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Will I be in for an electricity shocker?

Hi everyone,

Based significantly on the guidance from the GBA Website we installed a Fujitsu AGU15RLF, Floor Mount Evaporator and Fujitsu AOU15RLFFH Condenser in our Central Massachusetts 2,200 square foot colonial last week. It is exceeding our expectations one week in with cold and snowy weather.

We bought the house a few months ago and it was heated by electric baseboard we we've moved quickly to take control of the heating bill.

The energy audit identified the insulation as "pretty good" and we had air sealing done the same day.

We are also installing an oil hydronic system in the spring as my family was nervous that minisplits could not heat and cool the whole house. The house does not have an open floor plan and frequently has power outages. The oil system is not in yet.

Since the minisplit install last week, it has been successfully providing baseload heating capacity, 64-66 degrees on first floor and 60-62 in the bedrooms, very comfortably. We only use the baseboard to bump up to 68 sometimes when using a room otherwise the minisplit has been carrying the load.

We won't get our next electric bill for a month and I am curious if I could be in for a shocker of an electric bill and that the minisplit could be too good to be true! We are trying to follow the GBA tips, no set backs, keep on auto. Our past baseboard related heating bills were $600.

Thank you in advance for your feedback.

Asked by Peter Kelly-Joseph
Posted Mar 16, 2017 10:03 AM ET
Edited Dec 27, 2017 3:51 PM ET

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7 Answers

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1.

The snowy weather in central MA this past week has had some cool night time temps, but during the day it's been in the 20s. At mid-20s temperatures a mini-split sized for 0-5F outside design temps should be modulating nearly 100% of the time, and would have a COP of better than 3, and will soon be bumping on 4 as temps rise into the 30s & 40s. At 15-18 cents /kwh that's roughly on par with than gas at $1.30/therm-(delivered), and considerably cheaper than $2- 2.30/gallon oil.

Your combination tests at HSPF of 11.2, which you can probably hit for a seasonal average in a central MA climate if it's right-sized:

http://www.fujitsugeneral.com/us/resources/pdf/support/downloads/submitt...

That's 11,200 BTU/ kwh, so on average it takes (1,000,000/11,200= ) 89 kwh per million BTU/MMBTU, which at 18 cents will cost you $16/MMBTU. At 15 cents it's $13.35/MMBTU

A gallon of #2 oil has ~138,000 BTU/gallon of source fuel energy, but burned at 86% delivers only ~119,000 BTU/gallon into the house. So it takes (1,000,000/119,000=) 8.4 gallons/MMBTU, plus the pumping & burner power. At $2/gallon that's $ $16.80/MMBTU, plus the electricity, plus the annual tune-up to retain any hope of keeping it burning at 86%, and if it's pushing 3x oversized for the load (even the smallest oil boilers would be, if you're heating with a 1.25 ton mini-split), it will have considerable standby losses reducint it's as-used AFUE to under 80%, so you'll be lucky to net $20/MMBTU with the oil boiler.

A 95% efficiency gas burner delivers 95,000 BTU/therm into the house, or (1,000,000 /95,000=) 10.5 therms/MMBTU. At $1.30/therm that's $13.65/MMBUT, plus the air handler or pumping & control power. Condensing gas has a slight marginal-cost advantage over the mini-split with 18 cent electricity, about the same if it's 15 cents, but with a code-min 82% efficiency gas burner is more expensive to burn gas than to run an HSPF 11 mini-split.

If you've been heating predominantly with baseboards and using a total (all uses) of $600/month during typical winter coolth, assuming $100 of that was for stuff like hot water & lighting the heating portion should drop to about $150 for this billing period, and dramatically less for the March-April billing period. So you're looking at ~$250, a savings of $350 /month for the winter period.

Answered by Dana Dorsett
Posted Mar 16, 2017 2:16 PM ET

2.

BTW: Who is your electricity supplier, and what are your rates? (delivered: take the total bill, divide by the kwh.)

Answered by Dana Dorsett
Posted Mar 16, 2017 2:17 PM ET

3.

Wow, thank you for the details breakdown Dana. My supplier is National Grid and I pay $0.20 per kwh delivered. You didn't ask but the installer did do a Manual J and shared it with me. The sizing was primarily for the cooling load of ~20k btu.

Answered by Peter Kelly-Joseph
Posted Mar 16, 2017 2:39 PM ET

4.

Regular electric resistance heat as a Coefficient of Performance (COP) of 1.

Switching from electric resistance to a heat pump with a COP of 3, should produce the same amount of heat with 1/3 the amount of electricity. COP of 4 would equate to producing 4 times the amount of heat as with electric resistance.

Please update with your actual use.

Answered by Erich Riesenberg
Posted Mar 16, 2017 8:07 PM ET

5.

Erich, I assume that you are looking for my actual kw usage this month? Will update when the bill comes.

Answered by Peter Kelly-Joseph
Posted Mar 17, 2017 9:33 AM ET

6.

It will be great if the heat pump reduces the electrical as much as the COP implies.

Answered by Erich Riesenberg
Posted Mar 17, 2017 5:58 PM ET

7.

Hi Everyone,
I wanted to provide an update since I last posted to provide some data for discussion wider discussion.

I would hope that my experience provides some data that can be used as an “average consumer” considering a minisplit installation. I am happy to provide more data if anyone would find it helpful.

To add to the data provided earlier, we have a 1970 built 2000 square foot colonial in Massachusetts. It was air sealed by a MassSave contractor in March 17 and the before and after result was 2300/1800…not sure what units, ACH50?

The house has “pretty good” but likely old batt insulation since the house was electric baseboard heated. Since then I’ve been the homeowner wondering around with a caulking gun, sealing up 2 unused fireplaces and other basement and attic gaps.

Here is electricity data for discussion:
Jan 17: 2201 (all baseboard electric, billing ending Jan 6)
Feb 17: 3037 (all baseboard electric)
Mar 17: 2711 (all baseboard electric)
Apr 18: 1520 (minisplit installed March 9, indirect oil boiler DHW March 10, boiler baseboards installed May 1)
May 17: 556
Jun 17: 413
Jul 17: 507 (some summer air conditioning)
Aug 17: 555
Sept 17: 439
Oct 17: 443
Nov 17: 565
Dec 17: 996 (billing ending Dec 6, heated with minisplit except for few cold days around Veterans Day)

Other data points:
Second floor stays around 60 in 20s outside temps, 55 in teens outside temps. Bedrooms are heated from 7pm-7am to 62 with existing electric baseboards.

In cold weather (lows teens-low 20s) I estimate that we burn 6 gallons of oil per day = $14 at current prices with the split off.
I experiment with the minisplit set to 70 with boiler set to 62 and I estimated that it uses 30-35 kwH per day for days with lows in the 20s highs in the 30s. Meter readings between 50-60 KWH. It is installed on the first floor in the kitchen/dining room. At $0.25 per kwH, its $7.50 per day plus say 2 gallons oil for supplemental, say $13.00 all in. But the boiler heat is more comfortable and uniform in the first floor rooms.

I estimate around 30 kwH with a 17 kwh non-heating load (maybe a bit more in winter with holiday electric oven usage, old electric dryer used, 10 KwH for second floor baseboard.

DHW=$35 per month (15 gallons oil at $2.35)

I think my follow up questions are:
When the split can’t keep up, should I shut it off it completely or just bump the boiler thermostat up and run both?

Identifying a cross-over temp online is challenging, any suggestions?

Should I keep the first floor thermostats higher, say 68 at night to heat the second floor more and reduce need for baseboards or just use baseboards?

Answered by Peter Kelly-Joseph
Posted Dec 27, 2017 3:46 PM ET

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