Heat Pump for Heating and AirConditioning in San Diego

soylentgreensoylentgreen Solar Expert Posts: 106 ✭✭
We are in the process of having a good sized PV system (4.6KWAC) 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. This summer was rather hot, and since my wife and I work at home a lot, it's nice to have AC for those rare times when we need it. (This summer 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 and http://www.bankrate.com/brm/news/advice/20060217a1.asp). 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.

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 of course.

For cooling, I figure we'd probably use 100-200KWH for 1-2 months of the year.


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.

Comments

  • PhotowhitPhotowhit Solar Expert Posts: 5,364 ✭✭✭✭✭
    Re: Heat Pump for Heating and AirConditioning in San Diego

    Nice thing about having been grid connected is you can read off your electric bill how many Kw hours of electric your using?

    SO I take it this is a grid connected array? So we're talking about offsetting electric costs?
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • soylentgreensoylentgreen Solar Expert Posts: 106 ✭✭
    Re: Heat Pump for Heating and AirConditioning in San Diego

    Oh, yes, sorry I left out that key info: This will be a Grid Tied system.
  • dreesdrees Solar Expert Posts: 481 ✭✭✭
    Re: Heat Pump for Heating and AirConditioning in San Diego

    My thoughts after having a hybrid furnace / heat-pump installed on my house in San Diego County not far from the coast a couple years ago.

    1. Unless you live pretty far inland or your house is poorly insulated, you're really rarely running the heat during the coldest portion of the night since most will typically turn off set the thermostat to go off around bedtime and come on around the time you wake up. So in reality, it will rarely run below temps of 45F or so around here and will spend most of it's time running in temps around 50-60F. Good for a heat-pump!

    2. You don't want resistance backup heating. It will tend to come on when you don't really want it do.

    3. HVAC compressors can be noisy. The furnace is nearly silent. Not much of a concern inside, but might be outside depending on your neighbors and your tolerance for noise.

    4. If you size your PV system so that it doesn't completely offset your usage, you can still pretty easily run into the higher tiers of electricity in the winter. My system generates about 5,000 kWh / year. But in Dec/Jan it has generated as little as 215 kWh during a SDG&E billing period. Combined with using extra electricity running the heat pump, I pulled over 400 kWh from the grid that period which still put me into the expensive electricity for about 80 kWh worth. I probably should have run the gas furnace more that billing period. If your panels tilt south (mine are nearly flat) this won't be as much of an issue, but still something to take into consideration. It would be nice if the thermostats had more features for adjusting the switch between furnace and heat-pump other than outside-temp. Ideally I'd be able to monitor electricity usage and when I get to a point where it looks like I'm due to go over the baseline rates by the end of the billing period, switch to using gas only for heating.

    5. Adding A/C to a system is EXPENSIVE! A 95% gas furnace and perhaps a mini-split to cool your office will be much cheaper. If you're committed to adding whole-house A/C - it's not significantly more to get a heat-pump at that point.

    Hope this helps!
  • solar_davesolar_dave Solar Expert Posts: 2,367 ✭✭✭✭
    Re: Heat Pump for Heating and AirConditioning in San Diego

    When sizing a grid tie system most of the same things apply:

    1. Understand your loads, while less critical than on an off grid because you have a second source of power.
    2. Understand you goals, green, tier clipping, net zero ... PVWatts is a pretty good tool here.
    3. Conserve where you can, cheaper to save a watt than generate a watt.
    4. And maybe most critical, understand your net-metering plan and the various TOU plans available and how they might interact.

    If the best ROI is a goal then the net-metering plan is essential, your ability to just clip the high tier rate power consumption will create the best ROI, but generally you get addicted to NOT paying those utilities.

    Here the plan allows kWh carry forward, so on peak generation in winter allows high consumption in summer, but it doesn't help cover night time consumption. In your case it is a cash based plan, if you can get enough cash to cover early and then consume later is that OK? Net zero was my original goal, in order to do that I would need about another 4000 watts of solar, but my off peak consumption annually is less than $500. It is just not cost effective adding in another $16000 worth of PV to get to zero.

    BTW having electric cars is a pretty big load, we have used up to 400 kWh a month on our pair of plug in cars. If your profile for car usage is off peak charging it is a huge operating cost osaving, but if your usage drives you into high tier then size your solar accordingly. You can go to the various EV car web sites and find users in your area to get great ideas an rate plans that make the most sense. I think I saw SDG&E has plans for EV usage.
  • soylentgreensoylentgreen Solar Expert Posts: 106 ✭✭
    Re: Heat Pump for Heating and AirConditioning in San Diego
    drees wrote: »
    My thoughts after having a hybrid furnace / heat-pump installed on my house in San Diego County not far from the coast a couple years ago.

    1. Unless you live pretty far inland or your house is poorly insulated, you're really rarely running the heat during the coldest portion of the night since most will typically turn off set the thermostat to go off around bedtime and come on around the time you wake up. So in reality, it will rarely run below temps of 45F or so around here and will spend most of it's time running in temps around 50-60F. Good for a heat-pump!

    Good point, I'm not usually running it at 4AM :)

    2. You don't want resistance backup heating. It will tend to come on when you don't really want it do.

    OK.
    3. HVAC compressors can be noisy. The furnace is nearly silent. Not much of a concern inside, but might be outside depending on your neighbors and your tolerance for noise.

    Any info about how noisy? Our nearest neighbor's house is only 10 feet away, but they have zero windows on that side of the house. Our second nearest neighbor is about 80' away. I'm assuming that a two-stage unit means the outside compressor also runs at a slower speed, which would be good - or am I wrong about that?

    4. If you size your PV system so that it doesn't completely offset your usage, you can still pretty easily run into the higher tiers of electricity in the winter. My system generates about 5,000 kWh / year. But in Dec/Jan it has generated as little as 215 kWh during a SDG&E billing period. Combined with using extra electricity running the heat pump, I pulled over 400 kWh from the grid that period which still put me into the expensive electricity for about 80 kWh worth. I probably should have run the gas furnace more that billing period. If your panels tilt south (mine are nearly flat) this won't be as much of an issue, but still something to take into consideration. It would be nice if the thermostats had more features for adjusting the switch between furnace and heat-pump other than outside-temp. Ideally I'd be able to monitor electricity usage and when I get to a point where it looks like I'm due to go over the baseline rates by the end of the billing period, switch to using gas only for heating.

    Interesting - On the net metering plan, does SDGE do the math by raw KWH, or by KWH*price? E.g. if I over-generate in the summer, do I have "1000KWH" in the bank or is it "$250" that's a credit to my account?

    Panels will be at roughly 260 degrees tilt.

    I'm assuming that we'll have to get rid of our gas furnace when we get the air handler for the heat pump, but maybe I'm wrong about that. Need more info.

    5. Adding A/C to a system is EXPENSIVE! A 95% gas furnace and perhaps a mini-split to cool your office will be much cheaper. If you're committed to adding whole-house A/C - it's not significantly more to get a heat-pump at that point.

    We have budget for the more expensive solution, but I do appreciate the mini-split idea - one problem is that we have 2 offices, so we'd probably need 2 of them.


    Hope this helps!

    Tremendously, thanks!
  • soylentgreensoylentgreen Solar Expert Posts: 106 ✭✭
    Re: Heat Pump for Heating and AirConditioning in San Diego
    solar_dave wrote: »
    When sizing a grid tie system most of the same things apply:

    [...]
    If the best ROI is a goal then the net-metering plan is essential, your ability to just clip the high tier rate power consumption will create the best ROI, but generally you get addicted to NOT paying those utilities.

    Here the plan allows kWh carry forward, so on peak generation in winter allows high consumption in summer, but it doesn't help cover night time consumption. In your case it is a cash based plan, if you can get enough cash to cover early and then consume later is that OK? [...]
    BTW having electric cars is a pretty big load, we have used up to 400 kWh a month on our pair of plug in cars. If your profile for car usage is off peak charging it is a huge operating cost osaving, but if your usage drives you into high tier then size your solar accordingly. [...] I think I saw SDG&E has plans for EV usage.

    Agreed - for psychological reasons we'd love to zero out our bills completely, but realistically that's not going to happen nor is that the necessarily the best financial goal.

    On EV usage, we have some things in our favor : a part time & seasonal job so I'm generally not commuting in the summer months and only 2-3 days/week other months. We have a 240V blink charger and it's terribly convenient to just plug in and charge regardless of the time of day - I could probably learn to live with off peak charging, though there have been times when an unexpected trip came up in the evening and it was sure nice to have the car charged.

    SDGE does have an EV-TOU2 plan which is attractive (seems much better than the PV-TOU plan they offer). I'm not sure if it is better or worse than just staying on the regular DR plans (which have a $0.14 low tier baseline)
  • dreesdrees Solar Expert Posts: 481 ✭✭✭
    Re: Heat Pump for Heating and AirConditioning in San Diego
    Any info about how noisy? Our nearest neighbor's house is only 10 feet away, but they have zero windows on that side of the house. Our second nearest neighbor is about 80' away. I'm assuming that a two-stage unit means the outside compressor also runs at a slower speed, which would be good - or am I wrong about that?
    That shouldn't be a problem, then. You will definitely hear it, when outside and your nearest neighbors will be able to hear it with their windows open, but it shouldn't bother them without windows on the same side of the house. With my dual-stage system, the noise level isn't significantly different between the 2 stages at least going by ear.
    Interesting - On the net metering plan, does SDGE do the math by raw KWH, or by KWH*price? E.g. if I over-generate in the summer, do I have "1000KWH" in the bank or is it "$250" that's a credit to my account?
    Both. :) For most of the accounting, it's a dollar amount credit that is saved (you always have a $0.17/day minimum bill that has to be paid). If you are on a TOU plan, this means that you can build up credits even faster during the summer. You can only get a credit for excess generation at the end of the year if you have an excess production of energy.
    Panels will be at roughly 260 degrees tilt.
    That's the direction they will be facing - pretty close to west which is good for the TOU schedule. I assume your roof has a fairly typical ~20* tilt?
    I'm assuming that we'll have to get rid of our gas furnace when we get the air handler for the heat pump, but maybe I'm wrong about that. Need more info.
    Yeah, if you don't have much room you might not be able to fit both a furnace/air handler. My stack is about 8 ft tall. Let me know if you want references for various HVAC companies that I got quotes from.

    I wasn't able to find a HVAC contractor after 3 quotes that did proper heating/cooling load analysis - they all guesstimated and based on my own calcs tended to oversize the system (suggesting a 4-ton system even though 3-ton is plenty especially given that my ducting isn't sufficient for 4-tons of air - even 3-tons is pushing quite a bit of air through them).
    On EV usage, we have some things in our favor : a part time & seasonal job so I'm generally not commuting in the summer months and only 2-3 days/week other months. We have a 240V blink charger and it's terribly convenient to just plug in and charge regardless of the time of day - I could probably learn to live with off peak charging, though there have been times when an unexpected trip came up in the evening and it was sure nice to have the car charged.

    SDGE does have an EV-TOU2 plan which is attractive (seems much better than the PV-TOU plan they offer). I'm not sure if it is better or worse than just staying on the regular DR plans (which have a $0.14 low tier baseline)
    I'm on the special EV Project plan with the lowest spread between peak/off-peak rates - it's basically the same as the EV-TOU plans. 99% of my charging happens on timer - I have an end-timer on my LEAF which means it normally stops between 5:00-5:30 so the bulk of the charging happens on the super-off-peak rate and a bit on the off-peak rate. I originally always started charging at 12am when super-off-peak started, but keeping the SOC of the battery low when possible is better for battery life.

    I very rarely have to charge during on-peak and the significantly higher rates are only during 12pm-6pm from May 1-Oct 31. When I do, I still figure that I'm paying basically the same as it costs to buy gas for the Prius, so it doesn't bother me.

    I've thought about going whole-house TOU but if not done properly one can easily end up with a higher bill. Really need hourly consumption data for a long time to make these decisions, but the data is not easy to get.
  • BB.BB. Super Moderators, Administrators Posts: 31,061 admin
    Re: Heat Pump for Heating and AirConditioning in San Diego

    Assuming San Diego's power policies are somewhat similar to our's in Northern California (PG&E)--Your meter will spin backwards during mid-day. And forwards at night.

    The net result is read at the the end of the month.

    For example, in summer, I will generate around 250 kWH a month from Noon-6pm Monday-Friday which I paid to my account at ~$0.30 per kWH ($75 credit)

    My off peak usage will be 80 kWH @ $0.10 per kWH (call it $8 usage)

    My net for summer will be $75-$8=$63 credit for summer month.

    In the winter, my usage is more than my generation, so I pull from my "account" and only pay ~$6 per month minimum billing (I cannot avoid that charge).

    At the end of the year, any "money" left in the account is reset to ZERO. If there is a negative balance, I have to pay the amount due.

    If I have generated more kWH than I have used, I will get something like $0.05 per kWH for excess generated (zero to maybe $20 credit for me).

    Note, it gets more complex because we have tiered billing. These tiers also apply to the credit amounts too. But it is not easy to calculate--and years ago, PG&E temporally changed the calculations which dramatically increased off peak power costs (i.e., generated power would push us into a higher billing rate, which dramatically increased our off-peak power costs--so on paper, a large solar array penalized people just like a large A/C system would--very mad customers until plan was changed back).

    Make sure you understand the billing for your utility. In California, I believe you will have to have Time of Use Billing. If you have heavy power usage during working hours (10am-6pm roughly) for A/C or computer servers, etc.---And you have a "smaller" solar power system, it is possible that with TOU billing and a smaller array to actually end up paying a larger utility bill with solar vs your "no solar GT" bill. (This killed people that wanted to try a small system first to see how well it worked).

    Also, if you pay commercial rates--The whole numbers game for GT Solar is a different kettle of fish. And much more difficult to justify in many cases.

    Anyway--Do not assume anything. Ask questions and (if you are using an installer), have them show you what your billing will be (not always easy unless you have time of use logs for your home--should be available with the new "smart meters").

    -Bill

    In northern California, using a lot of power off peak will push us up into much higher tiered pricing... So be careful.
    Total Energy Rates ($ per kWh) PEAK [B]OFF-PEAK [/B]
    Summer 
    Baseline Usage  $0.31312  ( )$0.07921 ( ) 
    101% - 130% of Baseline  $0.33128  ( )[B]$0.09737[/B] ( ) 
    131% - 200% of Baseline  $0.48086 (R) $0.24695 (R) 
    201% - 300% of Baseline  $0.52086 (R) $0.28695 (R) 
    Over 300% of Baseline  $0.52086 (R)[B] $0.28695[/B] (R) 
    
    Winter   
    Baseline Usage  $0.11093  ( )$0.08262 ( ) 
    101% - 130% of Baseline  $0.12909  ( )[B]$0.10078[/B] ( ) 
    131% - 200% of Baseline  $0.27867 (R) $0.25036 (R) 
    201% - 300% of Baseline  $0.31867 (R) $0.29036 (R) 
    Over 300% of Baseline  $0.31867 (R) [B]$0.29036[/B] (R) 
    
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Heat Pump for Heating and AirConditioning in San Diego

    I've been planning to convert from a whole-house heat pump and A/C system (heat pump upstairs, gas furnace + A/C downstairs) to mini splits in most rooms, for 2 years. Now, however, with modern heat pumps getting over 20 SEER I'm leaning back towards just replacing the air handlers & pumps. With the house already ducted it will be cheaper, with only a minor loss in efficiency. Unfortunately I'll lose out on being able to zone the house room by room with the mini splits, but since it already has upper and lower zones that still helps some. You are in the same situation since your house is already ducted and wired for the pump. Modern systems combine gas furnaces with air handler and evaporator in a fairly small footprint. Mine is about 2 X 2 1/2 going by memory. Modern gas efficiencies are extremely high - upper 90's - but that also requires fan-forced exhaust because they take so much heat out to get that efficiency. A plus though is they only need 2 PVC pipes to do it and not a chimney.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
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