Geothermal vs. Natural Gas
I’d like a recommendation on whether to invest in geothermal or natural gas for our new house build in Maryland. We plan to start construction this summer (2021). And we expect to live in the house for 25 years.
The house will be 1.5 levels (second floor will be over the connected garage) and about 3200 sq ft. with 2×6 studs. It will have a vaulted ceiling throughout. Wall insulation will be greater than R30 and roof will be greater than R40. Windows will have a U-factor of about 0.3 (Marvin Signature). Maryland’s climate is classified as humid/sub-tropical and deep soil temperature averages 57 degrees. We like to keep the house at 71 degrees in the winter and 73 degrees in summer.
We’d prefer a geothermal system for heating and cooling. We believe the GSHP system will be 4 tons but we need to confirm that. It will utilize vertical wells.
Alternatively it would cost $13,500 to bring natural gas to the house from the current end of run. We would use nat gas for heating, domestic hot water, and to power a backup whole house generator. Of course we’ll need air conditioning units.
If we went geothermal we would use propane for our backup generator and the cost to buy and bury a 500 gallon propane tank is about $8-9,000.
So which system makes sense for us? Geothermal with propane for generator, geothermal with nat gas for generator and cooktop, or nat gas for heating, cooling and generator with central air conditioners?
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Replies
I think you are looking at this wrong. The geothermal field will cost 20-30k and the NG line will be 13.5k- those are sunk costs and dont include the actual equipment cost. In MD, an airsource heat pump (with enhanced vapor injection, marketed as cold-climate heat pumps, or hyperheat as Mitsubishi's brand name) will produce full rated heating to around 5°F which is WAY below your design temperature without the sunk costs and similar equipment cost.
Spend that 13.5k-30k you were going to spend on gas or geo to get from a very good house to a passive-house. A passive-house will be much more resilient (temp wont fall to dangerous levels if the power goes out) and have a much smaller HVAC system so you can likely forgo the large generator (maybe a smaller backup for a few hundred $$ and run on gas).
There is some cool tech hitting the market this year that will make solar without batteries able to run your house with a power outage (iQ8 microinverters from Enphase), so that might offset even more cost.
From the Chiltrix website: "There is no current ENERGY STAR program for air-cooled chillers. But it is interesting to note the CX34 air-cooled chiller exceeds the Energy Star efficiency requirements for variable speed water-cooled chillers (geothermal chillers) which are usually considered to be the most efficient type available."
I don't know if this statement is true, but if it is it would suggest there is no efficiency advantage to a GSHP over air source.
I'm a fan of air-to-water heat pumps, but I don't think that comparison means much. The COP of the Chiltrix depends on the air temperature, on the water temperature it is delivering, and on its power level. A ground-source heat pump has a much more constant operating temperature (and there are standards for what numbers to use). So I don't know whether it's apples to apples. Also, the energy star threshold is higher for water to air, and there are now modern variable-speed ground-source heat pumps that substantially exceed the energy star standard. For example, COP of a water-furnace 7 series in heating mode with 50 F water as the source gets COP of 6 at low speed.
I think that a well designed ground-source system really can do better than an air source, if you are comparing good modern equipment for both. On the other hand, the cost of PV to make up the extra energy needed for the air-source is less than the extra cost to go ground source. So it's only justified if you care for other reasons, such as minimizing the impact on the grid during a cold snap, or reducing the size of the generator or batteries you want to ride out a power outage.
And of course, the best way to build to ride out a power outage and reduce the electricity input needed is to make a great envelop, as the first comment pointed out.
Finally, I'll say that I think that anyone wanting to call what they are going "green building" should not be considering running natural gas to a new build at this point. And certainly not if it costs as much as it cost me to drill my two ground source heat pump wells.
You're probably going to be doing more cooling than heating in Maryland, unless you're up in the mountainous Northern extremes of the state. Friends of mine around DC and Baltimore have told me they usually run their air conditioning 10-11 months out of the year. It makes sense to concentrate on cooling performance in this case.
Using some of the money to improve your home's insulation will benefit both cooling AND heating. That's a big plus in your climate zone. Your heat pump's main advantage here would be that it is "reversible", and can also provide heat when needed. The energy efficiency focus should still be towards the cooling side of things, since you're likely cooling dominated here. You want to look at the entire system, which means average run times over an entire year here, not only the relatively short heating season.
I would look into induction for your cooktop and not natural gas or propane. In many cases, induction can do as well or better than a gas burner for cooking -- even for many fancy things. I had once questioned the ability to use a wok with an induction cooktop, but I believe it was Martin on GBA who said he had done it, and explained how. I was surprised to hear that, but I'm sure you could find the posts in the archives.
Regarding your generator, if you expect FREQUENT outages, the natural gas service might still make sense. I don't think natural gas is "ungreen" as some think, provided that you use it efficiently. The majority of new electrical generation over the past decade or two has been coming from natural gas fired generators anyway. If you expect relatively short outages, then propane's advantage is that it lasts literally forever -- propane never degrades. The disadvantage to propane is that you need a tank large enough to vaporize sufficient amounts of propane to ensure your generator runs correctly. This will often require a larger tank that you would have planned on if you were sizing things for runtime alone. BE SURE to check this -- your propane supplier can easily run the numbers for you, you'll just need the BTU consumption numbers for the generator you're planning on using.
Small diesel generators are another option that might save you money over a propane installation. Diesel fuel isn't as stable as propane, so you need to check it periodically, but it can easily last years in the tank if you're careful. The majority of large standby generators are diesel fueled and are pretty trouble free. Diesel generators aren't smoky and smelly the way you might think diesel would be if you're used to black smoke from trucks and things like that.
Once last thing, if you're considering a residential standby generator I would seriously consider the Kohler products over the manufacturer you see advertising heavily and selling through the box stores. Kohler has a far superior product. I don't work for them or anything like that, but design critical power systems and spec many, many generators at work so I've seen how things hold up over time.
Bill
Thanks to everyone for your advice to date. The building site is about a mile from the Chesapeake in an established neighborhood. Cooling will be required more often than heating but we do have 5 months where the low temperature is below 40 and often below freezing. We can upgrade our insulation plans, such as using 2x8 rather than 2x6 studs, if it makes sense. We plan to use Zip system sheathing and Hardie Board exterior.
We currently use an induction cooktop (I highly recommend it). If we decide to install natural gas we'll most likely have a gas cooktop with an additional induction hob.
We don't lose power too often but for those times that we do we want a whole house generator. I've narrowed the brands to Kohler and Cummins/Onan. I've used a Honda gasoline generator for years but want to upgrade to an automatic system.
Based on everyone's input I've looked into EVI systems with variable speed pumps and they sound great on paper. Given the relative newness of the technology I wonder how reliable they will be over years of use?
So back to my original question. Does it make sense to install a GSHP system, or a natural gas heating system, or an ASHP, or an EVI system with variable speed pump?
I would go with the air source heat pump, or the EVI system. Ground source heat pumps usually don't make financial sense after all of the installation costs are considered (the "ground source" part is very expensive to install).
Variable speed motor drives on compressors have been in use for a long time commerically so I wouldn't expect reliability issues -- it's a "new technology" for residential systems, but it's based on commerical technologies that have been around for a while (decades).
If you end up with a gas cooktop, be sure to use a suitable vent hood and allow for makeup air. That's something that can be an issue, and there've been a lot of posts on these forums about it.
Bill
Is this home going to be your primary or a secondary residence? If so have you confirmed with your lender, if you are using one, that they will allow you to build a house which is not tied to the grid?
I vote for the air source heat pump. Much less expensive to install.
"not tied to the grid" typically means not tied to the ELECTRIC POWER grid. Lenders often don't like things they perceive as "weird stuff", which means off-grid, such as the usual all-solar and battery setup. Lenders don't like manual things like woodstoves as primary heat sources, either.
What lenders ARE usually OK with are alternative, but conventional, systems. An example would be a propane fired furnace instead of natural gas, or a heat pump. Heat pumps are considered "conventional" systems, and they run automatically, so you don't have to worry about things failing and freezing while you're away.
Lenders want to make sure their asset is protected, since your home secures the loan. That's why lenders require insurance coverage for the home. As long as your home is using conventional systems that are well understood (natural gas, propane, or oil fired furnaces, or a heat pump or electric resistance heating system), you won't usually have any problems. I can't see it being a problem to not bring in natural gas for the purposes of obtaining financing for a residential home build.
Bill