Cost savings w/ A2W HP

kcopp

I have a customer w has a 2400 ft2 home.
It has a decent combi boiler in it. Radant floor staple up w/ plates floor. A few panel rads. Home is in SW Maine. uses approx 3-400 gal of LP per year for heat and hw. 80% of that cost is heating.
He wants to get off of LP totally. Looking at solar PV collectors.
Also wants to do an air to water heat pump.
Was asking approx electrical costs to run.
Anyone have any clue as to how to figure out this answer or an approximation of operating costs?
TY, kevin

Hot_water_fan

Air to water heat pumps are extremely uncommon. Air to Air are extremely common. I'd try to work towards air to air.

Electricity: $/MMBtu = $/kwh * 293 / COP

COP depends on the heat pump and operating conditions. 3-4 is good.

Looks like he uses about 30 MMBtu/year, which implies a heat load of ~12,000 btu/h. Seems low for a house that size. Does he use wood too?

Jamie Hall

If you can find an air to water heat pump -- they do exist -- which runs at your outside temperatures, the key to the running cost is in the "COP", which is simply the ratio of heat power out to electrical power in. That information should be in the manufacturer's specifications, but it varies with temperature difference between input and output and with the actual temperature of the input, and is often a little hard to pin down.

A very rough back of the envelope figure would be a COP of around 2 at winter temperatures in Maine, so you can figure on about 7000 BTUh output for each kW input. From there it's simple arithmetic -- figure out how many BTU you will need in a day -- the heating load of the house -- and divide by that 7000 to get the kW-hours of electricity you will use. That times your electricity rate will give you the cost per day.

It is possible that your client might be able to have a big enough PV array to power all the electrical needs of the house, including the heat pump, and taking into account the need to store that electricity for those week long cloudy spells. Don't forget to add in the cooking and hot water loads, if the client really wants to ditch LP.

Again, a useful back of the envelope figure for photovoltaics is around 20 watts per square foot output -- in full sun, and the area measured perpendicular to the sun. If the array is not oriented perpendicular to the sun, that must be reduced by -- very roughly -- the cosine of the angle between the perpendicular axis of the array and the direction to the sun, up to about 30 to 45 degrees off axis; from there the output drops dramatically.

Good luck. I think you may find that the total capital investment may be eyewatering...

Hot_water_fan

Your client won’t need storage if he’s grid connected. He may want it though. 

kcopp

He will be grid tied...not an issue. House is pretty tight. Spray foam and poly iso board all around. He does use wood at times.
He wants the floor warming. Its an old barn that he converted to a home. High ceilings.

Already has A/C via a ductless mini-split... maybe he could swap out the heads.
Maybe just use the LP when it gets really cold.

I did warn him about the cost upfront... Eyewatering sure thing... maybe pants filling...

Also just tried to have him go w a heat pump water heater and ditch the combi part of the boiler.
He was looking at the Sanden water heater but the output is only around 8000btu.
Im still not convinced on them... not a great value. imo.

Hot_water_fan

https://www.greenbuildingadvisor.com/article/air-to-water-heat-pump-retrofit

Here’s a good example of a cold climate and low water temp application. 

Still shocked by the low amount of LP! Good for him. How warm are these floors anyway? If his load is close to the fuel usage implied value, they must be like…73 degrees on the coldest day? 

retiredguy

I have a 2 neighbors who have geo-thermal ground source heat pumps that works very well here in Pittsburgh, Pa. The system is installed with plastic pipe buried 6-8 feet below the ground surface. their heating bills are 1/2 of all the other neighbors heating bills using natural gas. I do not have $ figures on the operating costs.

hot_rod

What are the LP costs up there? They seem to be running 3.50 average around that area

This issue takes you through the steps for hydronic air to water design.

Siggy has 3 air to water systems in his family now, going into the 3 rd winter on his own, north of Utica NY

I’d pipe a hybrid, leave the Lp boiler in case, or for extreme  cold periods

https://idronics.caleffi.com/magazine/27-air-water-heat-pump-systems

motoguy128

Might as well just invest in geo water to water.  In that climate.  Otherwise keep
LP.  Most of the winter the
air source will fall short on capacity and might have a net cop of maybe 1.5.  

Hot_water_fan

@motoguy128 the article i linked is someone in the same climate who had an annual COP of 2.5, no reason to spend extra for a ground source. 

Jamie Hall

"annual COP of 2.5"

I should hope. Considering that the COP in the shoulder seasons should be well over that. That says absolutely nothing useful about the performance of the system at 0 Fahrenheit in a high wind.

Looks great in a sales brochure for the suckers, though.

Hot_water_fan

@Jamie Hall I don’t understand the reaction. So what if it has a bad COP on a windy 0 degree day? How many days will the customer experience like that, even in Maine? My point was that geothermal is so much more expensive, take the sometimes low efficiency of the air source. Using resistance backup or LP for a few days/year isn’t the end of the world.

Jamie Hall

The bad reaction is this: unlike the folks who hang out on the Wall, most people will have difficulty grasping that the COP will vary, and will be mislead -- and I dislike misleading people, although it's done all the time.

As you say, using resistance backup or LP from time to time isn't the end of the world -- although with the electric rates in my area, electric resistance isn't far from it! -- but a consumer who isn't expecting to have to do it or, perhaps more seriously, whose installer hasn't managed to persuade them to have the backup heat installed, is not going to be a happy consumer, and that does everyone a disservice.

The problem isn't unique to the heating industry, although there are many examples in that industry.

Hot_water_fan

That makes sense! Thanks for explaining. Yes, installers should explain that colder months will be more expensive than warmer heating months and that’s because of lower efficiency at colder outdoor temps. Those efficiency losses, while real, rarely if ever justify the price premium of geothermal and will still be cheaper than LP. I think most people can accept that in about 5 seconds and move on. 

pecmsg

GEO had its place many years ago but with inverter driven equipment I don’t see a return on
investment  

Jamie Hall

I'm still waiting for some clever person to produce a two stage air to steam heat pump... first stage air to stage 1-stage 2 heat exchanger, operating to bootstrap from perhaps from a minimum of -30 or so F to around 40 F, (not sure of the refrigerant for that stage) and second stage from the first stage heat exchanger, using water as the refrigerant with all the condensing being done in existing steam heating systems (with modifications one could go up to say 240 F process steam, if needed)(water has some very interesting and attractive properties, if one looks at it as a refrigerant).

There is no thermodynamic reason why it can't be done. There are practical problems -- many existing two pipe or vapour steam heating systems have enough leakage to be pesky, and one pipe systems would probably have to be converted to a variant of a Paul type system -- but those items are, as they say, trivial details.

The market isn't huge, but there is enough built steam out there to be at least vaguely optimistic.

hot_rod

You can crunch the numbers for most any application, every application is different. No need to guess or believe marketing hype.
Get opinions from folks that have some actual run time in cold applications.