Propane vs. Heat Pump w/PV - SF Bay Area

matthewk4November 22, 2011

I live in the SF Bay Area, but up at 2600 feet elevation, so it gets a little hotter in the summer and a little colder in the winter than down in the flats. Still, a typical winter night (like tonight) is around 40 F, only occasionally does it get down to freezing and only for a week or so. Once in the last 10 years it got down to 28 F overnight.

My 11 year old 80,000 BTU 90% efficiency Bryant furnace just cracked its heat exchanger, filling the house with CO (and triggering the CO alarm, good thing we had one of those)

So now I'm faced with the replacement decision.

Option 1: New 80,000-100,000 BTU 95% efficient furnace and a new 4-5 ton A/C unit. Furnace fueled with propane, currently paying $3.61/gallon(!) for that. We occasionally get 1-2 day power outages, and the 18 kw generator runs the furnace blower but the heat comes from propane.

Option 2: 10 HSPF heat pump, no new A/C unit required. Heat pump powered by PG&E electric E-6 time of use rates ranging from $0.09794/kwh winter off-peak baseline up to $0.32295/kwh on-peak at the 300% tier. *However* we also have a 14 kilowatt (DC rating) grid-tied PV array on the house which currently offsets 80% of the PG&E bill, keeping us pretty far down in the rate tiers. During power outages, the PV is offline so all power would come from the 18kw generator.

We also offset some of our heating load during the winter with a wood stove burning (nearly free) wood that grows on our property.

My calculations say that even at the worst case power bill, the heat pump is $31/million BTU vs $41/million BTU for the propane at today's propane prices... and of course if we're not in the highest electric tier and we're not using it during peak, the price goes down as low as $9/million BTU for the heat pump. (Even less if the PV system happens to power the whole thing of course, and I'm prepared to upsize the PV array by another 5kw if it makes sense.)

But is there more to it that would mean that I really wouldn't be happy with a heat pump, or would pay a lot more than I'm expecting (e.g., maintenance) to go that route?

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david_cary

I think you generally would be fine with a heatpump. I live in a colder area than you and I have 2 floors with 2 separate systems - one HP and one dual fuel so effectively NG when it is really cold.

At 30 degrees, it is nice to have the NG. The system runs less but I don't really notice the warmer air coming from the registers unless I try to by putting my hand over them. Upstairs with the heatpump only, I have some issues with airflow. It is a zoned system that wasn't done all that well. I think with a good duct system where you don't really notice the air flow, then you should be perfectly happy.

From a dollar standpoint, if you have terrible ductwork, the heatpump may not be as cheap to run as you expect. This is not talked about much but if your ductwork leaks to unconditioned space, then a heatpump will cause more leaks by virtue of it running for longer periods of time. These leaks can represent 50% of energy use in a worst case scenario. Not only are btu's lost, but the house becomes negatively pressured and sucks more outside air in leading to more btu's lost. Logic would say that if the propane furnace runs for 30 minutes vs 2 hours for the heat pump, then there is much more leakage. The btu's actually leaked may not be much higher but the infiltration from negative pressure has to be worse (presumably 4 times worse).

So just make sure your ductwork is in good shape.

Also know that you might be in a small minority there which could be an issue.

    Bookmark   November 22, 2011 at 5:49AM
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tigerdunes

Matthew

Some observations.

The generator is powered by propane?

You need to see what the KW pull is for a heat pump. At the moment with existing setup, what else does your generator power during an outage?

Based on your location/climate, a heat pump would be a logical choice for heating versus propane heating.

Can you be a little more specific on how your tiered electric rate works? Without heating or cooling, what is a good average of KWH usage each month for your home?

How large is your home?

Have you thought of a dual fuel system? Possibly an 80% eff two stg furnace that could be used for those rare cold snaps and power outages?The backup furnace would eliminate the need for aux heat from heat strips.

How would you characterize the insulation qualities of your home?

Ductwork has been inspected? Any hot cold spots in your home?

Post back.

IMO

    Bookmark   November 22, 2011 at 7:57AM
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neohioheatpump

Propane is no bargain. Although your electricity can be expensive at times it sounds like it isn't all the time.
A new efficient heatpump can handle these types of temperatures no problem as long as your house isn't leaky. Your bills shouldn't be bad. When a heatpump is running constantly during colder temperatures is when electric usage gets higher.

I like the comfort of heatpumps at 30 and above. They do run more at a lower register temperature but his provides an even heat.

    Bookmark   November 22, 2011 at 9:18AM
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tigerdunes

Even heat?

Humm...

Depending on your ductwork system not a heat pump.

IMO

    Bookmark   November 22, 2011 at 9:58AM
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matthewk4

> The generator is powered by propane?

Yes.

> You need to see what the KW pull is for a heat pump. At the moment with existing setup, what else does your generator power during an outage?

Generator powers nearly everything. (The only things not on the generator are the septic enhanced treatment pumps, the pool equipment, and the hot tub.)

> Based on your location/climate, a heat pump would be a logical choice for heating versus propane heating.

The last few days have been fairly typical for the "cold" part of the year. In the last 72 hours, minimum temp 35, average temp 41.

> Can you be a little more specific on how your tiered electric rate works? Without heating or cooling, what is a good average of KWH usage each month for your home?

It is the PG&E E-6 rate plan. http://www.pge.com/tariffs/tm2/pdf/ELEC_SCHEDS_E-6.pdf

This past month our net usage was 28.4 kWh/day, but during the summer we generate more than we use.

> How large is your home?

2400 square feet.

> Have you thought of a dual fuel system? Possibly an 80% eff two stg furnace that could be used for those rare cold snaps and power outages?The backup furnace would eliminate the need for aux heat from heat strips.

That's what I'm thinking now. Definitely solves the power outage problem, though the woodstove does that nearly as well. Seems like a low or moderate efficiency furnace is about the same price as a standalone air handler anyway.

> How would you characterize the insulation qualities of your home?

Home was built in 1972, parts remodelled in 1980 or so... so not the greatest insulation, but we have installed double-pane windows everywhere and upgraded the insulation in some areas.

> Ductwork has been inspected? Any hot cold spots in your home?

Ductwork will be overhauled at the same time we install the new system. No particular hot or cold spots now though.

    Bookmark   November 22, 2011 at 12:02PM
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weedmeister

This may not be relevant, but can you say why your PV system does not provide power during a grid outage?

And why do you want to keep the current AC system if you go with a HP? Or did I misunderstand the 'no new AC unit required'?

Depending on age, the new HP (or AC) will lessen your electrical usage by a noticeable amount.

    Bookmark   November 22, 2011 at 3:43PM
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rmrc12

@weedmeister - I suspect his PV array is grid tied so the power from the PV goes to the grid as a credit but the power he uses comes from the grid (no battery power storage). If the grid is down, so is he.

    Bookmark   November 23, 2011 at 3:52PM
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TommieD

I have two Heat Pump projects completed within the last three months that are using under $40.00 per for November and December. One home is 1800 sq. ft. and the other home is 2000 sq. ft. Both homes have 16 SEER Goodman split systems. Both homes have are kept between 68 and 70 degrees 24 hours a day.
The Homes are in Calaveras County at 1500 feet elevation. The temperature at nights are under 20 degrees. Both units have not gone into emergency heat.
Both homes are saving about $200 plus a month from removing the Propane use.
I highly recommend a well installed Heat Pump system with a tight and well insulated duct system. The full system static should not be designed more than .3 in. w.c system static.
I do not see your home needing more that three tons.
I am testing the operation of these systems at over 350% more efficient than an electric strip heat. So for every 1000 watts the compressor is using the system will deliver more than 3500 watts of power or heat into your home.

    Bookmark   January 27, 2013 at 3:14PM
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SnidelyWhiplash

Matthew - I'm not sure how the pricing works with your PV credit, but if you're at 28 kwh per day, it's the cost of the NEXT kwh's that matter for incremental use like when you turn on a heat pump, not the average. If your pricing is such that you get anywhere near the 30 cent rates, I can't imagine a heat pump would be cost effective. But I'm just a homeowner, not an expert.

Tommie D - Help me with the math - at the 13 cent lowest PGE rate, $40 would be 300 kwh. The fan for my gas furnace uses 100 kwh or more during heating times. These customers must not have refrigerators and wash clothes in the nearest stream?

    Bookmark   January 27, 2013 at 5:11PM
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saltidawg

snidely,

A typical oil or gas furnace draws 1500 Watts while running.

    Bookmark   January 27, 2013 at 5:20PM
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SnidelyWhiplash

Thanks Salti, that sounds like it's in the right range. 1.5-3 hrs per day x (1500/1000) x 30.

    Bookmark   January 27, 2013 at 5:49PM
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hvacajun

Matthew-
Listen to Tiger dunes and go with a dual fuel, no doubt. Out of all these posts, I was wondering why no one mentioned the dual fuel system, but then I saw Tiger's. gas heat furnace(propane) with a heat pump outside. requires a dual fuel board. No big deal. This will be the best system for you.

    Bookmark   January 27, 2013 at 9:35PM
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