Indirect versus gas fired WH

jerry scharf_2

Ed,

The killer for a standard hot water heater is that it has a chimney right through the water chamber. When they have no electric connection, they have to depend on natural draft. When the water heater turns off, you get a nice constant air flow through the water heater cooling the water. With flow checks, the loss from the boiler piping to the indirect will be much less.

One test would be to heat an indirect and a water heater to 140F, shut them off and let them sit for 24 hours. The final temp will give you the standby heat loss. The indirect people say that it should only lose a few degrees.

Efficiencies will be all over the map for indirects, depending on the boiler and piping. You can mess up the piping and take a nice collection of high efficiency gear and still lose a lot of heat.

jerry

EBEBRATT-Ed_7

Any information available on the efficiency of indirect domestic hot water versus a standard gas fired water heater?

Obviously depends on the efficiency of the boiler the indirect is connected to versus the efficiency of the water heater. But are there other issues to consider???

Thanks for the help
Ed

Constantin

Well, short of doing a test yourself...

... comparing such efficiencies is going to be difficult. I know of no comprehensive comparison and GRI/GAMA has come out and stated for the record that EF and AFUE are not comparable. Besides, neither combustion test captures standby losses, which are particularly egregious with indirect gas water heaters.

I'd love to see "real" numbers myself...

EBEBRATT-Ed_7

Constantin, Thanks for the reply. As you probably are aware "gas networks" in MA. is giving out rebates on indirects for$300. Was just wondering about efficiency
Thanks,
Ed

EBEBRATT-Ed_7

Jerry, So I guess that comparing the combustion efficiency of a gas water heater versus the efficiency of the boiler connected to the indirect and combining this with your method of heating them both to say 140 to get the standby loss -may not be the perfict method but sounds like it would be in the ballpark.

Thanks,
Ed

Constantin

I didn't know that...

... then again, I doubt I would be eligible, seeing that I have oil as the main heating fuel. The folks at CSG tell me I have a raft of rebates coming at me due to the tight house specs, the high efficiency of the AC, etc. but I have yet to see them.

I wonder whether you could link the standby loss to the annual fuel consumption stated on the Energyguide stickers...You'd have to know the conditions of the Energyguide test (i.e. how many BTU's did they pull vs. were wasted), but it would give you something close to an seasonal efficiency rating.

Chuck_7

FYI - I turned off my 40 gallon gravity water heater for 8 hours in an effort to determine if there would be a payback on turning it off a water heater over night say in a laundrymat. The temperature only dropped about 15 deg. I figured the savings would be maybe $5 a year (very roughly). To me this means there would be no real payback for additional cost spent for efficiency. The savings should come from the equipment cost and operating simplicity (though it is always good to save energy if you can).

Joe Brix

Outdoor temp

and draft levels would play a big part on how fast the gas water heater cooled off. If you're heating season is at least 6 months, you'd probably get great efficency with an indirect as you'd be heating water with an already hot boiler. Controls like Tekmar, and I'm sure Vision and the Ultra's controls post purge BTU's: running the indirect circulator to pump BTU's into the indirect rather then just letting the heat fly out the flue after completing a heating zone call.

Constantin

Could you tell me more about your test?

... so your 40 gallon gas water heater lost 15 degrees overnight... in what conditions? How cold was the room, what were the conditions outside, etc.?

Perhaps I got my calcs wrong, but the below excercise would suggest that standby losses from a 40 Gallon water heater reach 5MM BTU a year, something on the order of $75/year in Boston, where gas costs $1.5 a therm.

This is built on the assumption that your conditions were "average"... Furthermore, I would argue that the standby losses would be proportional to the water heater temperature, i.e. they would drop as your test progressed.

Not captured in the standby losses is the additional exfiltration that a typical gas water heater termination offers to a building. Several million BTUs might go that way too, if the WH is installed in a conditioned area.

jp_2

test

I ran a similar test.

all winter, no heat in basement, ice on basement floor.

30gal DHW, used about 5 gal hot water per day.

I used about 900 cu ft per month included cooking.

my bill was about $15.00 I think. it doesn't seem to me that stack and tank loses are that high considering the basement was below freezing. yes I had pipes freeze twice on me.

I think the biggest loses on most DHW are during firing, my came out to be about 70% efficient.

I realize I should do the cals, city water dipped as low as 35F.

J.C.A._3

For your consideration....

How about adding in maintenance costs. Having an indirect reduces the number of appliances and their maintenance.(not to say that an indirect won't have any maintenance, but will NOT have an additional burner and control to worry about.)

The other advantage to an indirect is the lower temperature deviations. When a direct fired appliance is fired and no one is there to be using it, the fire is basically firing into a thermos bottle, expanding and contracting at alarming rates.(think 20° differentials on the controls) This stresses the glass lining in most units more than the use itself.

The third equation to me, at least, is the warranty. Why not go with a stainless indirect and get a liftime warranty and forgo the 6/10/12 year cycle of replacement to the "original" owner, especially if you plan on staying there a while.Seems to me it makes economic sense in the long term.JMHO. Chris

EBEBRATT-Ed

Good information from everyone thank you. I was just curious that if the gas co is giving rebates for these they must know something. They seem to be very popular. I thought that maybe a water heater mfg. or indirect mfg, would have calcs on this issue. - haven't found any yet.

Funny how the pendulem swings. A few years back the deal was not to run the wasteful boiler in the summer

hr

Add to that the warranty

5- 6 years on most residential WHs. Indirects go 15-20, some lifetime.

Sediment in the bottom of a WH shortens the life and reduces heat exchange efficiency greatly.

Many indirects have large clean out ports, and sediment at the bottom doesn't effect the coil output.

In a regular HW tank, getting the heat from the burner to the tank bottom is only part of the transfer. Getting it from the steel bottom to the actual water is more of an issue in hard water or sediment prone areas. Even a 1/4" thick layer will greatly impact the efficiency. This doesn't show up on a combustion analyzer stuck in the flue pipe

hot rod

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