Efficiency of an indirect water heater

hot_rod · December 2

Probably isn't anywhere near what many assume. AI seems to way off with their prediction of 99% :)

A coil in a tank of still water is not such a great heat transfer mechanism. Certainly not 90% boiler efficiency even if you have a massive or duals coil indirect for the boiler to feed.

I'd guess not much better than a boiler with a tankless coil, when there is no circulation.

The heat transfer number is not so easy to pin down with so many variables.

EK has it right using a separate flat plate, leveraging two moving, turbulent flows with a lot of surface area.

With close approach sizing of a plate HX you could generate 120 DHW with 125° SWT to the plate HX. So now you could leverage the mod con 90% efficiency. But still the crappy heat exchange inside the tank. A dhw recirc pump would change the transfer efficiency.

Condensing tankless WH would be a higher transfer efficiency moving turbulent flow again.

And of course the near 100% efficient electric tank water heater. Better yet a properly applied HPWH.

Nothing against indirect tanks, but the hype may be overlooking the thermodynamics.

Then there is the pie in the sky standby loss numbers. An indirect tank at 120F in a room at 80F skews those numbers. Put the indirect next to the combustion air grill and see how that less than 1° per hour works out. Ive been in boiler rooms with snow drifts below the combustion air grills.

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mattmia2 · December 2

I think you eventually get good transfer but that is because you start out with poor transfer and the tank temp drops so the approach gets much wider and the transfer gets better. I've thought about either pre-firing the boiler on flow to the shower or adding a brazed plate hx before the tank at the inlet and in the return to the boiler.

Kaos · December 2

A plate HX (or coil inside tank) is pretty close to 100% efficient, any heat lost on one side is transferred to the other side. But this is not the reason why indirect efficiency is low.

The issue with indirects is RWT. From monitoring my indirect, the return water tends to be above condensation temps, so at best an indirect is mid 80% efficient off a modcon.

The other issue is losses during non-heat season. The modcon and all the piping has to be heated up to about 160F and all that heat is lost after each cycle as there is nowhere to purge it.

This also assumes everything is plumbed and configured properly. How often do you see that in wild? Chances are the indirect is over pumped (thus higher RTW so lower efficiency) with high volume and uninsulated pipes. Add in some scale buildup on the heat exchanger and things get even worse.

So I would say on a yearly efficiency basis a modcon+indirect is comparable to a power vented tank.

It really makese no financial sense to install one from either BOM or operating cost. If you want efficient hot water of gas or propane, get a condensing tankless water hater.

EdTheHeaterMan · December 2

AI does have it wrong. The indirect efficiency is directly related to the efficiency of the heat source. Subtract a factor based on the efficiency of the transfer process including the surface area of the heat exchanging surfaces and any loss thru the insulated jacket to the ambient air. But to compare a tankless coil to an indirect is like comparing apples to pineapples. They both are apples but they are completely different fruits. And there are no pine cones involved.

From experience I can tell you that an oil fired boiler with a tankless coil that is maintaining 160° in the off season will operate the oil burner for a few minutes every 4 hours as a result of the vent connection to the chimney. That same oil fired boiler will fire every 20 hours for a few minutes to recover the loss of temperature in the tank. That is assuming no one uses any hot water for that 20 hour period. (like when you are on vacation). I had a timer on my oil burner just to test this theory. It is not a theory anymore. it really happened with my Buderus indirect and my Buderus boiler with Riello burner over 30 years ago.

That is why I recommend Indirect water tanks over tankless coils and tankless wall hung water heaters. You need to live like a European when you use a tankless water heater. Us Americans like to have as much hot water as we can get.

ethicalpaul · December 2

It really makese no financial sense to install one from either BOM or operating cost. If you want efficient hot water of gas or propane, get a condensing tankless water hater.

I couldn't agree more except get a heat pump water heater

hot_rod · December 2

https://forum.heatinghelp.com/discussion/comment/1823828#Comment_1823828

what us the factor to use?

My comparison to a tankless coil was just about the heat transfer of a coil in still water, not the boiler operating confition. Although that is a big $$ number no doubt

mattmia2 · December 2

https://forum.heatinghelp.com/discussion/comment/1823839#Comment_1823839

unless this lets you get a stainless indirect tank that will last several times as long as the other options.

EBEBRATT-Ed · December 2

Tankless coils are ok for two people but in this day and age it doesn't make sense to me to keep a hot boiler 24/7.

As far as indirect go I have an electric HWH. I have been on vacation for up to two weeks and come home to still have luke warm water after 2 week and mildly hot water if gone for 1 week and I shut it down when I leave. I assume indirects are insulated as good as a EWH.

So I think indirects are a good choice.

Tankless water heater seem nothing but trouble from liming up the HX due to high temp heat transfer and flow issues I know a lot of people who put them in and have ripped them out.

HPWH are expensive to purchase and who can fix one if they bite the dust? A homeowner certainly isn't going to invest in the refrigerant tools needed for repairs and neither are the plumbers who install them. Many have given up on them as they just turn into an EWH. They are an expensive throw away appliance. Even if you get a rebate you wont get a second rebate

hot_rod · December 2

water heaters are big business, something like 4 million electric tanks are sold per year

HPWH, around 190,000 in 2023

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ethicalpaul · December 2

nice graph @hot_rod

I’d advise everyone to watch that acceleration curve in the years ahead

mattmia2 · December 2

https://forum.heatinghelp.com/discussion/comment/1823861#Comment_1823861

Maybe htp can build one that will last long enough to pay for itself.

jesmed1 · December 2

@hot_rod Could you please comment on this efficiency comparison chart from Energy Kinetics? It's for different boiler types that also heat DHW. I don't doubt its correctness, but I wonder what assumptions are made for the conditions under which the non-EK boilers are running. Are these truly typical efficiency numbers for cast iron boilers with indirect or tankless coils?

We have a natural gas fired water heater with a typical draft hood and standby losses up the flue, so we're probably 70% or less efficient there. I would love eventually to go to an Energy Kinetics System 2000 with an indirect tank that would (a) eliminate our existing 2 large cast iron boilers and (b) improve our hot water efficiency. I know Roger from EK has commented here before that the EK efficiency comes from (1) low thermal mass, (2) low water volume in boiler, and (3) post purge.

Our combined system efficiency for DHW and cast iron boilers is probably around 70-75% now, and EK advertises 85% combined system efficiency for the System 2000 with DHW, so in theory we could probably improve our efficiency 10-15% by going to a System 2000 at 85% system efficiency.

hot_rod · December 2

https://forum.heatinghelp.com/discussion/comment/1823889#Comment_1823889

I don't know who compiled that chart. I usually look for independant lab test results.

Be handy to see mod con with indirect.

There are so many variables however, for comparisons like this.

Cycle efficiency tells more of a story. I did a post on calculating that recently.

Cycle Efficiency = total heat output over a period of time ÷ energy content of fuel consumed

Cycle efficiency will always be less than steady state efficiency, calculate it from the run fraction

Run Fraction burner on time ÷ total elapsed time

Mild temperature days with fixed output boilers is where it fall apart.

Using load info RF= heat required ÷ steady state output

A 30,000 load on a 125,000 boiler = .24 or 24%

Using this Brookhaven developed graph, (used with permission) that RF % puts you in the 70% range.

Therein lies the problem with zoned boilers in mild conditions, or less than design load. Which may be 80% of the heating season.

Don't dismiss the importance of modulation, even running out of condensing mode.

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EBEBRATT-Ed · December 2

HPWH is 2% of the market. Not popular and not perfected yet. When they are done experimenting and start to sell more the price may come down

GroundUp · December 3

I've been a long time proponent of indirects over anything else for fully occupied homes, but began questioning this efficiency not so long ago after paying attention to the delta across the coil in multiple different brands and sizes of indirect. My personal home, I use an outdoor wood boiler for space heating and have a plate heat exchanger piped into the cold inlet of my tank which I feel would be a great option in lieu of an indirect when using a mod/con, possibly with a parallel circuit to the space heating zones(s) so that any DHW draw during a space heating cycle would bring in preheated water to the tank. I've also been kicking around the idea of adding a small bronze or composite circ to mix the indirect tank (from the relief port to the drain port with tees) to keep a flow of water across the indirect's internal coil as well as heating it in a parallel fashion via plate exchanger to maximize heat transfer and efficiency. Probably a negligible increase on both accounts, but it's something…

hot_rod · December 3

On one hand you will increase the heat transfer of the indirect considerably by moving both flows

However the trade off is you lose the tanks stratification, which can add 7, 10 or more gallons of drawdown

With a wood fired boiler you could run the indirect to 140, 160f or more mix down to 120

I guess it depends on your dhw needs and usage. If you have a large soaking tub or multiple loads in a row, that extra drawdown can be nice to have

GroundUp · December 3

I was referring to a mod/con system, not my own.

HeatingHelp.com · December 3

We just had to delete several sexist remarks on this discussion. It's stunning to see how a discussion about water heater efficiency can lead to sexist stereotypes and a "joke" about violence against women. These attitudes are toxic and not welcome here. You're better than this. Follow site rules and be respectful of others.