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Efficiency of Indirect Water Heater?


I am trying to figure out an approximate relative yearly cost of an indirect water heater vs. a standalone water heater.

We are installing a Lochinvar Knight KNH055 for home heating. This modulating unit can produce up to 55K BTU, with a stated AFUE of 95%. We are considering installing a Lochinvar Squire 40 Gallon Indirect Tank.

Let's compare this indirect to a high efficiency propane Rheem (as an example). This 40 gallon model #xp40S12DM46U0 has a stated energy factor of 0.67.

I want to find the relative cost difference between these two systems, I'm less interested in getting the exact correct yearly cost. Below here I give a lot of details showing how I have tried to calculate this. I am a homeowner and not a HVAC professional, so all of this could be wrong -- please correct me!

Here is how I have tried to arrive at an estimate:

The energy guide for the Rheem says the Rheem will cost $590/year. This number is arrived at with the following assumptions:
* Propane costs $2.41/gallon
* Incoming water temperature is 58F
* Hot water temp of 135F
* Total daily hot water production 64.3 gallons

A few quick derived numbers from the assumptions:
* 1 gallon of propane = 91,333BTU from eia.gov
* $2.41/gallon / 91,333BTU = $0.00002639 propane cost per BTU

Using the energy.gov formula to find yearly cost:
365 days * 41045 / 0.67 energy factor * $0.00002639 = $590 yearly

(Note this 41045 number is mysterious to me and I did not take the time to understand it.)

Great, the $590 is the same number as the energy guide link. So I have verified at least I can re-create their thinking.

Let's try to apply this to understanding to the indirect's cost. Using the same assumptions as above, we have:
* 135F - 58F = 77F the required temperature rise
* 77F * 8.34 BTU per gallon of H20 = 642.18 BTUs req. per gallon H20
* 64.3 H20 gallons day * 642.18 BTUs/gallon = 41,292.17 BTUs/day
* Now here's where I am really making things up and have no idea if this is a correct way of looking at things:
* 41,292.17 BTU/day / 0.95AFUE boiler = 43,465.44 req. BTU/day input to boiler
* 43,465.44 BTU/day * $0.00002639BTU = $1.15 per day
* $1.15 day * 365 days = $418.67 cost per year of running boiler to produce 43,465.55 BTUs a day.

But that's clearly not a correct estimate of the indirect's usage, because there has to be:
1) standby losses of the indirect (which the energy star calculations for Rheem claim to include)
2) the heat exchange between boiler and indirect is not 100% efficient

With some time spent online searching, I cannot find clear values for either the standby losses, or the heat exchange efficiency should be modeled as.

So, ending with two questions:
1) Assuming my above calculations are a correct way of modeling this problem, my above formula for modeling the indirect's required BTUs should be:
41,292.17 BTU/day / 0.95AFUE boiler / STANDBY LOSS efficiency / HEAT EXCHANGE efficiency = amount of BTU required
What should I use for STANDBY LOSS and HEAT EXCHANGE LOSS?
2) If my calculations are incorrect please provide guidance as to how I should be looking at this.

Thank you for your time and advice.


  • livininmichigan
    livininmichigan Member Posts: 3
    edited September 2017
    I just realized that table 8A of this document states that the 40 Gallon squire has:

    * Standby loss of 0.9 F / hr
    * Tank Heat Source Friction Loss Feet W.C. 4.5

    Is the tank heat source friction loss feet wc a number I need to account for the efficiency loss?

    These numbers were found using the same 58F incoming water temp, and 77F rise, as I used above.
  • Zman
    Zman Member Posts: 7,573
    edited September 2017
    You should absolutely go with the indirect. Figure your energy use as 88% efficient (Lochinvar running at high temp) rather than the 67% for the water heater.

    Don't try to figure out the government rating assumptions unless you have lots of time on your hands. As it turns out, the testing is deeply flawed..
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    HVACNUT Member Posts: 5,864
    Totally agree with @Zman. Go with the indirect.
    A sensor is supplied, and controlled by the DHW program settings in the boiler.
    Setting the indirect temperature to 140-150 degrees and adding a thermostatic mixing valve will make the indirect more efficient by not directly depleting the tank. Therefore, fewer cycles.
    Specs. on page 5 show the 40 gallon to have a .9 degree per hour standby loss.
    I've seen tanks advertise .25/hr. Whether true or not, I don't know.
  • NY_Rob
    NY_Rob Member Posts: 1,370
    A few days after we commissioned our new mod-con last summer we had to go out of state to a wedding in NJ. Being the boiler and the 30 gal HTP SuperStore Ultra indirect were new and no one was going to be home for 24hrs... I turned the system off at 11am Sat morning just as we left the house.
    When we returned at almost exactly 11am Sunday morning (24hrs later) I turned the boiler back on. I thought the boiler would fire up immediately due to heatloss in the indirect- I have it set for a 10geg differential. Well after a half an hour the boiler still didn't fire up. It finally fired up once one of the kids got in the shower. It dropped less than 10F in 24hrs with no one drawing from it. Pretty good insulation I guess.
  • gschallert
    gschallert Member Posts: 170
    edited September 2017
    It boils down to the efficiency differences, 67 v 90. I switched from direct fired stand alone propane 40 gallon tank to indirect 40 gallon last fall. DHW costs have dropped plus recovery time is amazing. My standby losses (negligible) are under mfg spec, I added an insulating jacket to tank as well since it's in unheated basement.
  • flat_twin
    flat_twin Member Posts: 350
    With no space heating, DHW only we use 8-10 ccf NG per month with a 40 gallon indirect tied to a modcon boiler. NG is about $1.20 per ccf , so $10-12 plus $8 service fee for hot water every month for two adults.
    The indirect replaced an aging electric water heater. Our monthly electricity usage is down roughly 300 kwh or $40.
  • Gordy
    Gordy Member Posts: 9,546
    edited September 2017
    The only way you could possibly do better than a mod/con indirect setup, is a high turn down ratio combi. However the combi would suffer compared to an indirect with supplying multiple fixtures. Even with a 55k boiler coupled to the indirect. Most stand alone DWH are 35kish input. I directs certainly have lower standby losses than a stand alone out of the factory.

    The up front costs of the indirect are certainly higher, and one must consider the life of the indirect. However as Carl said it boils down to what efficiency the hot water is being made at. Mid 60's, or upper 80's.
  • Boon
    Boon Member Posts: 260
    So it isn't really related to efficiency but no one has mentioned the benefit of having only one fuel burning appliance. Am I putting too much weight on having a single fuel burning appliance vs two?
    DIY'er ... ripped out a perfectly good forced-air furnace and replaced it with hot water & radiators.
  • Gordy
    Gordy Member Posts: 9,546
    I don't think that's a big issue having multiple fuel burning appliances. Especially if there were already multiple ones.

    Some things do come to mind though, and that is venting, and make up air.
  • livininmichigan
    Thank you everyone for your wisdom and thoughts!

    @Zman thanks for your suggestion of using 0.88 for the equation. Gives me at least a relative idea.

    Also, not surprised to hear government testing is deeply flawed... :)

    @Boon and @Gordy I have read a lot of other threads on here extolling the many virtues of the indirect setup, which is why I am going to get one. :) I just hadn't seen any actual estimation of the relative efficiency gain (fully acknowledging the difficulty of getting exact numbers). I just wanted to have a rough idea of the payback period.