Boiler Expansion Tank placement

Streetwise

I am building a small radiant system that uses a hot water tank as a boiler. It uses two circulators on the output side, to two different zones. I have read that expansion tanks want to be on the suction side of the circulators. My question is if I could mount the expansion tank on the supply side of the boiler (in line with the zone returns). Since the output is split into two zones it would be much easier to do this while still keeping it on the suction side of the circulators. The system will be running at around 100º - 120ºF.

I should mention that I am replacing an older single-pump water tank system that had both the circulator and tank on the supply side that "pushed" water into the boiler. It was an excellent low-cost system that ran for 20 years on the tank.

Thanks.

Dave

EdTheHeaterMan

Place the expansion tank on a micro bubble type air vent on the supply of the boiler. then a Tee after the air scoop/vent that branches off to the circulator pumps. Use isolation valve flanges for the circ pumps.
In this diagram, from the Hydronics course I taught, there are 3 zones with the circ pump after the air separator/expansion tank location. By using a full port ball valve on the supply pipe just before the expansion tank and fill valve assembly (@DanHolohan called this a pumping away module) you could purge all the loops from one hose bib connection at the top of the supply riser. All you need to do to purge the next loop is close the valve of the loop you are purging, then open the valve of the next loop. Big time saver on purging several zones.

Note: In the presentation, this slide was animated to show how to water would flow from the fill valve to the radiators (because the full port ball valve on the supply was closed) then to the 3 closed valves to build up pressure for purging. Once you opened one of the loop valves on the return, the air in that loop would flush out of the loop and into the boiler, from the boiler is would fill with water until all the air left at the hose bib located just before the closed full port ball valve on the supply pipe. Then the first valve on the return would close and the second loop valve would open and the air purge would repeat on that loop. Then finally the second valve on the return would close and the third would open. This would finish the purge operation without moving the hose from loop to loop.

GroundUp

Yes, that's how all properly installed systems are done. The expansion tank is connected between the boiler and circ(s).

EdTheHeaterMan

GroundUp said:

Yes, that's how all properly installed systems are done. The expansion tank is connected between the boiler and circ(s).

There is a lot of science involved in doing it this way. Boyle's Law has to do with temperature, and Henry's Law Has to do with pressure. By locating an air vent at the lowest pressure and the highest pressure area of the system, then all the air that is dissolved in the water will be released and the air vent can evacuate it from the system.

By placing the inlet of the circulator pump in that spot you get both the highest temperature and the lowest pressure in the same place. So much easier to purge baseboard loops that way and the air stays out by design.

hot_rod

Do you have an air purger on the hot out of the tank? You should.

Since the water heater, like a cast boiler, has very little pressure drop, you could mount the tank on the return side, you are still "pumping way". ~Get it as close top the tank as possible to avoid pressure drop in the piping.