Indirect hot water heater with gas steam boiler?

Montykitty

I am converting from oil to gas heat. (steam) in my house.  I have received conflcting advice about whether it is advisable to install an indirect hot water heater.  Some contractors say no - that while the system will work initially, after 2-5 years the coils in the indirect tank will become plugged with sediment and cause failure.  Others say that this is not an issue.  Comments/opinions?

Charlie from wmass

If done correctly

The indirect will work out just fine. It tends to be the pump not the coil that fails. If you are quoted any wet rotor pump it is the wrong pump. If the pump is not bronze it is the wrong pump.

Steamhead

You have to use the right indirect too

the tank-in-tank type doesn't work well with steam boilers. You need one with a coil-type heat exchanger, and the coil must not be higher than the boiler's lowest possible waterline.



We use SuperStor indirects for steam boilers. Here's a pic of one, hooked up to a gas-fired Smith G-8. Note how Gordo, my partner, supported the circulator.

Jean-David Beyer

tank-in-tank type doesn't work well with steam boilers

Could you explain why a tank-in-tank does not work well with a steam boiler? I know you would not be putting steam in the outer tank. Is the problem that the top of the outer tank could be above the lowest possible working of water in the boiler? Or is it some other reason?

Steamhead

That's right

when running a hot-water loop off a steam boiler, it helps if the loop is completely below the boiler's waterline. This allows the weight of the water to create a bit of static pressure on the loop (or indirect coil) which helps keep the water from flashing to steam. Ordinarily we pipe a bypass in the loop to mix some returning water with the water from the boiler, and we Pump Away too, both of which also help prevent this flashing, but we want to design the possibility out as much as we can.



It is possible to run a hot-water baseboard loop above a steam boiler, but such a loop can be purged of air easily with the pump (if you're Pumping Away that is) so an air vent on the loop is not needed or used. Air vents can leak air back into this type of loop, so we don't want to use them.



The tank-in-tank heaters cannot be purged with the pump as coil-type heaters can, therefore they must be vented at the top to get rid of the air. You would have to seriously overfill the steam boiler to get the water up high enough to fill the outer tank, and the air vent could then leak air back into the outer tank. I once called Triangle Tube to ask about this, since we like their Smart tank-in-tank indirects, and these were their reasons not to use these units with steam boilers.



Not all coil-type indirects will perform as well as the TT Smart units, but the SuperStor units will, so that's why we use them with our steam boilers.



The SuperStor coil-type indirect is your best choice.