3 months - happy, but electric use seems high

We had a WaterFurnace NDZ038B14AC installed last November, using the existing gas furnace for backup. I'm happy with it -- it keeps the house warm, mostly runs on stage 1, and has not once gone to the gas backup. I got geo to get off of fossil fuels: mission accomplished.

However... I did not expect that our heating costs would go up. Of course gas prices are ridiculously low. So I went back to the GeoLink Design Studio report from our contractor. It predicted a cost of $944 for heating with gas, $739 with geo. At prices of $1.2/ccf and and $.13/kwh, that says we would be using 7.3 kWh/ccf.

Taking out non-geo electric use, our 3 bills since install show us using .95 kWh per degree day. Last year on gas we used .085 ccf/degree day. This gives a pre/post install ratio of about 11 kWh/ ccf.

It seems we're using 50% more electricity than GeoLink predicted. Is this normal? Anyone else have experience converting from natural gas to geo?

Some more info... We live near Milwaukee WI (53213) in a 1500sf Cape Cod, well sealed and insulated. GeoLink design was for a 40,000 Bthu heating load. It's a Waterfurnace NDZ038B14AC split system with desuperheater, buffer tank to electric water heater (old gas heater used as the buffer tank). Gas blower/ backup furnace is a 96% efficient York Affinity. 3 vertical loops, each 180', about 12' apart. Installer added a large return grate in the basement return air manifold, but no other duct changes.

The first full month on geo (November) we used 883 kWh, of which I attribute 743 to geo + hot water. The prior year on gas we used 70ccf gas, but it was a bit colder. Next 2 months were colder but similar ratios. We use very little hot water (last summer we used about 4 ccf/ month, cooking and hot water). Our installer's worksheet shows 53.2 degree EWT, 47.2 LWT, 60 degree EAT, 92 degree LAT. Just now on stage 1 I measured 66 degree EAT, 81 LAT (I assume it was stage 2 when installer measured). Last year on gas we set back the thermostat during the days and overnight. This year we leave it constant (69).

Our installer (Mark Doll, Professional Geothermal) was careful and attentive. He re-did the initial GeoLink analysis with more current gas prices, but it's the ratio of geo kWh to pre-geo ccf I'm interested in, and that did not change.

 

The hare is on the track

and the dogs are running full out trying to catch him.

I do not use that program. I do the math the hard way the same as you seem to be doing. What you need to find is a conversion table that will allow you to compare dino gas to KWH at what it costs you per BTUH.

Mark

 

Using EIA's heatcalc spreadsheet and your efficiency # for the gas furnace and your prices it shows about an 5% savings using geo with a cop of 3.3 (the default setting) per mbtu. That would be for the same temp settings though. Could you be seeing the difference due to your setback of the gas system last year? I read somewhere that an 8 hr setback could save around 5-15% on heating. That's not 50%, though. I used the EIA calculator below:

http://www.google.com/url?sa=t&rct=...v9H8DQ&usg=AFQjCNEI3O4-27x7XDhiFFJKOpjvhEZHsA

You also went from gas to electric water heater. An electric water heater might use 400 kwh on it's own without the desuperheater contribution. So if the desuperheater was providing 60% of your hot water, you still might see another 160 kwh extra electric use from the water heater per month. Maybe also part of the picture?

 

"The first full month on geo (November) we used 883 kWh, of which I attribute 743 to geo + hot water"

I'm somewhat skeptical that balance-of-house load consists of just (883-743)/30 = < 5 kwh per day.

<5 kwh per day for refrigerators, lighting, laundry, media, etc doesn't fly with me.

 

I understand our electric use is (was?) unusually low -- the year prior to geo we used 1554 kWh (4.25 kWh/day), the year before that we used 1988 kWh (5.4 kWh/day). Our peak month in the 2 years prior to geo was Dec. 2009, at 273 kWh. If we use that, then geo + hot water is 610, compared to 70 therms, or 8.7 kWh/ccf -- still about 20% above the predicted ratio of 7.3. But I don't see our other electric jumping up like that (Dec.'09 I attribute to overly enthusiastic use of pre-LED xmas lights).

I wonder about the hot water... I guess spring will clarify that. But based on our prior summer gas usage (4 ccf/ month for 3 straight months), I have a hard time seeing that making a big difference.

I'll look into the EIA calculator and the setback -- we might have had the "at home" temp a little lower too (68?), and I suppose the thermostats could have been a little different -- the house does feel nice and warm this year.

Thanks for your input!