Heat Pump in San Diego?

[soylentgreen]

Hi, first time poster, would appreciate your advice...

We are in the process of having a good sized grid-tied solar PV system installed, and have planned to intentionally over-size it to make room for further increases in electric usage (an electric car, HVAC).

I wanted to explain my thinking on the HVAC side and see if I'm doing anything stupid.

Background:

The house is was built in early 1980s and is wired & plumbed for Air Conditioning but doesn't have it. The gas furnace is original. Natural gas is used for heating, hot water, clothes dryer, and the fireplace (which is rarely used). The house is fairly well insulated.

Climate is near-coastal San Diego, so AC usage is typically only a week or two a year. However this summer was really hot -- since my wife and I work at home a lot, it's nice to have AC for those rare times when we need it. (We put in a crummy window AC and it did ok at keeping one room cool, but it was noisy and added about 100KWH to our bill.)

Our gas furnace is probably 65% efficient (based on its age). Our comfort heating load looks to be 30 therms per month, or 880KWHt/month. So if we just ran electric space heaters, we'd go broke. However, modern heat pumps can do about 3.0 COP (ratio of heat out to electric in) in a mild climate. So we'd need 880*.65= 650KWHt of heat, which would require only 215KWHe of electricity for several months of the year (if my calcs are correct)

Price-wise this works out to about $30/month for gas vs. $30-60 for electricity at today's prices. Thus if we were paying SDGE for electricity there'd be no financial benefit to buying a heat pump and using it for heat. But with solar, the balance changes. (for reference SDGE charges about $1/therm for natural gas and $0.14 to $0.33/KWh. Our solar system should be big enough to just zero out our bill on average year round, we believe)

For cooling, I figure we'd probably use 100-200KWH for 1-2 months of the year (that's a WAG number).

Questions:
* are my calculations roughly correct? any big errors?
* I assume that we'd get rid of the gas fired furnace entirely (our HVAC closet is pretty cramped) - is that dumb? Typical winter low temperatures here are about 40F, perhaps 35F on a couple days.
* any other issues with heating with a heat pump? As I understand it, they tend to deliver air that isn't very hot, so they run more often.
* I'm a big fan of humidity control (I can stand 80F when it's dry, but if it's 70F and humid I'm not happy), and we are concerned about noise - does that mean we should look into a 2-stage system? Any recommendations?

Thanks in advance

[catmanacman]

a heat pump would be perfect with your climate at 40 degrees outside temp the air will feel warm from the vents . . make sure that the auxillary heat is locked out to a low temp as you wont need it

[Tommy knocker]

Probably don't matter what we tell ya. You got it all figured out.

[beenthere]

Not sure what you mean by over size the PVs. Does that mean 10 KW instead of 4, or 15 KW instead of 10.

Best to find out what size heat pump you really need first, before making any calculations on PV size. Don't for get to include water heater and dryer if your going to electric with them also.

[soylentgreen]

Not sure what you mean by over size the PVs. Does that mean 10 KW instead of 4, or 15 KW instead of 10.

Good question - in this case, neither - we are limited by roof space, so "oversize" in this case means going to go for the higher efficiency panels (SunPower E20s) which should get us a little under 5KW system and generate roughly 600KWH/month-ish. That's about 50% more KWH than we are averaging now but we think that would roughly cover the heat pump usage if my #s are right.

[soylentgreen]

Probably don't matter what we tell ya. You got it all figured out.

I can't tell if that's sarcastic or serious...can you explain?

[Shophound]

Something to think about...if you put in a heat pump, you will probably use the a/c side of it more than one or two weeks a year. Why? Because it's there. Whereas before you might hold out on the window shaker until you just could not stand it, now you can flip a switch and everything's cool again.

When I lived in San Diego I serviced heat pumps. Few of them had emergency or auxiliary heat strips. And this was back in the 80's. I know heat pump performance and efficiency has improved since then. But if I had gas heat I would carefully weigh the costs of natural gas vs. heat pump. For a PV array I would lean toward keeping gas on the water heater, dryer, and maybe even the furnace. During my time in SD I lived in Linda Vista, El Cajon, and Lakeside. Only in El Cajon and Lakeside did I find a need to run a/c more than just a few hours a day and a week or two per year. Closer to the coast, most of the time having the windows open kept everything fine. Something you can only do where I live now maybe a week or two out of the entire year.

[dan sw fl]

Hi, first time poster, would appreciate your advice...

We are in the process of having a good sized grid-tied solar PV system installed, and have planned to intentionally over-size it to make room for further increases in electric usage (an electric car, HVAC).

I wanted to explain my thinking on the HVAC side and see if I'm doing anything stupid.

Climate is near-coastal San Diego, so AC usage is typically only a week or two a year. However this summer was really hot -- since my wife and I work at home a lot, it's nice to have AC for those rare times when we need it. (We put in a crummy window AC and it did ok at keeping one room cool, but it was noisy and added about 100KWH to our bill.)

However, modern heat pumps can do about 3.0 COP (ratio of heat out to electric in) in a mild climate. So we'd need 880*.65= 650KWHt of heat, which would require only 215KWHe of electricity for several months of the year (if my calcs are correct)

Price-wise this works out to about $30/month for gas vs. $30-60 for electricity at today's prices.
Thus if we were paying SDGE for electricity there'd be no financial benefit to buying a heat pump and using it for heat. But with solar, the balance changes.
(for reference SDGE charges about $1/therm for natural gas and $0.14 to $0.33/KWh.
Our solar system should be big enough to just zero out our bill on average year round, we believe)

For cooling, I figure we'd probably use 100-200KWH for 1-2 months of the year (that's a WAG number).

Thanks in advance

So, paraphrased,
Is the question,
you wish to invest in a ~ 4,000 watt solar system and a small heat pump [ total ~ $27,000 ]
and try to justify a savings of $200 a year on Heating (150 therms N.Gas) & Cooling (100 kwH) in SanDiego?

That SIMPLY does NOT work out even using CALIFORNIA _GREEN_ MATH.

[beenthere]

Something to think about...if you put in a heat pump, you will probably use the a/c side of it more than one or two weeks a year. Why? Because it's there. Whereas before you might hold out on the window shaker until you just could not stand it, now you can flip a switch and everything's cool again.

When I lived in San Diego I serviced heat pumps. Few of them had emergency or auxiliary heat strips. And this was back in the 80's. I know heat pump performance and efficiency has improved since then. But if I had gas heat I would carefully weigh the costs of natural gas vs. heat pump. For a PV array I would lean toward keeping gas on the water heater, dryer, and maybe even the furnace. During my time in SD I lived in Linda Vista, El Cajon, and Lakeside. Only in El Cajon and Lakeside did I find a need to run a/c more than just a few hours a day and a week or two per year. Closer to the coast, most of the time having the windows open kept everything fine. Something you can only do where I live now maybe a week or two out of the entire year.

Depends on what size array he puts in.

[beenthere]

So, paraphrased,
Is the question,
you wish to invest in a ~ 4,000 watt solar system and a small heat pump [ total ~ $27,000 ]
and try to justify a savings of $200 a year on Heating (150 therms N.Gas) & Cooling (100 kwH) in SanDiego?

That SIMPLY does NOT work out even using CALIFORNIA _GREEN_ MATH.

What if you add in that he could end up producing more electric then he uses. And gets a few dollars back from the electric company.

200 dollars heating saved.
100 dollars cooling saved.
1200 dollars yearly electric bill saved(doesn't include the A/C)
1500 dollars total yearly savings.

30% tax credit for the PV system brings its cost down.

[Shophound]

Depends on what size array he puts in.

True.

If I still lived in coastal San Diego I would still focus on the building envelope first before considering PV. No need to go super insulation crazy in SD, as the climate is temperate. However, paying attention to the envelope can reduce both the intensity and the need for energy consuming climate control. During my SD days I serviced many coastal homes with no a/c and only wall furnaces for heat. These old homes were leaky and marginally insulated; I can only imagine how even modest improvements in that climate would reduce costs and improve comfort overall to homes like these, and even the newer tract homes like the OP may have.

[soylentgreen]

So, paraphrased,
Is the question,
you wish to invest in a ~ 4,000 watt solar system and a small heat pump [ total ~ $27,000 ]
and try to justify a savings of $200 a year on Heating (150 therms N.Gas) & Cooling (100 kwH) in SanDiego?

That SIMPLY does NOT work out even using CALIFORNIA _GREEN_ MATH.

No, but that's because you left out 80% of our electricity usage!

My Math:
The PV system will be about $19k after rebates & tax credit, and should generate 7800 KWH/year. Our average utility bill with electric car usage and (estimated) heat pump usage will be about $150/month. PV system + Heat Pump should knock that down to about $15/month (mainly natural gas for the clothes dryer and water heater). So the savings is something like $130/month or about $1500/year. This implies a 12 year payback on the system. I've run the numbers, and you can make this payback period go up or down substantially depending on the values you plug in for cost-of-money vs. expected increases in electricity and gas prices, and estimates of how much more electricity we will use - we have one electric car but only drive a few days a week on average. So overall I'm pretty confident the PV system, as a pure investment, ranges from "mildly bad" to "screaming great!" depending on the assumptions one uses.

I'm intentionally not including the cost of the heat pump, but I'm not sure it's fair to include that entirely in the calculation, since (A) our furnace is 25 years old and probably needs replacing soon anyway, and (B) there will be a substantial quality-of-life increase.

[soylentgreen]

True.

If I still lived in coastal San Diego I would still focus on the building envelope first before considering PV. No need to go super insulation crazy in SD, as the climate is temperate. However, paying attention to the envelope can reduce both the intensity and the need for energy consuming climate control. During my SD days I serviced many coastal homes with no a/c and only wall furnaces for heat. These old homes were leaky and marginally insulated; I can only imagine how even modest improvements in that climate would reduce costs and improve comfort overall to homes like these, and even the newer tract homes like the OP may have.

The house is stucco with concrete roof and quite well insulated. If we lived closer to the ocean we wouldn't need A/C, and if we lived further away we would need it without question. We are kind of in the transition zone climate-wise.

A mini-split located in the office has been suggested, but I'm not sure if this would do the trick, especially in the winter when we'd want downstairs heating.

[Shophound]

Minisplit heat pumps are available.

[Tommy knocker]

No, but that's because you left out 80% of our electricity usage!

My Math:
The PV system will be about $19k after rebates & tax credit, and should generate 7800 KWH/year. Our average utility bill with electric car usage and (estimated) heat pump usage will be about $150/month. PV system + Heat Pump should knock that down to about $15/month (mainly natural gas for the clothes dryer and water heater). So the savings is something like $130/month or about $1500/year. This implies a 12 year payback on the system. I've run the numbers, and you can make this payback period go up or down substantially depending on the values you plug in for cost-of-money vs. expected increases in electricity and gas prices, and estimates of how much more electricity we will use - we have one electric car but only drive a few days a week on average. So overall I'm pretty confident the PV system, as a pure investment, ranges from "mildly bad" to "screaming great!" depending on the assumptions one uses.

I'm intentionally not including the cost of the heat pump, but I'm not sure it's fair to include that entirely in the calculation, since (A) our furnace is 25 years old and probably needs replacing soon anyway, and (B) there will be a substantial quality-of-life increase.

See what I mean now?

[soylentgreen]

See what I mean now?

Tommy, I've asked the moderators to look into your posts, as so far you've been 100% snark and 0% content. I'm happy to entertain a strong disagreement if you bring facts and data, or even if it's just plain old crusty old-fart opinions to the table.

But you are just doing content-free sniping here. I'd expect better on a "professionals-only" forum.

I would love for you to explain what you think, and why you think it. I'm here to learn.

[soylentgreen]

Minisplit heat pumps are available.

I thought that all minisplits were heat pumps... am I misunderstanding something? Are there non-heatpump minisplits?

[dan sw fl]

Several alternatives exist in the mini-split area.

For example,
two of Several Mfgs,


http://us.sanyo.com/HVAC-Multi-Split-Systems

http://catalog.mitsubishipro.com/cat...s-heat-pumps-2

[soylentgreen]

Several alternatives exist in the mini-split area.
For example,
two of Several Mfgs,

Ok, I think I understand my confusion : although technically all Air conditioners are "heat pumps" in the thermodynamic sense, in the industry the terms have special meanings, e.g.
• "Heat Pump" : a heat pump that works in both cooling and heating modes
• "Air Conditioner" : a heat pump that works only in cooling mode.

(which brings up the question: is there a "heat pump" that only works in heating mode? If so, what would that be called? Off topic I know...)

Now I think I understand Shophound's original comment - they meant that I could use a minisplit for heating, not just cooling.

I agree, and it's a good comment, though in this house the office is upstairs, so I think in the winter a minisplit used for heating wouldn't do a good job of heating the whole house. I suppose that might be workable - just heat the office during the day and leave the downstairs unheated.

[Tommy knocker]

Tommy, I've asked the moderators to look into your posts, as so far you've been 100% snark and 0% content. I'm happy to entertain a strong disagreement if you bring facts and data, or even if it's just plain old crusty old-fart opinions to the table.

But you are just doing content-free sniping here. I'd expect better on a "professionals-only" forum.

I would love for you to explain what you think, and why you think it. I'm here to learn.

Ok. My professional opinion is its a waste of money, the solar thing. Your #s may be good, didn't actually look, in a perfect world where stuff don't break. In my opinion your probably closer to 20 years on the ROR. Now you will disagree and though out a bunch of previously researched data you have there in front of you. My point was it seems you already have the data and made your decision. And I doubt anyone here is going to change your mind.