Energy monitoring/logging

drees · March 2010

Since installing a new dual-fuel HVAC system with heat-pump (3-ton Trane XL20i heat-pump, XC95 furnace), my bill has been up a decent amount (about 50-100 kWh/month) even though I haven't been using the heat-pump due to the low gas prices and expensive electricity prices.

I'm guessing that it's because the compressors have a heater to keep the "freon" out of the oil. Apparently this is very common with AC/heat-pumps. I'd like to monitor the power draw so I can verify the usage of the heaters and then be able to monitor other loads (such as dryers and ovens) down the road as well. The compressor has a 240V 30A circuit running to it - it'd be easiest to use some sort of clamp-on ammeter in the main service panel.

I could get a TED or WEL, but that means spending at least hundreds of dollars, though it will give me a lot more than what I'm looking for right now.

Looks like there is a Black & Decker whole-house monitor (EM100B) which may help, but may not help me isolate a particular device.

A clamp-on ammeter would work for instantaneous monitoring and can be had for $40 (but are those any good?).

Any other ideas?

Solar Guppy · March 2010

Re: Energy monitoring/logging

There is no heater in heat-pumps for that function, There are Aux/Emergency heat strips in the Air handler, its a function of your thermostat to when the heat strips get engaged, typically when the temperature requested is more than 1-2 degree than the ambient.

drees · March 2010

Re: Energy monitoring/logging

Solar Guppy wrote: »

There is no heater in heat-pumps for that function, There are Aux/Emergency heat strips in the Air handler, its a function of your thermostat to when the heat strips get engaged, typically when the temperature requested is more than 1-2 degree than the ambient.

My heat-pump is a duel-fuel system, so no electric strip heaters in the air handler.

Multiple people on the HVAC forums have confirmed that nearly all AC/heat-pump compressors have a crankcase oil heater that can draw 40-100w or so depending on a number of variables. The purpose is to keep the oil/refrigerant separated - if the refrigerant condenses into the oil it's lubrication values are significantly compromised resulting in shortened service life.

Solar Guppy · March 2010

Re: Energy monitoring/logging

Interesting, I read some Google references on this,

My Goodman draws less than 20 watts, which I had assumed was the Air handler electronics. The unit I installed last year does not have this heater, 100% sure on this.

From reading, most newer designs that use the scroll compressor design don't have a heater, only the older piston types have heaters and this is why I have never seen this.

GreenerPower · March 2010

Re: Energy monitoring/logging

Drees, just turn off your unit in the winter as the guys on hvac-talk are telling you. You don't want to disable the crankcase heaters.

http://www.gntrains.com/Documents/ACControls.html wrote:

crankcase heater is often installed to prevent the compressor from running out of oil upon startup. How does this work? If the compressor sits for a long period of time and the oil in the sump is allowed to cool off (this is especially possible if the compressor is riding along in a nice, cool stream of air underneath a railway car), it can pick up quite a bit of dissolved coolant in the oil (you might notice the oil level rise substantially when this happens). When the compressor starts up and immediately pulls a partial vaccuum on the oil in the sump, the coolant outgasses (much like a bottle of soda does, when opened quickly) and whips the oil up into a foam in the process. This foam is easily sucked out of the compressor sump and pumped into the system piping, causing the compressor to be starved of oil. Keeping the oil hot with a heater forestalls this problem.

There are 2 crankcase heaters in your units according to the schematic in this.
GP

LucMan · March 2010

Re: Energy monitoring/logging

drees wrote: »

Since installing a new dual-fuel HVAC system with heat-pump (3-ton Trane XL20i heat-pump, XC95 furnace), my bill has been up a decent amount (about 50-100 kWh/month) even though I haven't been using the heat-pump due to the low gas prices and expensive electricity prices.

I'm guessing that it's because the compressors have a heater to keep the "freon" out of the oil. Apparently this is very common with AC/heat-pumps. I'd like to monitor the power draw so I can verify the usage of the heaters and then be able to monitor other loads (such as dryers and ovens) down the road as well. The compressor has a 240V 30A circuit running to it - it'd be easiest to use some sort of clamp-on ammeter in the main service panel.

I could get a TED or WEL, but that means spending at least hundreds of dollars, though it will give me a lot more than what I'm looking for right now.

Looks like there is a Black & Decker whole-house monitor (EM100B) which may help, but may not help me isolate a particular device.

A clamp-on ammeter would work for instantaneous monitoring and can be had for $40 (but are those any good?).

Any other ideas?

Crankcase heaters average about 50KWH a month, some are self regulating and use more power the colder it gets. You can turn the breaker of to the condensing unit if you want, just turn it back on 24 hrs. before you want to use it again to boil the liquid refrigerant out of the compressor crankcase.
Do you have an ECM blower motor in your furnace. Check your fan settings. Is your furnace fan set to run continuously?
Buy a Fluke clampon it will work well for you in this application, a must have tool for checking voltage & amps.

drees · March 2010

Re: Energy monitoring/logging

Solar Guppy wrote: »

My Goodman draws less than 20 watts, which I had assumed was the Air handler electronics. The unit I installed last year does not have this heater, 100% sure on this.

My furnace draws 15W at idle, 40W when circulating/filtering air. I'm sure that's higher when the furnace or heat-pump is on as the fan ramps up a lot more.

GreenerPower wrote: »

Drees, just turn off your unit in the winter as the guys on hvac-talk are telling you. You don't want to disable the crankcase heaters.

There are 2 crankcase heaters in your units according to the schematic in this.

Thanks for that - as long as gas remains so cheap (about $1/therm) and electricity expensive (13-26c / kWh) I'll probably do that.

I didn't see a thermostat in the schematic for the pumps - from what I've been told the heaters will turn off depending on ambient temps and/or temperature of the pipes attached to the compressors.

LucMan wrote: »

Do you have an ECM blower motor in your furnace. Check your fan settings. Is your furnace fan set to run continuously?

I believe it's an ECM moter - it's variable speed. I initially had it set to run continuously, now just have it set to circulate mode which runs it 15-20% of the time I believe.

LucMan wrote: »

Buy a Fluke clampon it will work well for you in this application, a must have tool for checking voltage & amps.

Thanks - is there a specific model you recommend?

GreenerPower · March 2010

Re: Energy monitoring/logging

drees wrote: »

I didn't see a thermostat in the schematic for the pumps - from what I've been told the heaters will turn off depending on ambient temps and/or temperature of the pipes attached to the compressors.

If you look at the 2nd schematic page, there is a thermistor on J8 this one probably connected to the liquid line (LLS), T2 controls the relay to engage the crankcase heaters.
GP

drees · March 2010

Re: Energy monitoring/logging

GreenerPower wrote: »

If you look at the 2nd schematic page, there is a thermistor on J8 this one probably connected to the liquid line (LLS), T2 controls the relay to engage the crankcase heaters.

Thanks - obviously not experienced with reading detailed schematics!

Energy monitoring/logging

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