Heat transfer in an indirect water heater

Dave_NHDIY · March 5

Hi All, I currently have a combi and am planning to add an indirect water heater where the DHW is supplied via the coil with the large volume of water being heated by the FHW side of the boiler. I am planning to discontinue the use of the DHW portion of the combi. The large source is going to be used as a buffer tank for my radiant system. My question is not about this architecture as that's a whole other discussion.

My question is how to calculate the ability to transfer heat from the large water volume to the coil water?

The tank I have is a Lochinvar Squire 50gal. It's specs say at 180F boiler water and with 58F DW in it can provide 210gal of DHW continuous out at 135F.

Is there a relatively easy way to know how many gallons it can be provide out of the coil at 115F with the large volume at 135F?

mattmia2 · March 5

you should be asking how much power it can transfer and you seem to be asking how much energy has been transferred. unfortunately calculating the power transfer of a heat exchanger isn't simple and irt is kind of hit or miss if the manufacturer of the indirect even gives you the specifications needed to do the calculation.

you would need to know the hot side and cold side temps and something like the transfer rate of the hx and you would get power in usually btu/hr or watts in the rest of the world.

the first hour or gallons per hour rating that the manufacturer gives you is not the operating conditions of your system, it is at a specific boiler size and incoming water temp, it isn't a super useful number other than for comparing 2 tanks.

maybe @Larry Weingarten has a method to calculate wha you need (also what are you trying to calculate exactly)?

EdTheHeaterMan · March 5

Less than 2 GPM, and it may not actually work well.

To get 115° outlet temperature from a heat-transfer coil submerged in a 135° tank of water, that tank temperature cannot be allowed to drop much below 135° or the heat transfer will fall off quickly.

With only about a 20° temperature difference between the tank water and the outlet water, the coil will struggle to transfer enough heat.

To reliably produce 115° water with 58° inlet water, you would be better off keeping the tank temperature higher — around 145° or more — and using a mixing valve to temper the domestic hot water.

Dave_NHDIY · March 5

Thanks for both your comments. I obviously was worried that heat transfer would likely not be adequate. My plans for my system are described on this other thread https://forum.heatinghelp.com/discussion/203641/using-an-indirect-water-heater-as-a-buffer-tank#latest

So it's seems my possible solutions are

Using a higher tank temperature maybe 145F with some loss of efficiency but guaranteed unlimited hot water. I'm thinking I might have to go hotter than 145F to avoid short cycling, but maybe I can ignore this as short cycling due to the DHW side of a combi seems a given.
Continue to use the DHW side of the boiler and solve DHW capacity and temperature fluctuation issues using maybe a small seperate tank. A benefit would be on the FHW side the big tank could follow the outside with an ODR .

I'm thinking option 1 is probably my simplest solution. If you have any strong reservations let me know so I can factor them into my thinking.

mattmia2 · March 5

you could supplement the coil in the tank or just use an electric water heater as a storage tank and use a brazed plate hx designed for the close approach temp you want. i don't know how your system is planned so i don't know if you need another circulator for circulation through the hx. you could possibly use the hx in the combi for a similar purpose to heat the water in the tank.

if you are storing boiler water in the closed system in the tank a standard steel electric water heater tank should last decades.

hot_rod · March 5

If you are talking about a reverse indirect where the tank is boiler water and the coil is the DHW?

It takes a massive amount of coil surface area for that to work. Even with all this copper in the TurboMax they still need 180 or hotter to perform

Dave_NHDIY · March 5

Hi Bob @hot_rod so I'm assuming heat transfer specs are symmetrical meaning with my tank at 180F it would give me 135F at 210GPM through the coil. Is my assumption wrong about it being symmetrical? Do you feel 145F tank water would not given me enough DHW at 115F?

mattmia2 · March 5

energy transfer is measured in power, not temperature.

Dave_NHDIY · March 5

I see where a have used GPM in my comments I meant GPH

mattmia2 · March 5

it would be in btu/hr or watts.

hot_rod · March 5

An indirect WH tank is basically a tube and shell, or AKA shell and coil, heat exchanger. To get efficient, or quick recovery requires a lot of surface area exposed to the transfer fluid. Also having two moving flows greatly increases the heat transfer. That is why flat plat HX are generally 5 times better heat transfer. Lots of surface and two counterflow fluid streams. Everything you want to know about HXers here, and plenty of math to make your eyes water.

https://www.caleffi.com/sites/default/files/media/external-file/Idronics_29_NA_Heat%20exchangers%20in%20hydronic%20and%20plumbing%20systems.pdf

Screenshot 2026-03-05 at 11.24.13 AM.png

RascalOrnery · March 5

So are you fellows saying that the best way to get the most heat exchange out of a lower temperature in the boiler is to use a plate exchanger and a storage tank instead of an indirect containing its own coil / heat exchanger?

mattmia2 · March 5

if you want a low approach temp (low difference in temp between the hot side and the cold side of the hx), yes.

EdTheHeaterMan · March 6

https://forum.heatinghelp.com/discussion/comment/1890592#Comment_1890592

You are not the first person to want to do this. Your approach is novel, but you do not need to reinvent the wheel here. There are other tried-and-tested ways to get what you are looking for.

To be clear, you have a combi boiler that is wall-hung and uses gas, operating at above 90% efficiency as a result of condensing the flue gas and keeping exhaust temperatures below 200°F. Is that correct?

What is the brand and model number of your combi?

And you want to:

Use the low-temperature return water from the radiant system during the call for heat to maximize the efficiency of the modulating gas burner input.
You have a Lochinvar indirect and were hoping to use this to facilitate the project with that indirect water heater.
Add additional closed-system volume with a buffer tank. Hopefully, the Lochinvar indirect can serve as that buffer tank.
I believe you are calling the space-heating heat emitters the “radiant system,” but I would like to clarify that this radiant system is in your floor: PEX tubing of some type, whether staple-up, embedded within a gypcrete or a concrete pour, or some other design of radiant floor heating.

Does that sum it up?

hot_rod · March 6

https://forum.heatinghelp.com/discussion/comment/1890678#Comment_1890678

you can do a reasonable job creating dhw with a coil type indirect. Mainly because you gave cold incoming water into the tank and a high temperature 180f or more in the coil. So you have a large delta t working for you

If you want 120f from a coil in a tank of 145f water you will need a large transfer surface, better yet with two flows moving. That is not what a typical indirect is designed to do

The best way to use the tank as a buffer is to let the boiler heat it to the desired radiant temperature on ODR

Use the combi function for dhw as it was designed

EdTheHeaterMan · March 6

https://forum.heatinghelp.com/discussion/comment/1890678#Comment_1890678

Reinventing the wheel is something I try to avoid. An indirect water heater is made for heating DHW from 58° to something above 115°. That is how you are supposed to use it. If you want to do something else then look for the proper pieces to make that happen.

(says the guy that took a center flue 40 gallon hot water tank from a gas automatic and put it on the top of a small metal trash can with an oil burner combustion chamber on it. Slapped a Riello oil burner on the side of the trash can and connected it to my above ground pool filter pump. no limit, no relief valve, no idea of what GPH to fire the burner) 🤣

mattmia2 · March 6

seems like you could have just grabbed a used but good boiler you tore out of somewhere

Heat transfer in an indirect water heater

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