H20 HTR ratings?

Steve_35

> Unfortunately water heaters don't list an output <BR>
> number for BTU, so there is no real way to get an <BR>
> idea for how efficient a water heater will be in <BR>
> a space heating application. I remember reading <BR>
> someone did a combustion test on a water heater <BR>
> and got 81% - but remember that is steady state <BR>
> efficiency. When you factor in standby losses it <BR>
> will be lower.<BR>

You can get a fairly accurate output rating by multiplying the continuous output in gallons per hour times the temp rise for that output times 8.25. That will give you the rated output. But as you say it is steady state, not including standby losses which are significant in most gas fired water heaters.

JimmyJam

Water Heater ratings

JimmyJam

Water Heater ratings

How does an energy factor of .64 relate to AFUE? I am bidding on a small radiant project that the others are quoting water heaters and I will not be, however I would like to differentiate differences between 92% and .64 E.F.

Thanks!

Steve_35

You can compare the

input to the output to get steady state efficiency. EF doesn't equate to AFUE unfortunately.

Mike E_2

Unfortunately water heaters don't list an output number for BTU, so there is no real way to get an idea for how efficient a water heater will be in a space heating application. I remember reading someone did a combustion test on a water heater and got 81% - but remember that is steady state efficiency. When you factor in standby losses it will be lower.

A .64 EF for DHW can't be converted to 64% AFUE since the EF factors in alot more standby losses up the flue since a water heater has so much idle time.
So a water heater in a space heating application that occasionally cycles has to be less than steady state efficiency - 81%, but above 64% since there isn't quite as much standby losses.
I came across a source (of course I can't located it now) that claims an EF of .69 equals 77%. I don't know how they came up with that, or if it is accurate.


EF and AFUE both basically measure the same things, but in slightly different ways.


EF is defined as measuring:
1. Recovery Efficiency
2. how efficiently the heat from the energy source is transferred to the water Standby Losses
3. the percentage of heat lost per hour from the stored water compared to the heat content of the water Cycling Losses
4. energy lost during the transition of the off/on cycle

AFUE is defined as:
The measure of seasonal or annual efficiency of a furnace or boiler. It takes into account the cyclic on/off operation and associated energy losses of the heating unit as it responds to changes in the load, which in turn is affected by changes in weather and occupant controls.

Darin Cook_2

There is alot of intepretation involved

An electric water heater has EF ratings in the 90's. Is it really the most efficient tank to use? No one thinks that is the best tank to use. Unless you have access to cheap municipal power. AFUE is a skewed number given to equipment under controlled lab conditions that do not occur out in the field. Even getting combustion efficiency numbers off of a digital combustion analyzer is not entirely accurate since those numbers are based upon stoiciometric cumbustion which does not happen in anyones basement.

All of those ratings give you a number to work with. You must correctly interpret the information presented. I have a saying, "Heating is a exact science that we cannot not do exactly". Though we must try our best to get as exact as we can.

Darin