Is a hybrid water heater more efficient than oil?
October 11, 2012 11:48 AM   Subscribe

Help me analytical-types! Do I upgrade my current water heater to the GE Geospring Hybrid for practically free??

Okay, here's my story.
I live in MA.
My house is 20 years old and heated with oil forced-hot-water.
My water heater is a furnace coil with 80-gallon storage tank.

This year we installed Mitsubishi split-system AC/Heat Pumps in every room.

The expectation is that we will be using these systems for heat until the temps are in the low 30's or lower. However my furnace will still be running all year 'round to maintain hot water for the house.

My understanding is that even in this case my hot water heating is STILL more efficient than a standard electric water heater.

However, this new GE Geospring hybrid heater is claiming 60%+ more efficient than a standard water heater.

National Grid is giving $1000 rebates for upgrading to this system making the upgrade close to free for me.

Is this a good idea? Will I see a substantial savings over my current water heating system? Trying to work the numbers is beginning to blow my mind so I figured I'd release my dilemma upon you smart people.
posted by Thrillhouse to Home & Garden (8 answers total)
You might look into tankless water heaters. You don't heat the water until you want it, so you never pay to keep it warm. I installed one when my old water heater went tits up, and it saved me about 50% on my electric bill every month. When I did the research, I found that the gas tankless heaters were better than the electric tankless heaters on some technical ground having to do with the electronics, not the heating theory. That may have changed during the last three years I've had this unit.

When you read about this, keep in mind that the various sizes of heaters measure the gal/minute relative to temperature. That is, 114 degree units deliver at, say ten gallons per minute, but at 15 gallons per minute you get 110 degree water. But the flow is indefinite: you never run out of hot water, because there is no storage tank. Units range from small to huge, depending upon how many showers and dishwaters you want to have in simultaneous operation. They use the same heating theory that percolators and steamer use: a smallish volume of water moving through a very hot chamber. The temperature is adjustable.

The units are small, about the size of a suitcase, and the only caveat that I encountered was that the gas unit had to be ventilate properly (not a problem in my case). In theory you can put several of the units in a large house, so that you can reduce the flow-time it takes for the hot water to get to, say, an isolated upstairs bathroom. They can even be mounted on an outside wall.
posted by mule98J at 12:03 PM on October 11, 2012 [1 favorite]

This is hard to answer specifically without more information. Are you concerned about efficiency in terms only of cost or are you concerned about environmental footprint as well? If it is just cost, you need your electricity cost and your oil cost, as well as the efficiency of your oil heater.

The efficiency of a normal electric water heater is 100%, not accounting for heat losses from storage. The efficiency of the GE model is somewhere around 240%, as it is a heat pump. Depending on the age, of your oil heater, it could be very roughly 60-95% efficient. However, oil is generally cheaper than electricity per unit of energy, so you'll need to provide some numbers to work this out.

My guess is that the GE system will be cheaper to operate. There are some other factors to consider though. The GE system is a heat pump. If it pumps heat from an unheated area, that's great, but if it is taking heat out of your house to run, then it isn't helping you during heating season (but it would, of course, help you during cooling season).
posted by ssg at 12:07 PM on October 11, 2012

There may be all kinds of benefits to upgrading. Not only will you get rebates from your provider, you may also get state and federal rebates as well.

That said, I too have a tankless hot water heater and with rebates, it was pretty much free. I love that thing to distraction. Unlimited hot water is great.

It's a bit of a myth that you save on energy with tankless though, because you take longer, hotter showers with it. (YMMV)

Even so, I really, really like my tankless.
posted by Ruthless Bunny at 12:24 PM on October 11, 2012 [2 favorites]

I'm concerned MORE about cost savings.

One factor that I'm not sure I need to consider is the fact that the whole furnace will be running from roughly April-Nov for the exclusive purpose of heating water. This HAS to impact the overall efficiency, doesn't it?

If it helps, according to the DoE average elec rates in MA are much higher than the rest of the country at 14 cents/ kWh and oil is about $3.50 /gal.

Unfortunately there are no tankless that qualify for this $1000 Nat'l Grid rebate... It would take a long time to make up the difference. I'd prefer that too...
posted by Thrillhouse at 12:32 PM on October 11, 2012

At some point your current hot water heater will need to be replaced. If you have a chance to replace it now for free, you'll avoid having a sudden expense come up and not have your hot water supply interrupted.
posted by yohko at 12:37 PM on October 11, 2012

Find an energy auditor; they should be able to help you decide. I did a quick google search:
posted by theora55 at 5:03 PM on October 11, 2012

One factor that I'm not sure I need to consider is the fact that the whole furnace will be running from roughly April-Nov for the exclusive purpose of heating water. This HAS to impact the overall efficiency, doesn't it?

The whole calculation depends a lot on what sort of system you have and how old it is. I'm guessing you have an oil-fired boiler that is original to the house and it has a heat-exchanger coil in it for the hot water. In which case, yes, you are using a fair bit of oil keeping the water warm in your boiler over the summer.

Fuel oil has an energy content of about 41kWh per gallon, so assuming your boiler / coil system efficiency is about 70%, you pay about $0.12 per kWh for the actual heat in your hot water. With the GE heat pump system, you would expect to pay about $0.06 per kWh if electricity costs $0.14 (the heat pump has an energy factor of 2.4). So, the GE unit will run at about half the cost.

However, there are a few factors that can change things.

One, the heat for the heat pump has to come from somewhere. So, in the summer, the heat pump takes heat out of the house, reducing the A/C load, which obviously works out well for you. In the winter, the heat pump would be taking heat out of the air, which you've heated with your boiler. This doesn't make much sense! Fortunately, you can switch the hot water tank to electric only mode, and it functions just like a regular electric hot water tank.

So, let's break it down further:

If you are running A/C anyways, the heat pump hot water tank just helps to cool the house, though slightly less efficiently than a normal A/C unit. The cost to you to heat hot water is very near zero in this case.

If you aren't heating or cooling the house, the cost is as above, roughly $0.06 per kWh for hot water (though this will slightly cool the house).

If you are running your heat pumps to heat the house, you need to add that cost into your cost for hot water (since the hot water tank uses heat from the house). This isn't much, let's estimate is is a couple cents more, so $0.08 per kWh.

If you are running your oil heat, then it no longer makes sense to use the heat pump hot water heater, so you switch to electric mode for the winter and you pay $0.14 per kWh.

Overall you will come out ahead with the GE heat pump hot water heater, except in the cold of the winter.

The other factor is that your oil boiler plus coil system is not very efficient in the summer (as you assume), so you are probably paying quite a bit more for hot water in the summer (estimates seem to vary on this, as it depends on the system), but a little Googling suggests 40% or so efficiency, which means your hot water cost would be around $0.21 per kWh.

I'd suggest that the GE heat pump hot water tank would be a very good idea. It should save you significantly on your hot water heating cost. Just remember to change it to electric only mode when you are heating your house with oil.
posted by ssg at 5:55 PM on October 11, 2012

SSG, Thank you so much for writing that all out. Thats what we're going to do. I'm very data-oriented and that maps it all out the way I needed it.
Thanks again!
posted by Thrillhouse at 4:15 AM on October 12, 2012

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