Heat Pump vs. Geothermal Heat Pump

[David_Cary]

As someone who has built and considered geothermal. It doesn't pencil out in moderate climate areas.
It is also true that there are large inefficiencies and lack of competition in moderate climate areas which make the cost prohibitive.
In NC, we had (still have?) a state tax credit so you were getting 65% or so of your entire HVAC bill back. But when Geo was $80k and standard (decent HPs) is $15k, even the huge tax credit doesn't work.
Yes with hot water and assuming less maintenance/longer life, you can imagine in a low interest rate environment then maybe.
But, even without a solar state tax credit, that makes so much more sense in today's financial/rate structure environment.

My HERS is -7 without geothermal, with solar. My total HVAC install cost was $15k. I have a heat pump hot water heater too so we can call it $17k. 3800 sqft. Pool.
My last bill had 350 kwh of credit - because of less driving. I lose all that credit May 31 (when it will be 700 or 800 kwh).

Now if net metering goes away, batteries stay expensive, electricity reaches CA rates, and NG is outlawed - then absolutely Geo makes sense.

BTW - my last energy audit which showed comparison costs showed a $200 a year savings with Geo over conv HPs. Even if you get geo down to $5k over HP, that is still 25 years ignoring time value of money.

Drilling a well alone in my area costs $8k. You can't just get below a slab - you will cool the local earth.

 

[willow_hiller]

Has anyone factored in system longevity when comparing air source to ground source?

We're running a 20 year old heat pump right now, and we end up using over 6,000 kWh annually to operate it. And add to that about 3,000 kWh per year from a fiberglass tank traditional water heater (9,000 kWh per year total).

For my house size, I figure a geothermal system could bring the annual energy usage from heating/cooling down to 1,500 kWh per year, and if we opt for one with a desuperheater that supposedly can reduce water heater operating costs by half (down to 3,000 kWh total per year).

Meanwhile for about half the price (or less) of the geothermal system, we could just install a new air source heat pump which would probably use 2,000 kWh per year (5,000 kWh per year total). So on up-front costs and efficiency it doesn't seem worth the replacement... but I've also read that a geothermal heat pump lasts about 10 years longer than an air source heat pump (25 years vs 15 year), and the ground loop itself can last for 50+ years. So maybe if you stay in the home longer than 15 years, the investment is worth it.

 

[Feathermerchan]

Can you get an air source heat pump with a desuperheater on it?

 

[willow_hiller]

Can you get an air source heat pump with a desuperheater on it?

I think it's technically possible, but not feasible (would not sufficiently heat water). My understanding is that a ground-source heat pump has higher pressures involved in the compressor and gives off more waste heat as a result.

EDIT: Found an article. It used to be a thing, but higher SEER ratings on air source heat pumps made it inefficient: Where Have All the "Hot Water Generators" Gone?

 

[SageBrush]

fiberglass tank traditional water heater

 

[nwdiver]

So maybe if you stay in the home longer than 15 years, the investment is worth it.

On some level you also need to take into account the time value of money. $1k spent today costs significantly more than $1k spent in 10 years because you're losing the time value of that 'lost' capital. I had been mulling over a battery bank that lasts ~8 years for $3k or one that could last >20 years for ~$10k.... until I realized I could just put the $7k in an ETF, recycle the batteries with new ones every ~8 years and still come out ahead. Economics matters.

The limitations of air source is also to some extent based more on the refrigerant used than the energy available. -10F may be cold relatively speaking compared to 40F in the ground but on an absolute bases that's 250K vs 278K so you're increasing the amount of energy by ~11%. Using a better refrigerant could yield the same results as ground source at a lower cost. R-744 which is just CO2 maintains a higher COP at far lower temperatures than traditional refrigerants but isn't commonly used because the equipment is too expensive. Economics matters.

If you're in an area that sees <0F temperatures more than 20 or so days a year then ground source might make some sense.

 

[SageBrush]

-10F may be cold relatively speaking compared to 40F in the ground but on an absolute bases that's 250K vs 278K so you're increasing the amount of energy by ~11%.

I'm not sure I am following. I get the use of Kelvin and an absolute scale, but I thought Carnot efficiency is proportional to temperature gradient.

E.g., if you are trying to move heat to 100F (311K) then there is a (311 - 278) = 33 K temp gradient for the GSP to work against while the ASP has (311 - 250) = 61 K gradient. That would imply almost a 1.8x energy use by the ASP compared to the GSP for the heat transfer.

Or so I thought.

As an aside, one thing to VERY cognizant about with GSP is the energy cost to move the heat transfer medium through the long series of pipes. The less honest of the vendors ignore that detail.

 

[nwdiver]

I'm not sure I am following. I get the use of Kelvin and an absolute scale, but I thought Carnot efficiency is proportional to temperature gradient.

Yep... that's what I meant by changing the refrigerant instead of seeking more warmth.

 

[SageBrush]

Yep... that's what I meant by changing the refrigerant instead of seeking more warmth.

Oh --- I agree with changing the proportionality constant !

I am not sure about the calc that suggested the GSP is only 11% more efficient at heat transfer.

Disclaimer: I am somwhere between a GSP skeptic and detractor, but not because I doubt thermodynamic efficiency in a balanced system. The issues come up with operational cost, maintenance, and installation costs. In a fashion GSP suffer from the same issues you often highlight with nuclear -- attractive on paper in terms of energetics, crappy investment in real life.

 

[nwdiver]

I am not sure about the calc that suggested the GSP is only 11% more efficient at heat transfer

True... it is more complicated than that. But there's only ~10-ish% more energy at 40F than -5F. Which is a bit counter-intuitive.

 

[SageBrush]

But there's only ~10-ish% more energy at 40F than -5F. Which is a bit counter-intuitive.

Agreed !
Although that does not translate directly to efficiency.

We should malign GSP for the right reasons.

 

[willow_hiller]

This will spark an entirely different debate, but the fiberglass tank (Rheem marathon) water heater is actually a recent install. It is relatively power hungry (EnergyStar says 3,569 kWh per year), but it's entirely non-metallic and has a transferable lifetime warranty with unlimited replacements.

We figured that having a water heater that will last several decades (and not flood the basement) would somewhat offset the power consumption, and I'm not entirely convinced by the hybrid (heat pump) water heater claims of efficiency. In the summer, our basement is cool enough by itself, and in the winter I feel like having a mini-heat-pump on top of the water heater would steal heat from the room.

We were also really excited to learn that one of the geothermal installers in our area actually picture their system with the Rheem Marathon as the preferred tank to work with the desuperheater:

So we may be able to get the best of both worlds with efficiency and durability.

 

[nwdiver]

and in the winter I feel like having a mini-heat-pump on top of the water heater would steal heat from the room.

You could duct the exhaust outside.

This will spark an entirely different debate, but the fiberglass tank (Rheem marathon) water heater is actually a recent install. It is relatively power hungry (EnergyStar says 3,569 kWh per year), but it's entirely non-metallic and has a transferable lifetime warranty with unlimited replacements.

We figured that having a water heater that will last several decades (and not flood the basement) would somewhat offset the power consumption, and I'm not entirely convinced by the hybrid (heat pump) water heater claims of efficiency. In the summer, our basement is cool enough by itself, and in the winter I feel like having a mini-heat-pump on top of the water heater would steal heat from the room.

We were also really excited to learn that one of the geothermal installers in our area actually picture their system with the Rheem Marathon as the preferred tank to work with the desuperheater:

View attachment 544336

So we may be able to get the best of both worlds with efficiency and durability.

Have you looked into Sanden?

 

[Feathermerchan]

Ducting exhaust outside would mean bring in outside air somewhere or the house would collapse.
A cu ft out => a cu ft in.

A heat pump water heater in a house will always use the heat from the house to heat the water. By definition.

Yes newer more efficient A/C units have less BTU available to heat water. I had forgotten about that. Plus where you live you don't do much A/C. Not compared to where I live.

 

[nwdiver]

A heat pump water heater in a house will always use the heat from the house to heat the water. By definition.

It's feasible to duct the inlet and outlet but you're likely to just force it into resistance mode. Probably more efficient overall to used air that was efficiently heated to heat water then duct the cold exhaust outside. Not perfect... but better than using resistance heat... which is a REALLY low bar.

 

[willow_hiller]

Plus where you live you don't do much A/C. Not compared to where I live.

Not nearly as much AC, but plenty of heat. In some analysis I was doing recently I pulled the daily min, max, avg from the nearest NOAA weather station from our house. Here's just the last two years:

Minimum of 4 F, maximum of 99 F. And half the reason our current heat pump energy consumption is so high is because it's on its last legs, so we have our system set to switch over to emergency heat at 35 F to avoid killing it. A good quality, modern air source heat pump could definitely meet all of our needs, so we're not ruling it out yet. Collecting some quotes this week to make the final decision. I think if we could snag a ground-source installation for around $15,000, the Federal tax credit and the Maryland state $3,000 grant could make it very appealing.

 

[Zythryn]

As someone who has built and considered geothermal. It doesn't pencil out in moderate climate areas.
It is also true that there are large inefficiencies and lack of competition in moderate climate areas which make the cost prohibitive.
In NC, we had (still have?) a state tax credit so you were getting 65% or so of your entire HVAC bill back. But when Geo was $80k and standard (decent HPs) is $15k, even the huge tax credit doesn't work.

That is crazy expensive. This cost may be out of date, or due to peculiarities of your particular area/needs.
Geothermal in moderate climates is typically where it is easiest, although ASHP probably would suffice well.

Where I am at with cold snaps getting down to -20 to -30 degrees Fahrenheit GSHP work great.
Yes, they are more expensive than less efficient options, but typically not to the level yours was.

 

[nwdiver]

Not nearly as much AC, but plenty of heat. In some analysis I was doing recently I pulled the daily min, max, avg from the nearest NOAA weather station from our house. Here's just the last two years:

View attachment 544344

Minimum of 4 F, maximum of 99 F. And half the reason our current heat pump energy consumption is so high is because it's on its last legs, so we have our system set to switch over to emergency heat at 35 F to avoid killing it. A good quality, modern air source heat pump could definitely meet all of our needs, so we're not ruling it out yet. Collecting some quotes this week to make the final decision. I think if we could snag a ground-source installation for around $15,000, the Federal tax credit and the Maryland state $3,000 grant could make it very appealing.

Have you looked into split units to have zoned heating? It's not for everyone but I save A LOT of energy keeping one part of the house at a comfortable ~72F and the other half at ~50F in the winter.

 

[SSedan]

I live near Green Bay and a few years ago went from baseboard electric and a garbage high velocity AC system in a leaky 1970s 2400sq home to Fujitsu minisplits air source system.
Two outdoor units each with three indoor.

Once it is staying below +15f we end up Turing a few.of the baseboards back on.
This past winter was.mild the only two baseboards that ran were the bathrooms.

House is way more comfortable and the electric budget was going down till I went and bought an electric car. Have a 1996 electric water heater so I ordered a hybrid heat pump Richmond water heater just yesterday. May end up running it in pure resistance much of winter given the location vs where the nearest air source heat pump outlet is.

The Fujitsu do an admirable job given our climate but I do have to have a secondary source as I have seen -26f actual here, house is old and not well sealed, and one room has a lot of glass, though it is late 1990s glass and double pane, though some of the house is single with storm windows.
A newer construction home and the Fujitsu would work even better. I know that once it is well below zero they aren't saving me that much but that is a short period and the house is more comfortable.

Our system was like $22k with 5-12k BTU units and one 18k. Was installed early winter so I believe we were charged a premium for that.

 

[dhrivnak]

I'm sure everyone is familiar with Elon's ramblings about creating a Tesla Home HVAC that seem to get more serious every day. After learning about heat pumps, which was a new concept for me, I also learned about "geothermal heat pumps". Obviously the heat pump is more efficient when transfers to 55 deg earth vs 100/32 degree air.

So I'm confused why is there so much talk about improving the efficiency of heat pumps themselves, while at the same time ignoring "geothermal heat pumps" which are more efficient no matter how much the heat pumps improve.

It just seems to me like people don't know what geothermal is - I didn't know myself until recently. I thought you needed a volcano for geothermal anything. Sure, you need to dig or drill a hole, but I think that that is overrated. Big deal, we have machines that can drill holes. New homes can have geothermal installed even more cheaply, and there are new buildings made every day, but I bet that few have geothermal.

When I built my house my high efficiency Heat Pump was $7000 a geothermal was quoted at $17,000. That was a big delta. and with my house already being passive solar, built with Insulating Concrete Forms (R-50) and SIP roof (R-40) and low E Argon windows, I did not see a payback as my HVAC loads were under $300/year.