standby losses

Gregg Jackson

Does anybody know what the "average" standby loss of a direct fired water heater is?

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Stand by

Looks to me most of the new tanks are foam insulated. Probability a good R-value. The temperature of the space where it is located would be something to factor in.

Got a low combustion air vent in the room? The flue losses would be a bigger number than standby with a hole in the bottom of the tank, a hole in the top, and a constant up draft in the flue piping, and plenty of cold outside air coming into the room!

I suppose you could track the gas meter for a day with no hot water useage, and other loads, and come up with some numbers??



hot rod

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Boilerpro

I believe I've read that....

> Looks to me most of the new tanks are foam

> insulated. Probability a good R-value. The

> temperature of the space where it is located

> would be something to factor in.

>

> Got a low

> combustion air vent in the room? The flue losses

> would be a bigger number than standby with a hole

> in the bottom of the tank, a hole in the top, and

> a constant up draft in the flue piping, and

> plenty of cold outside air coming into the

> room!

>

> I suppose you could track the gas meter

> for a day with no hot water useage, and other

> loads, and come up with some numbers??

> hot rod

>

> _A

> HREF="http://www.heatinghelp.com/getListed.cfm?id=

> 144&Step=30"_To Learn More About This Contractor,

> Click Here to Visit Their Ad in "Find A

> Contractor"_/A_

Boilerpro

I believe I've read that....

there is an efficiency rating for water heaters similiar to AFUE. For a standard residential gas fired chmeny vented unit I believe this efficiency number was about 55% at best. Assuming about a 75 % firing efficiency when new, you lose about 26% of the heat produced. Anopther reason to use an indirect fired water heater.

Boilerpro

KevMcG

off this topic a bit

When you mentioned indirects you got me curious
I try to read the wall everyday (set as homepage) In the time I have been lurking here I don't recall seeing any opinions on Vaughn stone lined indirect.
I have installed a few over the years and liked them. We had one installed in a GC's house. One time he was working around the boiler and it was to hot in the area so he shut the boiler off. He said he went 2 days before he realized it was off and that was with 5 kids in the house and a wife showering. I was impressed with heat retention of the stone.

Unknown

Water heater losses

Tanks in cold damp cellars with many air changes (6 ACH) tend to have a standby loss of 20 to 25%.

When the cellar is heated or warmer with controlled air changes (less than 1 ACH) it can drop to 15% to 20%. Newer tanks that are more heavily insulated can be about 10 to 15%.

Many have taken to wrapping tanks in insulation, you want to be careful in doing that as you can restrict air needed for combustion and dilution air to the draft hood.

Center flue water heaters also have a high loss due to design, One of the reasons for spiral baffle (in additon to slowing down flue gasses for better wiping action) is to cut down on standby loss.

We used to put thermal dampers in the flues of water heaters to stop some of the standby loss. That can be increased when you have a boiler or furnace into the same vent or chimney, when the boiler or furnace shuts off the chimney is heated and the draft can now pull more room air thru the water heater causing more standby loss. A way to help with that a little bit is to make sure the water heater flue is above the heating equipment flue.

Bottom line underfired water heaters are not very efficient as an overall means of heating hot water. Years ago we have full floater tanks and back flue tanks which were more efficient. Indirest water heaters do not have all of these draw backs and tend to be a much more efficient way to heat water. Of course they have to be running all summer and unless ther is a way to modulate the control of BTU's that can off set some of the efficiency.

We used to use boilers for hot water in the winter then switch over to underfired in the summer, we used the tank as a storage vessel in the winter.