How Much Exchange Coil

[ejhills]

Hello All-

I'm considering 450 gallons of Hot Water Storage and am confused about the spec on the heat exchanger.....

Whats the math look like?

Ed

 

[Nofossil]

The math is mind boggling. Most folks go with various rules of thumb. For immersed coils, 100' of 3/4" is a good starting point. I have a bit less and wish I had more. If you can get finned stainless, it's much more effective. Some folks have gone with large amounts of PEX. In general, the more the better. Another approach is external flat plate HX with a circulator. These are very high performance - much more bang for the buck, but requires another circulator and the power to run it.

 

[ejhills]

So it occurs to me that the heat exchanger does not need to be this huge assembly that sits inside of the storage tank. It could be a much smaller unit that resides outside the tank. If that were the case an additional pump would be needed imagine.

Where did you buy your exchange coil for your hot water storage tank?

Ed

 

[Nofossil]

So it occurs to me that the heat exchanger does not need to be this huge assembly that sits inside of the storage tank. It could be a much smaller unit that resides outside the tank. If that were the case an additional pump would be needed imagine.

Where did you buy your exchange coil for your hot water storage tank?

Ed

The flat plate HX would be an external exchanger and would require an additional pump as described.

My indirect DHW tank came with a finned stainless coil inside - that's part of what makes them expensive. My external storage tank has handmade coils using both soft and hard copper - there are photos on my website.

 

[in hot water]

I had a good discussion with the Heat Transfer product folks at the RPA show recently. They claim about 10K btu is about all they can pull from the coil in their dual coil indirect tanks. The problem being the coil resides in still water. Adding a circ pump to the tank greatly increases the heat transfer to the coil, they suggested a factor of 4. But you lose stratification in the tank.

All things considered an external HX, ideally with counter flow piping, will be a much better bang for your bucks.

In addition it is serviceable, able to be changed, upsized if required for additional loads.

Coils in a tank have a place, but external, while requiring an additional circ, will outperform a coil in still water.

Engineers tell me it's fairly hard to model exchange and stratification in a tank like that. I visited a manufacturer of HX tanks recently. They had sensors attached at 2" intervals on the tank exterior with a data logger trying to get a handle on what really goes on. There is some fuzzy math involved in that calculation, from my observation.

I also videoed a bare tank with an infared camera last year to "watch' that stratification taking place in real time, pretty amazing.

External HXers can be easily sized and output plotted almost exactly. I have seen some great buys online for plate type HX. even with the additional circ it may pencil out better for you.

If performance for the $$ is the goal, I'd steer you towards external HX with two pumped flows.

hr

 

[pybyr]

Hot rod - thanks for your great insights and suggestions as usual &

Hot Rod & others - how critical do you think it is to set up the circulators on a plate HX to have counterflow under _all_ circumstances (both heating the storage and pulling heat back out of the storage

thanks

Trevor

 

[in hot water]

If you contact the Hx manufacturers they have software that can quickly model various "what if' conditions. I hate to guess and give you mis-information when then best answers are a mouse click or phone call away.

The Folks at Flat Plate know GEA PHE Systems can and will answer questions like that. (broken link removed) is a place to start.

hr