How do I calculate the energy usage of radiant heating?

October 23, 2012 8:43 AM

How do I work out the energy usage of built in radiant heating?

My circa 1967 house has built in electric resistance ceiling heat.

It is quiet, effective and generally great. Except when the bill comes.

Since it is zone-based, I've been toying with the idea of just putting a couple of oil-filled space heaters in the rooms that get used.

Thing is, I'd like to calculate before hand whether this is likely to save me any money.

The space heater is easy, just plug 1500 watts into the cost calculator and we're done.

But how can I work out the wattage of the built in heating?
As I said, it's set up in a zone system, with 1 wall-mounted thermostat per area, with (I think) 4 breakers in the panel.
There are no detachable plugs in the system, so no place to plug in a kill-a-watt.

The three ideas I've come up with are:
A) removing the thermostat on the wall and putting a voltmeter inline. I don't think it's a low-voltage thermostat, but I don't know for sure.
B) Figuring out some way to measure at the circuit breaker and just turning on the area I am concerned about.
C) Throwing all the circuit breakers in the house so there is no load, and then just turning on the heat circuit and observing the outside meter spin.

Any of these sound plausible or practical?
Any better suggestions?

posted by madajb to home & garden (11 answers total)

I came in here to suggest option "C", although if you don't want to do that I think I've seen some solutions for "B", though pricier per-circuit than I was willing to commit to.

You could go the "A" direction, but you don't want a voltmeter, you want an ammeter, and you want a fairly beefy one as there's a possibility that your heaters pull more than 20 amps. There are ammeters that work by clipping a lobster-claw thing around the wire to form a current transformer, I don't know how accurate they are, but that's probably safer than wiring a heater circuit through your Radio Shack multimeter...

But...

Both the oil-filled space heater and the resistive heater in the ceiling are using the same mechanism for generating heat, and aside from the ceiling heater possibly losing more heat from the ceiling, and that heat having to propagate downwards (yay for TVA and assumptions that electricity would be essentially free for forever!), they both have to do the same amount of work to pull a room up to the same temperature.

If you're depending on the localized heating of the space heaters vs the whole room/zone heating of the house heater, then there may be a win, but really you want to go with something more efficient generally: heat pump, or combustion based. And, yes, both of those are more expensive.
posted by straw at 8:54 AM on October 23, 2012

If you imagine yourself doing much household electrical work, you might find it worthwhile to buy a multimeter with a built-in inductive current probe. It's not as accurate as hooking up an ammeter in series, but is much easier to use and might be close enough for the kind of comparison you're after. (You'd just go out to the breaker box, remove the cover, and put the notch in the top of the meter around one wire of the circuit you want to measure.)

One other thing to keep in mind is that an in-ceiling radiant heater will lose significantly more heat upward and out of the room than the space heaters you're considering, so even if the power consumption is lower the space heater might be more efficient.
posted by contraption at 8:56 AM on October 23, 2012 [1 favorite]

option C is pretty easy and what i came here to suggest... once you get your straighline comparison based on consumption i can add that althgouh both heaters convert electricity into heat, the oil heater has the radiator effect going for it after it stops consuming electricity and I've found them to be very econominical to operate for the entire winter season.
posted by chasles at 9:12 AM on October 23, 2012

You could go the "A" direction, but you don't want a voltmeter, you want an ammeter

Right, yes, thanks for catching that.
posted by madajb at 9:12 AM on October 23, 2012

One of these (wireless electricity monitor) will do Option B for you.
posted by EndsOfInvention at 9:15 AM on October 23, 2012

Sorry for the derail, but when I had resistance ceiling heat I installed programmable line voltage thermostats in the zone(s) that I wanted to heat, and left the rest off. Not sure if this is an option, but I used this model:

http://www.amazon.com/King-ESP230-R-Electronic-Line-Voltage-Programmable/dp/B000PB32EU/ref=pd_sxp_f_pt

Of course, soon after, my utility offered giant rebates (along with a federal tax credit) for a Ductless Heat pump and my winter electric bill dropped from > 200 to < 80 per month with much better heat. After rebates, I had mine paid off in 1 year (or so). Without, it would have taken longer (much longer, actually).
posted by jeffch at 9:42 AM on October 23, 2012

I was going to suggest getting a clamp-around ammeter from Home Depot and using that on the mains cable as you switch the heaters on and off to determine their current draw. However, I think the wireless monitor suggested by EndsOfInvention is a more elegant solution. You could attach it to your mains and then compare the draw with the heaters on, then off, and then add in 1500 or 3000 W of hypothetical oil heat to see which would be better. And you'd end up with a neat whole-house power monitoring station to boot.
posted by Kadin2048 at 11:28 AM on October 23, 2012

jeffch -
I essentially just turn on the heat in the two zones I am in the most. We have upgraded thermostats, though not quite as fancy.
My utility does offer rebates for heat pumps, but I've got no interest in any sort of forced air heat. I could never go back after the wonder of radiant.

Kadin2048 -
They are neat. Unfortunately, they seem to rely on clipping a sensor around a feed line, something that isn't possible in my panel.
I seem to recall one that clips around the meter itself.
Anyone have experience with those?
posted by madajb at 12:30 PM on October 23, 2012

I had a wireless display with a head-end inside and a reader that clipped around the meter, it simply counted rotations of the dial, and wasn't fine grained enough to be generally interesting.

It didn't work when PG&E replaced the old analog meter with a modern digital "Smart Meter". And now I can read that stuff of PG&E's web site.

For your purposes, I'd shut everything else in the house down, take a stop watch outside, and read the meter manually.
posted by straw at 3:32 PM on October 23, 2012

Option C is correct for determining the wattage of each zone.

This is not thermodynamically exactly correct, but will get the point across: the problem you will have is that the colder rooms will suck the heat out of the single room(s) that you are trying to heat. Whether you save money will depend on your tolerance for feeling drafty, given that it will cost essentially the same per BTU.

My utility does offer rebates for heat pumps, but I've got no interest in any sort of forced air heat. I could never go back after the wonder of radiant.

You *could* do hot water radiant with a heat pump source.
posted by gjc at 4:26 PM on October 23, 2012 [1 favorite]

the problem you will have is that the colder rooms will suck the heat out of the single room(s) that you are trying to heat. Whether you save money will depend on your tolerance for feeling drafty, given that it will cost essentially the same per BTU.

I think that since I am heating just individual rooms with the door closed, the heat circulation in the house should be essentially the same.

You *could* do hot water radiant with a heat pump source.

Yeah, no rebate for that, around here at least.
posted by madajb at 7:53 PM on October 23, 2012