Standby losses

Pete_Engle

Does anyone here have any references that provide the standby losses for a "typical" gas-fired water heater?  I'm trying to figure out how much gas a 40 gallon standard (low) efficiency water heater will use, just sitting there idling, with no hot water use.  

Larry Weingarten

Here's a start!

http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/99/990707.html



It's called WHAM and it allows you to play with the variables around a heater.  Actually Energy Factor is of some use here also.  For a decent gas heater it's about .60.  That's 60% of the energy in the fuel makes it into the water leaving the heater.  Figure in combustion efficiency and the difference is loss.  Another rule of thumb is that modern heaters have standing pilots of about 400 btu.



Yours,  Larry

Pete_Engle

Standby losse

Thanks.  I have taken a quick look at WHAM, and it doesn't immediately look like it does what I need, but I'll look again.  The 400 btu number for a pilot is useful, and it's a part of what I need.  I'm hoping to find out how much gas a water heater is likely to burn on pure standby - no water use at all for an extended period.  Most of the information I've been finding is an attempt to put it in a percentage, kind of like EF.  With zero usage, the efficiency is zero.  But I don't care about efficiency - I need the actual gas burned over time.



Thanks anyhow.  THis is a start.

jp_2

do a heatloss

take an actual tank, figure surface area, find out R value of tank insulation and there you have it!

Pete_Engle

Not quite so fast

If it were an electric water heater, a heat loss might not be that hard, if you could get the R-values from the manufacturers.  The insulation is thick enough that you'd have to use a thick-walled cylindrical analysis, but that's doable.



The combustion chamber and vent are a bigger problem.  The metal chunks stuffed in the tube are there to improve heat exchange between the exhaust gases and the water, but they work in reverse during standby.  The combustion chamber is an unusual shape, and the airflow along the surface will be complicated, not to mention the variations caused by the pilot light.  Most of the heat loss is through the interior flue, not through the jacket, and that would be very hard to model. 

TonyS

If you have one handy

A water heater that is.... put a thermometer in it and check what the heat loss is an hour.

then just figure how many pounds of water x degrees per hour.

Post your results, I would like to know too.

jp_2

not so fast, part 2

ok, lets look at this closer.



internal flue, 3in in dia.  and heated by the pilot.  delta T maybe 30F?  125F water, pilot heated air, say 95F ? just convective lose here.   I will try to measure temp of the flue.



that "thing" inside the flue really isn't attached, at least tanks I've taken apart, that "thing" comes out easily, so I'd guess it has little effect on conducting heat away.  seems to be there to disrupt flue gas laminar flow.



outside tank , much larger surface area than inside.  older tanks maybe 1.5-2 in fiberglass. delta T, summer or winter? my basement usually was 45F in winter, so call delta T 125F - 45F or 80F.



still think jacket lose is negligible?  I think its the other way around.

Pete_Engle

not so fast again

OK, heat loss might be greater for the jacket than the internal vent (what's that pipe actually called, anyhow?).  Then again, maybe it's not.  Far too many guesses and variables.  Sure, we could have fun doing some calculations.  For an evening of fun, I just love to drag out the thermo textbooks and look up surface film coefficients.    This just seemed like something that somebody's actually measured over the years.  Getting the number without work would be WAY more fun.

EricAune

pipe description

Interesting discussion.



The internal flue gas passageway is called the flue.  Inserted in the flue is the flue gas baffle.  The flue gas baffle is designed to slow the rate at which the flue gas exits the combustion chamber and in return more heat is absorbed into the tank water. The baffle is designed so that the gasses remain above their dew points as to eliminate condensing.  This, of-course is standard design for a atmospheric vented water heater and also works in reverse during stand-by cycles.



Power vented tank type water heaters still have the baffle, however mix excess air at the vent housing with the flue gasses to decrease the temperatures allowing it to be vented with plastic piping.

 

my two cents

 

jp_2

according to slant fin:

7-12 degrees per hour.  I also found 2.5% heat loss per hour......



http://www.slantfin.com/documents/406.pdf



I'd still like to know, flue loss compared to all around skin loss.



I've measured and calculated water heater efficiency at 70%, not including standby.

TonyS

415 therms of gas

for standby with a burner efficiency of 70% and a 10 degree per hour temp drop...that is alot of gas! Is my math correct?

They say only a 1/2 degree with their indirect but they fail to mention the temp drop of the thermal mass of the boiler they are using.

jp_2

I agree

that does seem pretty high, like heating an extra 100 -200 gal per day at 70F rise, just to maintain 40 gal of water?

Larry Weingarten

How about this?

If you had a way to measure the burner on time to make up for standby loss for a 24 hour period (say 1/2 hour), you could multiply that by the firing rate (say 35,000 BTU/hr), add it to the BTU usage of the pilot (say 400 BTU/hr times 24 hours) and have a good guess about daily standby gas use.



Yours,  Larry

TonyS

I would think standby loss

is a constant, whether the water heater is running or off, the heat loss through the jacket stays pretty much the same. Just a temp drop per hr times pounds and hours should be pretty correct.

Pete_Engle

standby loss

probably is a constant, or nearly so.  But except for the reference posted above (which sounds a bit high), I have not yet gotten any data about typical temp. loss per hour.  I could certainly use that data to calculate  an approximate Btu/hr.  So far no luck from either AO Smith or Bradford White.  I've left messages for a few technical support people, but it seems they are not eager to have that information made public.



Sounds like a good subject for a DOE grant & paper, huh?

TonyS

It does sound high

But even if we go down to 5 degrees an hour on a 40 gallon water heater it would be 332 pounds of water x 5 degrees cost us 1660 BTUs an hour or 39,840 a day. At 80% efficiency that is one therm of gas every 2 days or more than one gallon of propane. It is an enormous waste of fuel!

jp_2

not a constant

pete, surely you remember your differential equations class?



 one time while studying I plotted the room temp vs time in a typical FA heated house.   the rate of temp fall in the room was amazing non linear.    even at the 4-5 degree differential, nonlinearity was very noticeable. 



that said, an average is easy to take and "good enough" for any type of rule of thumb or "standard "



effected by,



wind- air flow through flue

water temp

ambient temp



but you all know that !



 just sit in the basement for an hour and see how long it take to cycle,  you need to measure water temp too.    your measurement will be as good as anyone else's.

jp_2

DOE

pete, if i remember right, standbylosss is figured into the EF energy factor.  but its been a long time ago when i tried to figure out what that number actually meant.