective" (TED) and have it connected to my condo's main power and also have a "Spyder" monitoring several individual circuits. Ever since, I have seen a constant draw of 1.7kW coming from my furnace.

Since it's summer and our system's fan is set to ON to improve circulation, I have to assume this is the blower fan's motor. When we're away at work or vacation and even I've made a point to power down everything that can be powered down, I still have the 1.7 - 1.8kW draw. Someone please check my math, but over 30 days, that 1.7kW load comes out to 1,224kWh. This amounts to about 1/2 of my monthly electricity bill, which was 2,550kWh last billing cycle (the previous year at the same time it was 1,290kWh).

This is way too much for a blower motor (or even the whole furnace, considering it's summer) to be drawing, right?

I realize this is a complex problem, so I've tried to include some relevant information below. Thanks for reading!

Other Info

I have contacted the utility company and they have verified my meter is accurate; they are also billing me for the correct meter (rather that one of my neighbors). My TED data is exactly proportional to their measurements (I must apply 1.5 factor to TED data to make it match utility's) due to the fact I've never bothered to calibrate it. I'm just realizing this means that blower fan may actually be drawing 1.7 * 1.5 = 2.5kW (or 1,830kWh per 30-day month).

My utility does provide the ability to view daily usage, and I've done this to verify my TED is accurate once the calibration is taken care of.

I live in a multi-unit building that was remodeled in mid-2000s; the HVAC is original to that remodel (so 10+ years old). I know some of the other units have had their heat pumps replaced. I have a service contract with a heating/cooling company to make semiannual maintenance visits. In February of this year they replaced the blower capacitor and charged $200 for it. March's bill is the first that begins to show significantly increased usage.

Not much has changed to my condo in the past year (same number of people, no remodeling, etc.) I did replace the water heater but that circuit is monitored separately by the Spyder and I can see it cycle on/off during the day. We have also purchased a portable AC unit (Honeywell MN10CES) for our 3rd floor, which gets very hot in the summer. The unit has a stated cooling wattage of 1,050W (verified accurate with Kill-a-watt).

The unit is only turned on when we are at home, and so wouldn't be a factor in that 1.7 (or 2.5?) kW load I see when we're away. It also doesn't run the compressor at night because the temperature naturally gets below the 78F setpoint. Even if that portable AC unit was actively cooling 24/7, it should only be about 750kWh.

  • So you're saying even when you turn off the fan, your Spyder still sees a big draw on that circuit? What if you trip the breaker? By code, the furnace should be on it's own circuit, but maybe your's is not? – topshot Sep 17 '16 at 1:11
  • @topshot My HVAC system (a Lennox) doesn't give a way to explicitly turn OFF the fan (i have options for on, auto or intermittent), but if I raise the AC setpoint above current temperatures and set the fan to intermittent then I sometimes catch it turning off and I see the 1.7kW load go away. And I did try just tripping the breakers (2 for furnace, 1 for heat pump) and the 1.7kW also disappeared. So seems nearly certain that the blower is the culprit. – nabingham Sep 17 '16 at 11:30
  • 1700W is a LOT for a furnace blower motor. It must either be way oversized, very inefficient, always @ max RPM or being overloaded by restricted airflow or bad bearings. You're certain you don't have a (de)humidifier, electric air filter, etc or some other load that is also on this circuit? Turn off the disconnect on the furnace (usually a switch or breaker mounted to the cabinet) and open the cabinet. See if you can easily spin the fan and don't hear any odd noises. Inspect/clean your air filter. Verify you get "normal" airflow out of all your vents. What is the second furnace breaker for? – topshot Sep 17 '16 at 19:45
  • A motor using 1.7 KW would be a motor sized at 1-3/4 horse power. I do not know how big your house is but it is not that big. An 1800 sq ft house would probably utilize a 1/2 h.p. motor. something else is running or is tied into that circuit. Just for info. 1000 watts (1 kw) input equals approximately 1 h.p. – d.george Dec 29 '16 at 14:18
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It is very unusual but not unheard of to have a furnace hooked up to a heat pump. Usually you would have an air handler and air handlers usually have backup electric heating strips. These heat strips Combined with the 300-400 watts from the blower could easily suck 1700 Watts. The strip heat should not be on when the fan is set to on and the temperature is turned down but it is not hard to miswire the unit. You should also verify the heat pump is not coming on too as this is not too hard to miswire either.

  • Thanks for replying, despite the old thread! My investigation continues albeit slowly, as I have been out the state on business for the past few months. I will try and get back to this post when I’m back home and have time to check things out. Luckily the electricity draw is essentially zero while I’m away... – nabingham Oct 23 '17 at 14:13

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