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Old 07-31-14, 02:48 PM   #1
TimJFowler
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Default Nat. Gas vs. Solar Thermal Water Heater

Hello all,

I am searching for a replacement for our 40 gallon tank Natural Gas water heater. The current water heater was a cheap replacement ~12 years ago (~.50 energy factor) so almost anything will be an efficiency improvement. I've done some research [e.g. http://www.greenbuildingadvisor.com/...-water-heaters ] and given our particular constraints and requirements I've narrowed the choices down to two general categories:
A) Conventional Natural Gas Tank Water Heater
B) Solar Thermal Tank w/ backup heat source.

Here is my thinking, requirements, issues:

1. The water heater is located in a utility closet in the central hallway and the replacement needs to go in the same location. I'm not interested in re-plumbing the house to move the water heater, and there isn't room for a much larger water heater.

2. We are a family of three and don't use a lot of hot water (for 32 days in June/July we used 5.7 therms of nat. gas for cooking and hot water combined). So a tankless water heater doesn't make much sense for our use.

3. A heatpump water heater "creates cold" which could be useful in summer, but during the winter would create a cold spot in the center of the house.

4. A condensing gas water heater is relatively expensive (compared to a conventional gas burner) and would need a drain plumbed in.

5. We have cold winters where temperatures regularly go below freezing overnight. So, any solar option should be freeze-proof.

6. We live in a sunny locale but a backup heat source would provide peace of mind for us.

7. Any more expensive option should have a realistic payback period (i.e. within the warranty period).

So that leaves me with
A) an efficient, conventional nat. gas tank water heater for ~$580 plus installation:
- ex. Rheem Performance Platinum Series: Powered Damper Series - Rheem model # XG40T12DM40UO
OR
B) Solar Thermal w/ nat. gas backup for ~$4500 plus installation:
- ex. Rheem SolPak with Gas Assist Heat Exchange Tank Series - Rheem model# RSG75-48BP

I'm looking for suggestions, alternatives and any other info that I've missed. Are there less expensive, but reliable / warranteed solar thermal options that meet my requirements?

Thanks,
Tim

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Last edited by TimJFowler; 07-31-14 at 04:17 PM.. Reason: add model #'s
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Old 07-31-14, 03:21 PM   #2
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Your rheem links aren't working for me.
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Old 07-31-14, 04:18 PM   #3
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Your rheem links aren't working for me.
I added the model #'s which should help. The links work fine on my end.
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Old 07-31-14, 04:50 PM   #4
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My take on this debate starts with what the fuel sources for your electricity is. If you burn mostly coal to make electricity in your area, then use high efficiency gas as your backup but if there is a high degree of solar/wind/gas used to make power then I would use solar with an electric backup.

Either way, nothing compares with a cheap 55% gas tank on cost but should that be the baseline? If so, you will never get close to it doing a solar system.

Personally, I would get a well insulated electric tank, put one of the DIY 1 ton heat pumps on it for cheap. Add solar after if you want.
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Old 07-31-14, 05:11 PM   #5
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My take on this debate starts with what the fuel sources for your electricity is. If you burn mostly coal to make electricity in your area, then use high efficiency gas as your backup but if there is a high degree of solar/wind/gas used to make power then I would use solar with an electric backup.
Currently our electricity generation is 62% coal and only 6% renewable. That will change, but the electric utility is dragging their feet.

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Either way, nothing compares with a cheap 55% gas tank on cost but should that be the baseline? If so, you will never get close to it doing a solar system.
The particular nat. gas water heater I'm looking at has a .69 energy factor, and I'll insulate it for some small % improvement.

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Personally, I would get a well insulated electric tank, put one of the DIY 1 ton heat pumps on it for cheap. Add solar after if you want.
Hmmm... I'll have to price that option and figure out how I would plumb the heat pump.
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Old 07-31-14, 05:15 PM   #6
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There is a whole thread on building the DIY heat pump here somewhere. It isn't that hard and the amount of dehumidification/cooling/heating of the space is modest but the result is gas like cost to run without the venting.
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Old 07-31-14, 05:39 PM   #7
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Either way, nothing compares with a cheap 55% gas tank on cost but should that be the baseline? If so, you will never get close to it doing a solar system.
This seems to be the heart of the matter. While the operating costs for solar thermal approach zero, the upfront costs are comparatively huge. I would like to use solar energy for hot water but it is 8X more expensive upfront (w/o labor) than a "good" nat. gas heater.

If we lived off-grid or had propane heat this would be a more balanced equation. But, we live on the grid and have "cheap" gas and electricity.

Tim
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Old 08-01-14, 01:23 AM   #8
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There may be a third option, but it would depend on your future plans. It also has the disadvantage of there not being a fuel backup if your electric grid went down. Many people are trying to go as electric as possible because they can subsidize their electricity use with PV power. This would have the same effect as zeroing out the future expenses for water heating. That's if New Mexico is a grid tied state?

I personally plan on going that route. I've already bought an electric Marathon water heater which is a fiberglass design and is supposed to last a lifetime because it doesn't have the usual rusting out problem. They are about a thousand dollars. I think it is wise to think not only about the grid going down and causing cold showers. Cold showers can also be caused by mechanical failure. Solar hot water heaters have a very poor track record in this regard. Also regular metal water heaters last only about 10 years and then rust out. So a Marathon fed by PV is a very viable option compared to what you are considering. Of course, PV can be used for a thousand other things also which makes it more flexible even than cheap natural gas.
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Old 08-01-14, 02:14 AM   #9
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Perhaps you could use solar to preheat your incoming water before the water heater. Depending on your average attic temperature and time of water usage a 50' roll of copper in your attic might be enough to harness some solar. You mentioned not replumbing the house and most folks consider it a liability...but if you have the attic space, move the water heater to the attic directly above its current location. you will also gain a closet.
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Old 08-01-14, 05:06 AM   #10
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Quote:
Originally Posted by gtojohn View Post
Perhaps you could use solar to preheat your incoming water before the water heater. Depending on your average attic temperature and time of water usage a 50' roll of copper in your attic might be enough to harness some solar. You mentioned not replumbing the house and most folks consider it a liability...but if you have the attic space, move the water heater to the attic directly above its current location. you will also gain a closet.
If you move the cold water directly up to the attic to be warmed, what will you do with the condensate that is generated?

Other than that it is not a bad idea.

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