Hot Water Heater Standby Loss Cost: How Figure?

Constantin

Come on Jerry, you know you can't leave me hanging like that!

I'd love to find something tidier, something that take a beating and still look good, something that can handle up to 180°F water, somethign that can be cut so as to accomodate the weirdnesses induced by installers that did not forsee the possibility of 1" pipe insulation.

Please share a brand so I can see if Metropolitan pipe can supply me with it.

Tom_22

What's the formula?

How does one calculate the cost of maintaining hot water in a hot water heater when it is not being used, given the standby loss in BTU/hr and the cost of gas per therm?

For instance, I was looking to compare the AO Smith Pro-Max High Recovery natural gas fired hot water heaters in 75 and 100 gallon tank sizes. http://www.hotwater.com/frame.html?topage=/RESIDENT/reshome.htm

FCG-75: 998 BTU/hr standby loss.
FCG-100: 1148 BTU/hr standby loss.

Gas cost: .7681/therm
Delivery: .65770/therm.
Total cost: 1.4258/therm

S Ebels

Here's a practical way

Of looking at it.

1 therm = 100,000 btu's, divide that by the btu loss per hour and you will find out how many hours it takes each one to lose 1 therm.

100,000 / 998 = 100.2 hours

100,000 / 1148 = 87.1 hours

So with the FCG-75 every 100.2 hours of sitting there costs you $1.42

With the FCG-100 that same $1.42 happens every 87.1 hours.

From there you can figure the loss by the day, year or over the ? life expectancy of the water heater.

hr

Lots of variables

the room temperature where the heater is located will be a consideration. The wider the delta t the greater the loss through the jacket insulation.

How much draft in the flue piping drawing heat from the tank.

Heat trapper nipples? If not some heat could be moving into the hw piping as a loss to the space.

Size of the pilot flame.

Turn off all the other gas appliances, and the water valve to the tank, and record the gas meter dials. This should indicate how much energy was used for the given period.

hot rod

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Tom_22

Thanks SE,
So a comparison of the yearly cost for the heat loss through the tank walls would be $125. for the 75 gal and $142 for the 100 gallon. If the price of gas went up some, the differnce would be a little more, but...

Interestingly, that is an insignificant difference.

Calcs: There are 8760 hours in a year.
75 gal 87.6 (100 hour periods) x $1.42 = $125/yr
100 gal 100 (87.6 hour periods)x $1.42 = $142/yr

Tom_22

> the room temperature where the heater is located

> will be a consideration. The wider the delta t

> the greater the loss through the jacket

Hi hot rod,
The manufacturer didn't seem to give any consideration to the delta T. They just give the loss in one BTU/hr figure.

> How much draft in the flue piping drawing heat from the tank.

These are both the same model so one might guess the flue pipe loss would be about the same for both. Is it advisable to put a damper on the flue or anything?

> Heat trapper nipples? If not some heat could be moving into the hw piping as a loss to the space.

What's a heat trapper nipple? Google didn't tell me.

> Size of the pilot flame.

I guess this would probably be the same for both units. Don't any HWHs come with electronic ignition though, eliminating a pilot light? (I didn't look into that yet.)

Constantin

A general rule of thumb for quality indirects is...

... ½°F loss per hour of standby. That equates to about 1kBTU/day per 10 gallons of capacity. My Vitocell has about the same capacity as the FCG-75, yet it's daily loss is on the order of 7.5kBTU vs. 27.5kBTU you listed for the FCG. In other words, the Vitocell loses about 4x less heat than the comparable AO Smith unit.

Below is a quick calculation that you can replicate in excel. The combustion efficiency refers to how the lost BTU's are made up. The 85% refers to the Vitocell being paired with a efficient boiler (i.e. Vitodens or Vitola), while the gas water heater is assumed to be a standard 70% efficient model.

hr

Jacket loss

can be calculated from a HDS program I have. It allows you to input different room temperature, tank diameter, and insulation R value to get the loss. Pretty handy for comparing jacket insulation value.

Yes, some water heaters are available with spark ignition, generally the power vented type. these also minimize flue loss. But at a cost and noise tradeoff

Heat trapper nipples are simple floating balls or rubber flaps in the connection nipples to slow, or eliminate HW ghost flow into the piping. Usually the higher cost "energy saver" models come standard with them installed.

hot rod



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Constantin

Yet....

.... that jacket loss does not include the flue pipe of a gas water heater, right? IIRC, the jacket losses in HDS are for indirects only.

I just dug through the Viessmann Vitocell B300 TDM. Apparently, its measured daily losses assuming 68°F indoor and 149°F tank temps are 7,167 BTU per day.

S Ebels

There it is in black and white

A good indirect that size will save a person about $160 per year. Another good reason to ditch the water heater and go with a boiler fired indirect. Considering the life expectancy of an average HWH vs an indirect, it's a no-brainer.

Constantin

Yup...

... and another reason why my contractor is enduring my replacing the pretty (3/8" thin) foam-based insulation with 1" thick fiberglass/paperfaced stuff. Yes, it get's wrinkly, ugly, and all that, but 1" of fiberglass will beat 3/8" of Armaflex or whatever any day of the week (except when wet). Those pipe losses really start to add up once the tank is well-insulated...

I went for an indirect not only because it has a longer life-expectancy but because it isn't a source of CO, it doesn't require maintenance, and because it's one less appliance to worry about...

jerry scharf_3

There is better pipe insulation out there

Constantin,

You are just going to have to stop shopping at Home Depot. Also, my indirect has 3" of urethane foam, neener neener. :)

Seriously, there are better foam pipe insulation products. I am using one that rates at R4.6 picked up at a local supply house. I'm not sure I believe their number, but as you say, it's better than the thin stuff. And it's nice to cut. There was a tip on the wall once to cut the stiffer foam with a chop saw, it works great.

And I agree with all the reasons you listed.

jerry