Efficiency of an indirect water heater

The only reason I have an indirect is because I picked up a used electric tank for $50 and put a plate heat exchanger with a cheap recirc pump on it for a simple DIY one.

I would have never bought a real indirect. Even with free labor just BOM cost plus all the bits is simply not worth it.

Unfortunately up here in the great white north the cost of a HPWH is silly, so I have to settle for running my DIY unit.

Like anything hydronic, they are fun to play with though :)

You can always get a power vented water heater. These have significantly smaller stack losses. There are even some models that have a fresh air intake so they don't use building air for combustion.

Around me, the norm for smaller multiplex is either one large natural gas water heater or a bunch of resistance tanks, one for each unit. Ie triplex with a boiler:

Resistance tanks will last a long time if you change the anode once in a blue moon. My repurposed resistance tank is 22 years old and on its 3rd anode.

I'm glad you like it, but if I am guilty of being too pessimistic, I think you are a little too optimistic :)

I'm not sure how you can say your boiler doesn't run all year when you say it runs 8 minutes every two hours. The boiler is losing heat 24/7, to me that is waste.

Can you explain how the boiler isn't in your building space? Do you keep it in the barn? Thanks!

@ethicalpaul come on, your statements lead me to believe that you have never been in a home with an indirect tank and the only ones you have seen are in pictures on here. Most of the homes in my area have indirect water heaters, I work on them all year long and I have never been in a basement that I felt was any warmer as a result of the indirect water heater being heated. Nevermind it "heating up the entire house". People prefer them because they are reliable, last a lot longer than typical tank or tankless water heaters and they never complain about running out of hot water. Maintenance? What, replacing an anode rod every few years? That's nothing compared to the maintenance a combi needs or a tankless water heater needs.

I've lived with a tankless coil boiler, electric water heater and an indirect water heater. By far the indirect water heater has been the best.

Ok, so of the three options, tankless coil boiler, electric water heater and an indirect water heater. When I replace boiler, I should definitely ditch the tankless coil boiler. Either electric water heater or an indirect water heater will be an upgrade/savings

To each his own, the indirect works well for me coupled with a condensing 80k btu boiler even if it doesn’t condense. Produces plentiful hot water which is in sharp contrast to the 50 gallon electric WH it replaced.

Better yet, you show us the data showing any mod/con under any circumstances performing at under 85%. Unless everybody else's combustion analyzer and calculator are broken, 90+% is almost always the case.

No, but in addition to the standby losses of the tank, an indirect also has all the standby losses of the boiler, which must be re-heated in order to put heat into the tank. Surely someone before me must have thought of this negative aspect.

Not all Heat Losses are equally bad. Some are large and significant. Some are trivial. It is vitally important to know the difference. This is why we need to measure and quantify the heat losses and respect the facts as shown by the data. Otherwise we are chasing down every rabbit-hole.

ModCon Boilers are designed to minimize jacket heat losses. My ModCon Boiler is in a sealed boiler room. The heat losses from my boiler are trivial, especially when running only 8 minutes every 2 hours. My boiler definitely does not "heat my house" in the summer. But the boiler heat losses are included in the boiler thermal efficiency.

I know a plate heat exchanger is good, primarily because of the small size and large surface area - but why do you need one in the first place ? Adding an extra Heat Exchanger just complicates the system, increases the ∆T and reduces efficiency. I prefer to use an Indirect type that does not need an external heat exchange - because it has an internal heat exchanger which will always be more efficient.

My Indirect is a Smart stainless-steel jacketed tank-in-tank type with a very large heat transfer area. It is a very-efficient heat exchanger. This large heat transfer area allows the Boiler to run at a lower ∆T and higher efficiency when making DHW. No external plate heat exchanger required.

Equipment selection and system design make the difference on system efficiency and reliability. This forum should really be discussing "Best Indirect DHW System Design".

Thanks I will say that when I am thinking about the most heinous indirect setups, they are typically with oil burner boilers that are not very efficient and are hulking masses of iron with poor insulation.

Mod-cons have their own concerns of course of (lack of) longevity and maintenance but their efficiency is apparently pretty good.

It depends on the boiler. An old boiler that has a large water volume is a lot different than the modern three pass or even the wet base pin type boilers. Something like my WBV-03 doesn't run long enough when heating the indirect water heater to make any noticeable difference in temperature in the basement.

As far as maintenance versus an electric water heater, it's definitely less. My indirect doesn't even have an anode rod, so the only maintenance is flushing it once a year and replacing the scale stopper cartridge I installed. My electric water heater needed anode rod replacements, flushing and I had to replace the elements once. It would run out of water if back to back showers were taken. My 41 gallon indirect never runs outs, no matter how much demand is put on it.

I certainly never suggested a tankless

Do not confuse "efficiency" with "heat transfer rate" and the measures used ti increase heat transfer rate.

Efficiency is "Heat Out divided by Heat In". A coil in still water will almost always be more "efficient" than a heat exchanger with two moving flows under turbulent conditions. A coil in still water can be 100% efficient if there are no external heat losses. It is just kind of slow, so people try to speed that up, and sometimes confuse that with "efficiency".

A heat exchanger with two moving flows under turbulent conditions will speed up the heat transfer rate,, but that does not make it more "efficient". As an external device, your heat exchanger has heat losses. That is why it is insulated - to reduce those heat losses.

The heat loss per hour of an Indirect is a measure of the standby heat losses. It is pretty low.

Heat losses can be measured, added up and subtracted from 100% to determine "efficiency".

I do not use an Indirect that heats a coil in still water. I do use an Indirect that is a jacketed stainless steel tank-in-tank with a very large heat transfer area. As you increase the heat transfer surface area you can reduce the ∆ T. The very large heat transfer area allows me to run quite low 130º F boiler water temperatures, further enhancing boiler "efficiency".

What boiler water temperatures do you use with a flat plate heater ? A large flat plate heater can use a lower boiler water temperatures, allowing higher boiler "efficiency" - but there is a cost trade-off.

Hihot_rod-

I agree that an Indirect that uses internal coils for heating, is like a shell and tube HX design.  The problem is that you just can't pack enough tubes into one to make it fast and efficient. I would not use such an Indirect design for this reason, TurboMax or not.

An Indirect that is a jacketed stainless steel tank-in-tank is nothing like a shell and tube HX design. No Tubes - No comparison. The heat exchange all happens inside the Indirect DHW Tank without any internal tubes or coils. Very simple, because the heat exchange all happens inside the DHW Tank where there are no heat losses to the outside. Very efficient.

All Indirects ar not created equal. The Triangle Tube Smart Stainless-steel tank-in-tank Indirects provide more efficient heat transfer in the Indirect, and the large heat transfer surface reduces the ∆ T on the boiler, further increasing boiler efficiency.

An Indirect that is a storage tank with an external plate heat exchanger cannot be more thermally-efficient than a tank-in-tank Indirect, because the heating transfer takes place outside the Indirect where it is subject to heat losses, and they typically are designed for much higher boiler temperatures like 160º F..

Reduse temperatures, improve thermal efficiency, and save money. The cost savings really do add-up over the years.

Doug

Temperature efficiency of the heat exchanger doesn't matter here. If the approach temp is 10F or 1F, the only thing it changes is how hot the SWT and RWT will be. That has a very small effect on boiler burner efficiency and no effect on the rest. An indirect with 95% or one with 90% temperature efficiency has the exact same thermal efficiency.

As for temperature efficiency, even an indirect with the lowest temp delta will eventually have some scale buildup and this will make a much bigger difference. My DIY indirect used to have a coil inside the tank, after a couple of years I lost about 1/2 the recovery because of scale buildup. The reason I went with the plate HX is that it can be easily cleaned.

In my case the thermal efficiency of the indirect did not change because of the increased approach temp. The only thing that happened is max BTU/h it can absorb decreased so recovery took longer (boiler was running at 1/2 output but 2x as long).

If you are looking for temperature efficiency, you can back calculate from the spec sheet. ie:

https://www.heat-flo.com/storage/products/indirect-water-heaters/indirect-water-heaters-specification-sheet.pdf

These usually list continuous output which is what you want to use. They also have the surface area so if you do the heat exchanger math with delTm, you can figure out the flow rates and temp rises you can get at other points.

Normal Indirect operation is a series of re-heat cycles to maintain the DHW Temperature in the 120º F range.

I prefer my tank-in-tank indirect with 40 sq ft heat transfer area, and my 130º F boiler water temperature, because it is more efficient overall - saves me a lot of money every year. My Recovery Time is 8 minutes so that is not an issue. I have watched it operate for 15 years.

Equipment sizing is very important. Sometimes over-sizing certain heat transfer equipment can save a lot of money on energy-efficiency and long-term operating costs. Ever notice how much more energy-efficient a ModCon boiler is at low firing-rates than at high firing-rates ? That energy-efficiency difference is pretty large. That is because the boiler heat-exchanger is oversized for that low firing-rate. Equipment manufacturers have to make compromises. We can find the sweet-spot.

So go with your 3 sq ft flat plate heat exchanger and use 180º F boiler water to heat it if you like. I understand what you are doing, but I do not consider that to be the most-efficient way, or the lowest-cost way to operate an indirect to make DHW.

Doug