Hot water heater 96% vs modern condensing boiler

john_236

I have a house that I need to get a new hot water heater for and am adding about 14,000 btu of radiant floor heat to. Trying to compare options. Heating the floors only needed about 1/2 the year. I am leaning toward a 96% hot water heater vs high effency condensing boiler. The hot water heater will have to have a plate heat exchanger because the radiant floor heat will be used only on demand and reguire antifreeze solution as in section of house exposed to freezing. In comparing is one better to run a condensing boiler to make hot water by adding a storage tank or isn't the water that the boiler uses in heating that tank heated up to 180 to make the hot water and doesn't that bring down the efficency to less than the 96% the water heater would supply. What are the advantages and draw backs to each. Thanks

Steven Eayrs

Not sure I follow.......

So.....you are comparing a 96% water heater, and a condensing boiler. At 96% I would imagine the water heater is also a condensing unit? The biggest problem with using a water heater for space heating is that you are not using it for what it is designed for. "cold water in and hot water out". In other words the heat exchanger in the water heater is designed and sized to suck as much out of the flue gasses as possible, and can only do it when its cold water in and hot water out. A heating system has a small delta T, or say 10-20 deg, and is made to operate with hot water out, and only slightly cooler water returning.
Does that make sense.

Might also help you to read another post, a couple below yours titled "Mechanics of short cycling", and its not talking about a bicycle w/ small pedals. (-: Your water heater WILL short cycle if used for space heating.

And not sure you understand condensing boilers, or water heaters. In order for it to condense you will only see the boilers running up to 180 degs. or so, if the dhw (thru a indirect fired tank) actually demands it do so to keep up. The whole concept of condensing, means that the unit will run as low as possible to meet the load. THe cooler it runs the more it condenses.

Another way of putting it, if using a water heater for space heating, you might as well throw the eff %'s away. They only run efficent when ran as designed.
Not saying you won't be able to make it work, but have seen many installed, that were tore out a couple years later, and a boiler wasn installed.

As always am sure there are other opinions and in fact some who have managed make it work decent, by adding enough stuff to the water heater that is costs about the same as a boiler. But why install something for what it isn't designed for on purpose? Stick to the purpose the engineer designed it for, and you won't be sorry later.

john_236

flowup

Thanks for the input. The hot water heater is the Vertex by a.o. smith .http://www.hotwater.com/lit/spec/res_gas/AOSRG45101.pdf It has hot and cold recirculating taps on its side for this purpose. I do agree that maybe the 96% effency is based upon the cold water entering it to condense the water vapor from the gas and that is what is making it the 96%. I need more hot water from my current hot water heater and am also adding a radiant floor to the house. I wanted to do this with one system. If I use a condensing boiler for the demestice hot water with a seperate tank is that over kill during the summer Is it going to cost a lot to run. I have limited venting space for running a hot water heater and a bolier, need to choose one for both. Thanks

Leo G_103

I

wou;d say that your plan sounds wonderful and simple. I like it.

Leo G

Larry Weingarten

If you have...

... a way to keep the water used for space heating as cool as possible, you'll keep the Vertex in condensing mode more of the time. Maybe some pretty European radiators rather than PEX under the floor?

Yours, Larry

Jamie_5

I vote for the water heater

With only 14K BTUH on the space heating side, every boiler I am aware of would be oversized except on the lowest fire of some mod/con units. You could add a buffer tank, but as Tom Tesmar says, when you stand back, it starts to look a lot like a tank heater.

Use a heat exchanger to run the floors and go with the water heater.

mark ransley

You rate water heater by EF, Energy Factor, not by the rating you think you are getting of 96%. That unit may be near 84 EF. www.energystar.gov has all ratings. Boilers are more efficent by design than HW tanks. Antifreeze will lower efficiency. A Takagi Th1 is 93-94% efficient. I think you are better off with a boiler designed for what you want to do and fix your freezing issue another way like with a circulator pump on, and insulation. Learn about EF and you will be suprised, AO smith doesnt like to give out EF numbers on the Vertex

Larry Weingarten

Energy Factor...

... is a tricky topic. It may only be used for equipment that fires at 75,000 BTU or less. The Vertex is 76,000 BTU. Look at any commercial equipment. EF is not used. Me thinks it's a game that's played. Makes apples to apples harder.

Yours, Larry

mark ransley

How is it a tricky concept, most tanks sold are 55-60 EF so only 45-45 cents of every dollar is spent heating water. The Vertex is EF rated, Tankless start at 82 EF and go to 94 EF no tank is above maybe 86 EF, its just not well known.

Evil lution

Triangle tube

Try the link below. It's a combination boiler DHWT. We have installed a few that work good for the smaller applacations. The only hard part is the programming it gets a little confussing.

http://www.triangletube.com/TriangleTubeProductList.aspx?CatID=7

Unknown

EF is only applicable when you are using a water heater only as a water heater.

When you use it as a heating appliance, EF is not accurate, and not applicable. it is calculated on a very small heat load, where standby loss is 10% to 30% of total energy usage. Of course, that's total of a small amount of usage.

In a heating application, even cheap water heaters perform above 75% efficiencies... maybe even close to 80%.. even though their "EF" is more like 63%.

With a 14kBTU load, a mod/con is very unlikely to make any sense. And, for that matter, a vertex is very unlikely to make sense either, and a cheaper water heater is likely the cost effective, long term option.

and the takagi won't like running a system that small either. no way it will perform at that rated efficiency in "short cycle city", and you'll have to spend $20/month in additional electrical usage to run the pump and fans, or more, in most areas of the country. Never mind the more expensive initial cost, only to be further outstripped the first time you have a service call.