Water heating options...anyone tried condensing water heater?

[Shawn Clark]

As some of you are aware, I've been thinking some about long term approaches to reducing my nat. gas load from water heating. I've done some of the easy stuff: insulated as many hot water runs as I can reach, blanketed the tank, and gone to a front loading washer (already had the Energy Star dishwasher.) I've ordered one low flow shower head for testing. If it works satisfactorily others will follow. This carries great promise for minimizing hot water use.

However, once these things are done, end use conservation and such will be at roads end. So this brings me to considering the water heating itself. Working through it I come to the conclusion that a condensing nat. gas water heater (w/tank) would be the most cost effective. Do any of you have one of these?

I'm in a true 4-season climate and have four hot water users. Therefore I anticipate that solar water heating would be limited to replacing about 50% of the load on an annualized basis. (Perhaps 60% or more would be possible, but I'm trying to remain conservative.)

Before, I had also considered a demand based pairing, but my rough calcs suggest the energy savings for that wouldn't be as large as I hoped, while the capital cost would be. The energy factor for an electric water tank runs about 0.92, which indicates only about an 8% loss due to the storage tank. Interestingly, the energy factor for most of the listed tankless water heaters is only 0.82, although a few are listed all the way up to 0.98.

So that leads to the next consideration: why not electric water heating? Energy Star gas water heaters are in the 0.62 efficiency range and my non-Energy Star tank is listed as 0.58--before I jacketed it, which should bump it up well into the Energy Star range. Unfortunately, the efficiency of the electrical generation is low, about 33-60% max depending on source (the higher value being combined cycle with steam.) Plus transmission losses average another 7% in the U.S. So relying on gas even with the low water heater efficiency value still clobbers electricity even in the best case (0.92*0.6*0.93 = 51% and more in the most likely older coal plant case 0.92*0.33*0.93 = 28%.)

Other arrangements such as heat pump water heating and the like are not well suited for this climate.

Okay, then while solar would be great, it is not stand alone in this climate and natural gas looks like the most efficient/cost effective option here for the time being. But why not get the most efficient gas burner possible, a condensing system? It's going to be pricey, but still cheaper than tankless. The current problem is that they don't seem to have much market penetration yet and are not even Energy Star rated. The Energy Star ratings for these are supposed to come out this year...and the minimum will be 80%, I read somewhere that some will hit 89% overall efficiency. So when it comes time to replace this water heater that is the route I hope to follow...but I hope the existing tank will last long enough to work out the kinks and novelty (to installers) of the condensing units.

[icarus]

Shawn,

I know nothing about condensing burners for water heaters.

As I'm sure you and I have written about before,,, I am a huge fan of Pre-heated solar,,, supplemented with demand. The newest generation of Tagaki/Rinnai/Paloma demand water heaters are great products. Yes the up front cost is significant,, but with tax credits they come in line pretty quickly. Venting issues are expensive if you have a long run of vent,, but they can be direct vented out a side wall if you have a place to do so.

My simple solar pre-heat system supplies ~70% of our hot water (for 2) on a year round basis. After about April 1 it does 100%. The takagi handles this perfectly,,, not caring what the inlet temp is.

There are other issues,, for example distance to fixture,, but these units are so small (even for a big out put) the idea of using more than one as a point of use is pretty attractive,, in lieu of a circ system,, at similar cost.

Icarus

PS My system is in far NW Washington state.

Icarus

[SageBrush]

Keep on posting Shawn. These topics are great!
Would you mind providing details why you think solar heat will only cover 50-70% of your use ? I think of that question as a cost/benefit, not a technical limitation. I am pricing out a system in the next few days, so know that increasing heating capacity is really only an extra collector cost; the plumbing, controller and pumps are a fixed cost.

Now, you may have to cover extra collector panels in the summer to prevent overheating, but I doubt that is a big deal to you given your obvious motivation to cut carbon emissions.

[Shin Chan]

Geothermal heat pump - Wikipedia, the free encyclopedia

Geothermal_heat_pump

If you had the room to do so, why not think about a slinky-coil geothermal system too.

[jayman]

Here in Manitoba, we have the cheapest power rates in Canada, among the cheapest power rates in North America. Cost-wise, an electric hot water tank is a good bet. Brand new homes are built with electric furnaces, if that's any indication of how our power rates compare to natural gas

I like natural gas water heaters strictly for their recovery rate. I really hate running out of hot water in the middle of a soak, and have never had that happen with a gas hot water tank.

In my home, I have a gas furnace, gas hot water tank, gas fireplace, gas heater in the attached garage. The stove and dryer are electric. FYI my gas bill just for heating hot water is $45 a month

I've never heard of condensing hot water heaters. They seem like a good bet. I've never seen them around here

[icarus]

It seems that the going rate for a condensing water heater is ~$2700 usd. A Takagi/Rinnai runs ~$800

The condensing unit runs ~95% ef,, the demand ~85%.

What I don't know is how much you can/should calculate in for standby loses. With a demand tankless standby is zero,, but there has to be some standby loss with a tank type. Lets just say it is 5% for the sake of argument. In that case $1900 for 5% advantage would be a pretty difficult pay off. Factor in longevity and I'm not sure it would ever pay off. (Understanding that not everything is about the payoff time!) A conventional tank with life expectancy of 10-15 years, a full stainless demand might last 15-30? I am still running a number of Palomas that are more than 20 years old.

For that $$ difference you could put 3 demand water heaters as needed at point of use heaters.

[qbee42]

Keep in mind that depending on where your water heater is located, tank losses are not really losses during heating season.

Tom

[hyo silver]

Good point, Tom.

And if those incandescent light bulbs are only 5% efficient at producing light, doesn't that make them 95% efficient at producing heat?

[icarus]

Originally Posted by qbee42

Keep in mind that depending on where your water heater is located, tank losses are not really losses during heating season.

Tom

Good point,,, but on an annualized basis,, they are still loses. Also perhaps not the most efficient way of heating space.

As for Hyo's point,, heating your house with light bulbs is not much worse than heating with resistance heating!

Icarus

[Shawn Clark]

Originally Posted by icarus

It seems that the going rate for a condensing water heater is ~$2700 usd. A Takagi/Rinnai runs ~$800

The condensing unit runs ~95% ef,, the demand ~85%.

What I don't know is how much you can/should calculate in for standby loses. With a demand tankless standby is zero,, but there has to be some standby loss with a tank type.

Brand new A.O. Smith's condensing (which are considered premium) can be had for about $1600. The Energy Star estimator I came across suggested a price under $1000, but I assume that is once initial low volume premium's disappear. I'm not familiar with the Takagi/Rinnai. I was a bit shocked/turned off by the prices I've seen on various demand water heaters and the reviews they have gotten.

Standby losses probably are not more than 8% since electric water heaters have about 92% efficiency. The A.O. Smith also has extra nozzles that can be used with recirc...or perhaps thermal solar. I like the design of the A.O. Smith with the simple helical coil. We'll get a better idea on relative efficiencies when the first set of condensing water heater energy factors are published.

Older gas water heaters suffered more from air flow up the flue (uninsulated internal wall getting a draft) and from little insulation on top and bottom of the tank as well as from the thermosiphon in the inlet/outlet lines which typically could not be insulated. (You have to go out and find some noncombustible type material to do this--which is not carried in many of the big box builder stores anymore.) Some traps can be installed to reduce the effect as well. Since they were non-condensing as well, the efficiencies for standard dimensions have been been about 58-62%.

Demand or tank I hope to go to sealed combustion/direct vent as part of my drive to tighten up the home. So I put the costs of this aspect of the install as about equivalent.