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

Your rheem links aren't working for me.

I added the model #'s which should help. The links work fine on my end.

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.

Currently our electricity generation is 62% coal and only 6% renewable. That will change, but the electric utility is dragging their feet.

The particular nat. gas water heater I'm looking at has a .69 energy factor, and I'll insulate it for some small % improvement.

Hmmm... I'll have to price that option and figure out how I would plumb the heat pump.

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.

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

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.

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.

This is and issue. As solar PV gets less expensive, SDHW looks less viable especially with the mechanical issues of an improperly installed system. I have a number of 25 year old systems still working fine and some failing after 5-6 years. I won't install tube systems anymore because a higher percentage fail and they fail faster as well.

I have found that the best SDHW systems are ones that are PV powered (20w panel) and have no other controller to muck up (which is what has failed in 40% of the systems I see). They also are not SUPER efficient, generating high temps like a tube does. Doax made a system that I think could work well for many years.

NM is a grid-tie state.

I am considering Solar PV. Of course, we have a few issues there as well. Our roof ridgeline (simple pitched roof) runs North-South, so any solar panels would require racks for good solar alignment. We also have 3 mature trees on the west side of the house that provide good afternoon shading in the summer.

I'm crossing my fingers that our city is able to work out a community solar PV garden. That would allow individuals to own PV with optimal siting and hopefully at a price discount.

Tim

Our attic is well ventilated so it is warm in the summer, but much less so in the winter. Plus there is the condensation issue.

Anyone have experience with Heliatos Solar and their Solar Hot Water kits?

The GH Kits are advertised as freeze rated to -17°F, which should be plenty of range for our climate. I like the overall simplicity of the system, expandability, retrofit to any standard water heater and the price.

This setup is appealing, but I assume there are drawbacks and issues I'm not familiar with.

Thanks,
Tim

Well, that definitely makes it hard to have PV. Good luck with the community solar PV. I hope it works out for you.

That SDHW system you mentioned is kind of cobbled together. Not too professional.

I say use a pv-t panel array running a drainback loop. You could rig the loop to a mini heat pump if you wanted to. Buy a long lasting water heater of whatever heat source you want. The pv-t panel array would pay for the whole rig eventually.

Here are the results of our PVT design, done last year.
It is, in my opinion, good for pool heating but not quite so good for DHW although still a valid technology.

The pic shows two identical panels, one with the grid PVT and the other just PV.

Being in New Mexico, I don't imagine pool heating would be necessary most of the year. It could turn into a big hot tub pretty quickly in the summer. However, a pool makes almost as good a heat dump as a cooling tower for less watts.

We sometimes run the pool pump at night through the panels to cool it down when it gets too hot but that is with plastic panels. Not sure how the PVT would work.