Best way to use excess solar to offset gas heat

[compwizpro]

For me personally: to get the most out of every WattHour I generate. A heatpump easily gets you two to four times the heat output compared to direct electric for every unit of energy you put in.

This is my thought as well regarding heat pump over resistive heating. Based on the fuel costs, it would take me over 4 times the electricity to produce the same BTUs as my furnace which is a worse conversion than the sell back cost of power back to the utility. So the only real benefit I would get is being able to heat whatever room I'm in more.

The portable heat pumps I mentioned seemed to be the lowest investment in case my idea doesn't pan out, would get about 2 - 3x the BTU per watt than resistive heating, and minimal installation / impact of the house. But I've typically stayed away from portable units for cooling due to the extra noise from the compressor being on the inside. I'm also aware they only operate down to ~40F outside ambient so I wouldn't be able to run them in the deep winter months, but thats also when my solar is expected to underproduce so it wouldn't make sense anyway.

A 1 - 2 ton mini-split would be ideal from a cost / efficiency standpoint, but I don't have a good place to put the head unit that wouldn't be an eye sore inside / distribute the air evenly enough.

I like the idea of a monobloc heat pump with a water to air heat exchanger in the duct as another poster mentioned and they look relatively easy to install based on a few videos i've seen. I would need to find one I could buy in the US ideally in the ~2 ton range and look how I can integrate it into my existing thermostat (I have an Ecobee). Some people mentioned to use the different heat stages to trigger the heat pump vs gas. Also I would need to make sure the extra coil inside the plenum doesn't restrict the airflow too much to avoid burning out the blower.

All great suggestions!

 

[Doggydogworld]

Do you have a gas water heater? I'd look there first since that can reduce gas consumption every month you have overages, not just the months you need space heating. The air-to-water heat pump suggestions look good, if not too expensive. Some folks dump excess solar electricity directly into resistance water heater coils.

Since we're only talking about excess solar, I'd use the 4.3 cent/kWh price in your electric vs. gas calculations. Unless I'm missing something about your electricity rates, the 16.6 cents doesn't apply at all during months you have excess solar. And you'd presumably turn off your new "excess solar energy dump" system in the winter months when you don't have excess solar.

I don't see a benefit in using a portable heat pump for cooling the house in the summer when you already have central A/C. That just trades one electricity use for another. What's the benefit? I'd much rather put those excess summer solar kWhs into hot water and reduce gas usage.

 

[compwizpro]

That is my plan but my water heater is only ~7 years old and will replace it with a heat pump water heater once it fails.

The 4.3cent/kWh was a reference to the opportunity cost if I consumed the excess myself vs. selling it back. It was a reference to if I were to use the excess for resistive heating vs. selling it back, it would take me over 4x the electricity to equal to the BTU output using gas so it basically becomes a wash except I don't need to buy any extra equipment to sell the excess back to the utility.

Correct on the portable unit for cooling, the efficiency ratio nearly what my central AC is. The only advantage I see using it for cooling is I turn the temp down at night for sleeping and could use the portable unit to cool the room instead of turning the whole house temp down but that only works if I can tolerate the noise.

 

[Ampster]

I replaced my centralized gas fired furnace with an LG mini split. It has kept the two areas warm where we congregate without having to use gas to heat the whole house. I can turn on the other zones if needed.
Long term my solar system was sized for the above elimination of natural gas.

 

[timselectric]

My current water heater is also gas. And also only about 6 years old. I'm keeping it and putting heat pump water heated tank in front of it. This way it's still there as a backup option. I am a big fan of redundancies for everything.

 

[OzSolar]

FWIW: Bottom line is that the ROI on trying do anything with kWh's lost in the shoulder months rounds to zero. Those are some really cheap kWh since you are already throwing them away and unless you can suggest a practical way to put them in some sort of storage media for use in December when you really need them there's not really much of a bother to try.

 

[mvonw]

There are several available on the European market, for example:

Browse Air-To-Water Heat Pumps Units at ClimaMarket

The most efficient solution for your cold and heat pump needs. Install Air-to-Water Heat Pumps and enjoy heating, cooling and hot water.

www.climamarket.eu

Air to Water Heat Pumps | SPRSUN

SPRSUN is one of the leading air to water heat pump systems in China. Their DC inverter air to water heat pumps are designed for domestic hot water, heating and cooling.

sprsunheatpump.com

Heat pumps - Wasserman.eu

See ⁨Heat pumps⁩ at Wasserman.eu online store

www.wasserman.eu

Not sure about US distributors - I did a quick search, but didn't really find anything. Maybe some well known brands like LG have a US distributor, but again, didn't immediately find one.

Mine is a small PW030-DKZLRS-A from https://www.powerworld-e.com/content.php?id=212
The reason for this one is that I can run it off of my small winter inverter when needed without issues. Any excess solar just gets dumped in the buffer tank (for heating or cooling). It's also cheap, uses R-32, and installing a monoblock is just a matter of hooking up the water (glycol) lines.

I got a quote from a U.S. distributor for about $1000 for a midsize storage heater. It was wired for US. 240 volt. I have ordered one to be wired into my SolArk’s dump load.

 

[compwizpro]

Are you able to share the us distributor name along with the unit you got a quote for or is it the one in your post?

 

[mvonw]

Are you able to share the us distributor name along with the unit you got a quote for or is it the one in your post?

I'm always a little reluctant to post vendor information, because I have no idea if they are reputable, but I got the quote from
libertyelectricproducts.com
My quote was for $998 USD for the Eco208 unit.

 

[goldserve]

Setting up mqtt and letting the openevse take up all the excess solar I am not allowed to export has been great. Free miles on the ev, powered by excess solar when the export agreement is not favorable to sell excess or system was expanded and agreement was not amended

 

[OffGridInTheCity]

We have a 2600sq ft, built in 1997 (pretty good insulation) in Southern Oregon / zone4 (mild) climate. Originally I'd hoped to run the house 100% on my PV array and we do consume all the PV we produce - a lot of it running (diverted to) the heat-pump.

We went with a top tier Lennox Forced Air Heat-Pump w/natural gas secondary a few years ago for electric heat (and cool). The Lennox Heat Pump is Model XP25-048 with Heat COP hi = 3.14 / low = 1.88 and HSPV (IV) = 8.2. The matching furnace is model SLP98UH090XV60C.

We keep the house 70F at night, 75F daytime. The heat-pump works down to 20F (lowest we've ever gotten) and has no trouble maintaining 70F but really struggles to raise the temp from 70F to75F if outdoor is <30F. We raise the temp to 75F starting at 5:30am - not the most efficient time - and it can take several hours when <35F outdoor. Max power required is 18a @ 240v for the outdoor + another 600w? for the fan so a max of ~5,000w. It only runs 5,000w continuously at the extremes - raising temp when <35F or cooling when > 100F.

Here's the electric + natural gas therms for the last 3 winters - playing around with all electric vs natural gas assist. This winter has been harsh.


Using "1 therm = 29kwhs" it's interesting to note that my 2021-2022 mild winter used significantly more 'overall kwhs' because of the natural gas component than the all electric winters. Not sure it's apples to apples kwhs wise as I don't know efficiency of burning natural gas - but interesting that all electric heat-pumps may well indeed be a bit more efficient in mild climates.

 

[Supervstech]

kWh across the board, heat pumps are the most conservative, and with 20+ SEER equipment with variable output abilities, they are amazing producers. Full heat output in subzero conditions, i like the pricepoint of the BOSCH systems for forced air equipment, and daiken for the minisplit stuff.
several much higher efficiency models abound for best use of solar too.
There are 97.5% efficient gas furnaces available, but pricy. 92 to 95% efficient is more readily available and affordable, if you arent producing enough winter solar power, and grid is natural gas powered, high efficiency gas furnace is the best option.
For tweaking the small overproduction times in winter to heat, a small minisplit is the best bang for the buck. If they EVER put inverter compressors into a portable unit that may be a better option...

 

[wattmatters]

A 1 - 2 ton mini-split would be ideal from a cost / efficiency standpoint, but I don't have a good place to put the head unit that wouldn't be an eye sore inside / distribute the air evenly enough.

You can get split system indoor units which are hidden in bulkheads or appear as vents. Splits are by far and away the most efficient means of pushing heat into a space using electricity. Just get heat pumps rated for your climate.

I have a smart PV diverter for heating water.

You may have an EV one day and there are going to be options to vary the power delivered to the charger.

I don't understand US aircon systems. They go to the trouble of installing a big fat heat pump, only to restrict it to operate in one direction (space cooling) when frankly it's pretty trivial for it to be able to pump heat either way. Seems such a wasted opportunity.

 

[timselectric]

I don't understand US aircon systems. They go to the trouble of installing a big fat heat pump, only to restrict it to operate in one direction (space cooling) when frankly it's pretty trivial for it to be able to pump heat either way. Seems such a wasted opportunity

Natural gas is cheap in the states.
And heat pumps didn't work well in cold climates, until recently.
Money is always the deciding factor.

 

[upnorthandpersonal]

If money is not the deciding factor, and if you want maximum efficiency, you need low temperature heating systems such as a hydronic radiant floor heating. You minimize losses, and it's one of the most energy efficient systems to heat a building. I know this type is still considered a luxury in some countries and is expensive, but here in Finland it's pretty much the default option in any new building for a very long time already (together with heat pumps, geothermal or otherwise). Keep in mind that we don't have a natural gas network, so these technologies were much easier to adapt across the board.

 

[compwizpro]

If they EVER put inverter compressors into a portable unit that may be a better option...

So the Toshiba RAC-PT1412HVWRU portable unit I found does claim to use an inverter compressor with a CEER of 12.3 which doesn't seem terrible for this type of unit. The Home Depot link for the same unit says in heating mode it can produce 12000 BTU. Even if it was pulling the max power of 1500W (which I found the equivalent Madea unit only pulling 1000W for heat), that's still a COP of 2.3 (1500W = 5,115 BTU) if I'm doing that math right. It's not a ton of heat but I would only have it run in the fall / spring months where I still have a surplus of solar output.

We went with a top tier Lennox Forced Air Heat-Pump w/natural gas secondary a few years ago for electric heat (and cool). The Lennox Heat Pump is Model XP25-048 with Heat COP hi = 3.14 / low = 1.88 and HSPV (IV) = 8.2. The matching furnace is model SLP98UH090XV60C.

This is my goal long term. The furnace and AC were replaced a few years ago by the previous owner so I still have a ton of life left in those units. But once they need replacing / when it makes financial sense, I was going to go with something like that but hopefully a lower minimum outdoor temp for heat and at least a 2 stage blower. It's not uncommon to get below 0F for at least a week or 2 and stay in the 10s - 20s for at least a month. Those are also the times I will be most likely not have a solar surplus so sticking with a gas backup probably makes more sense at those temperatures.

You can get split system indoor units which are hidden in bulkheads or appear as vents

I looked into this more and it looks like they make floor mountable units as well. Although they look more expensive, I think that would be a more feasible option. A unit that could either fit inside an existing duct or be spliced inside an existing duct might be potential option as well depending on the cost.

 

[Mattb4]

So the Toshiba RAC-PT1412HVWRU portable unit I found does claim to use an inverter compressor with a CEER of 12.3 which doesn't seem terrible for this type of unit. The Home Depot link for the same unit says in heating mode it can produce 12000 BTU. Even if it was pulling the max power of 1500W (which I found the equivalent Madea unit only pulling 1000W for heat), that's still a COP of 2.3 (1500W = 5,115 BTU) if I'm doing that math right. It's not a ton of heat but I would only have it run in the fall / spring months where I still have a surplus of solar output.


...

Thanks for mentioning the Toshiba. I was unaware that such a thing existed. I am adding to my list of potential future things to get.

 

[OffGridInTheCity]

So the Toshiba RAC-PT1412HVWRU portable unit I found does claim to use an inverter compressor with a CEER of 12.3 which doesn't seem terrible for this type of unit. The Home Depot link for the same unit says in heating mode it can produce 12000 BTU. Even if it was pulling the max power of 1500W (which I found the equivalent Madea unit only pulling 1000W for heat), that's still a COP of 2.3 (1500W = 5,115 BTU) if I'm doing that math right. It's not a ton of heat but I would only have it run in the fall / spring months where I still have a surplus of solar output.

Interesting..... we have something similar. It's a Honneywell MM14CHCSCS portable 12,000BTU (heat) heat-pump - https://www.honeywellstore.com/stor...tu-cooling-and-heating-honeywell-mm14chcs.htm It says "Energy Efficiency Ratio (EER): 10.3 BTU/(W.h) 3.0 W/W"



I got it to keep a single room very warm during the night when the house is at lower temps and emergency backup. It's single exhaust and takes in 'indoor air' and heats that so it's not subject to outdoor temps. I don't think it's very robust in terms of operating temp ranges like a mini-split and I can't find detailed specs like you do for a mini-split.

In the morning we sometimes let it blow toward the heat-pump main home air intake in the hallway and it noticeably assists the whole house heat pump when it's struggling to raise the house temp in the morning due to <30F outdoors. This is it's 3rd winter and has been trouble free.

 

[compwizpro]

Interesting..... we have something similar. It's a Honneywell MM14CHCSCS portable 12,000BTU (heat) heat-pump - https://www.honeywellstore.com/stor...tu-cooling-and-heating-honeywell-mm14chcs.htm It says "Energy Efficiency Ratio (EER): 10.3 BTU/(W.h) 3.0 W/W"

I saw that unit used by others, but I couldn't find it on their website. It's annoying the manufactures websites don't contain all the energy efficiency / usage information. I would similar thing as you where use it to keep or main living space warm / have it set slightly higher than the main furnace set point so the furnace only kicks on as a backup. I would opt for a dual hose so I'm not drawing in outside air.

Thanks for mentioning the Toshiba.

This is apparently the same unit but made by Midea. It doesn't mention efficiency but the energy guide CEER values are similar. I'm hoping the inverter compressor makes it more efficient and quieter, at least that's what they advertise.

 

[OffGridInTheCity]

This is apparently the same unit but made by Midea. It doesn't mention efficiency but the energy guide CEER values are similar. I'm hoping the inverter compressor makes it more efficient and quieter, at least that's what they advertise.

The Honneywell unit is soft start but it's pretty loud - like a old fashioned window air conditioner level of sound.