Number crunching

her system quote claims 135% of usage, so that would help with the heat pump electrical usage.
she's on natural gas, which is skyrocketing. so a price comparison would be interesting.
either way, i will advise her to leave the furnace in place if possible.

 

What utility company?

To address this problem, make sure your battery can not be charged from the grid (an option) nor discharge into the grid (an option). In effect, the battery is only connected to the house load and never can load or return power to the grid. This means the battery can never be a load or source to the grid. But this assumes you are not "selling" your excess solar to the grid.

Another work around is to have two EVs. One is used for daylight errands and the other is charged during the sunlight hours. The next day, swap the cars so the fully charged one runs errands and the 'drained' one takes the solar charge.

Another option becoming more available is V-to-G converters. In effect, the EV (the utility doesn't have to know about) buffers the excess solar. However, this has to be 'engineered' just it may defer the forced transformer upgrade.

My Huntsville utility had to be dragged to a permit to operate. So I lost about three months of solar because the "permit wasn't posted!" Part of the TVA system, they are NOT friends to home solar. Regardless, here are my specs:

• 4.8 kW - peak solar production
• 13 kWh battery - solar ONLY!
  • 5 kW max rate
  • no grid charging to battery
  • no grid discharging from battery
  • measured round-trip efficiency 86%
• Significant loads
  • 7.5 kW - Tesla (44 kWh), derated to 3.7 kW but online adjustable.
  • 7.3 kW - BMW i3-REx (28 kWh), derated to 3.4 kW
• Emergency power generator
  • 16 kW - only when grid is down and battery drained to 5% SOC
  • Does not charge solar battery

In 2016, I upgraded from 100 A (25 kW) to 200 A (50 kW) service when transitioned from Prius to EVs. That was also when I added the 16 kW, emergency generator that is natural gas fueled.

My November 1 to 19 metrics:

• 339 kWh - solar production, saved $0.14/kWh (cost avoidance of high usage rate)
• 217 kWh - grid production, $0.12/kWh

Bob Wilson

 

Are you doing this purely for lowest cost, or also to reduce carbon emissions?

I'm often willing to pay a bit extra for the later.

 

There are "all electric" heat strips, there are electric heat pumps with heat strip backup and there are dual fuel heat pump/ hydrocarbon solutions.

In many parts of the country oil is not an option, metro areas have natural gas while propane is the only hydrocarbon for most of us.

Dual fuel is an excellent option for the rest of us since heat pumps are at least 1/3 of the operating costs of heat strips alone and propane costs as much as heat strips to operate standalone.

Dual fuel means the system can switch automatically and can be set to do so based on an economic or performance balance point.

You get ac by default with a heat pump.

In my mind the downside of heat pumps, with inverter based low ambient compressors or standard one or two stage psc compressors, is they are complex and don't have the same kind of truck stock serviceability as hydrocarbon based heat solutions. These days, parts can be a week out if available at all for most inverter based systems. Inverters are now shipping in traditional one or two stage units in the form of variable speed blower and condenser fan motors.

In addition heat pumps are lucky to maintain rated btus at 32f and most lose btus as it gets colder. Meanwhile the heat load is increasing.

Hydrocarbon heat can easily supply rated btus at any temperature and are usually oversized to begin with.

The other advantage of hydrocarbon solutions is the relative ease of generator backup which is nearly impossible for a week of ice storms when depending exclusively on a heat pump.

Overall I like dual fuel which costs a little more upfront but provides reliable comfort in winter and nice ac in the summer.

 

So far in shoulder season with day time temperature not lower than 25F and daytime high above 32F, the heat pumps works wonder. It is quiet, efficient, comfortable, and economical compared to using whole house oil boiler hot water heating system. As long as our electricity consumption does not go above net-zero, I will keep using electricity only to heat our home. But if the heat pump usage trips the net-zero into red, then oil boiler will be turned on. But I am curious how far down in low temperature our heat pump can function and at what efficiency. As I commented, the spec says it can work down to -31F and max HSPF2 up to 14. I am closely monitoring electric consumption of each unit and our outside and room temperature now. If the efficiency drops substantially or it can not heat adequately when outside temp gets colder, then that will indicate the heat pump alone is not suitable for whole house heating in our area. We will see how it goes. Today was the coldest day so far since we had the heat pump installed in May this year. Today morning time low was 23F and day time high temp was 36F. We still have a few more hours left today, but looks like we are going to use more than 20kWh total on heat pump operations alone. With other consumption added, ~25kWh solar generation today is not going to cover it. We started digging into the summer saving credit already. And we still have 3 months+ of real winter ahead of us.

 

Good thing there is no man-made, global warming or we'd all have to save money on our heating bills over the next months.

What brought this home was seeing recent TV reports of "record setting temperatures" in November. Reports by TV weather women and men who in Dixie aren't allowed to mention global warming.

Bob Wilson

 

Heat pumps commonly used to be rated at 47F, though some colder climate models now seem to be rated at a lower temperature, e.g. 32.

In theory, ALL (not just "most") heap pumps should lose heat output and efficiency as source temperature drops, that is a very direct result of the thermodynamic equations on which they work. Though I have seen some versions with a significant flat plateau over some temperature range. The makers provide datasheets listing their performance at numerous operating points.

This variable output, dropping as temperatures fall, is why home system design and equipment selection are significantly more complex than for traditional heat systems, requiring greater expertise. As outside temperature falls, a house needs more heat at the same time a heat pump delivers less, so the curves intersect at a "balance point" temperature, which the customer and designer can select. The lower the chosen balance point, the higher capacity and greater cost of the needed equipment. Below that point, supplemental heat is needed, though the heat pump still contributes what it can. There is also a minimum operating temperature, below which the heat pump delivers nothing at all.

When choosing a mini-split heat pump for my all-electric home's largest area (not meant to heat the whole house) almost 17 years ago, I had the advantage of having kept two decades of electric utility records, plus daily (and a few hourly) meter readings during several of the strongest cold snaps. These, set alongside detailed performance tables of the models being considered, proved invaluable in choosing system size. I picked a unit that would put the 'balance point' for that portion of the house in the mid 20s (with a minimum operating point of +5F) and kept the original zoned wall blowers for colder conditions. And kept the wood stove for power outages. It was also the largest to meet local incentive requirements, the sales-critter had initially recommended a step larger unit that did not qualify. I was happy with the results. Later, a significant attic and floor insulation upgrade noticeably lowered the balance point.

Three years ago, I added a second unit at the other end of the house, in the master bedroom (mostly to get A/C there), smaller but rated to operate down to -5F. With both units operating, the whole house has since not needed any supplemental heat, even when outside fell a bit below +10F. And the zone in between is definitely more comfortable.

... That is the best I've heard for commercially available units. When I was last checking out choices, most of the coldest climate models worked down to only the -15 to -20F range, depending on maker and model line. Much better than when choosing my first, but not needed for this mild coastal zone.

Just remember that it won't provide its top-line rated capacity anywhere near that temperature. Supplemental heat will be needed much earlier. And if I remember correctly (from several years ago, I took leave from PC for a while and haven't caught up with your postings of the past year), your house may have control problems running heat pumps and oil at the same time. I hope you found a workable approach.

 

Advanced heat pumps pioneered by Mitsubishi's hyper heat models can maintain their rated btus down to -20f and output some heat at even lower ambients.

However how they do it is not in the brochures and the techniques are a combination of complexity and marketing trickery.

The primary reason is variable speed compressors and blowers can run faster than "100%" to avoid losing "rated" capacity. The units get louder and use far more power at very low ambients. Efficiency can be worse than electric strips at extreme low temperatures.

They also forego traditional reversing valve defrosts by switching to hot gas bypass which uses a separate refrigerant circuit to defrost the outside unit while still providing heat inside.


Typical Hyper Heat Low Ambient Heat Pump in Heat Mode

A "hyper" unit may be rated for 24,000 btus at standard conditions (usually 47f) but often can achieve that 47f capacity at 50% compressor speed. As the outside temperature decreases, the unit ramps up its compressor to maintain rated capacity. At some point the compressor is running at 100% speed and rated capacity and it gets colder outside. The hyper unit then goes overspeed (hyper) holding full capacity sometimes down to -15f. Typically the outside unit gets louder and actually is drawing far more current. The power use is opposite of a conventional unitary system that uses less power as it is getting colder outside.

Hyper units have two more engineering features at their disposal. The use of electronic expansion valves (EEV) and computer controlled vapor injection. Some EEVs control the superheat and replace the function of traditional metering orifices (pistons) and TXVs (Thermal Expansion Valves). EEVs are computer controlled, stepper motor driven, refrigerant metering devices. The unit's computer can regulate the EEV to minimize wasted superheat and optimize the refrigerant flow based on realtime refrigerant and ambient temperatures. This adds btus and is needed for optimum variable capacity.

Vapor (flash) injection adds another EEV and plate heat exchanger (HX) to divert a small quantity of refrigerant after it has gone through the indoor coil. It injects a precisely metered portion of flash gas back to the compressor to avoid excessive discharge temperature when the compressor is in its special "hyper" speed mode. Vapor injection easily enables at least 10% more capacity exactly when it is needed, eg during the critical below freezing temperatures down to the unit's minimum rated ambients.

Finally, in most parts of the country propane is cost equivalent to electricity, so insulation and air sealing becomes more important to use heat pumps effectively. In order for heat pumps to effectively transfer heat to and from the outside air, proper installation is critical for long term reliability and more training and skill is required to repair a "hyper heat" model, something sorely lacking in today's tradesmen. Truck stock repairs are unlikely making dual fuel systems an optimum solution. With dual fuel the complexity of hyper heat models is unnecessary and tradesmen are more likely to be in their comfort zones.

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