How high do you guys mount expansion tanks for hydronic heating?

[Jeremy]

Hydronic pumps run all the time, they do not cycle and are not pressure-actuated like a demand water pump. The reason for the expansion tank is to provide room for the coolant and any entrained gas to expand and to provide additional thermal mass for the system.  In this regard it is similar to an expansion tank on the top of an automotive radiator.

-Sean
http://OurOdyssey.BlogSpot.com

Ok...so when the OP talks about trying to reduce 'short cycling' - if that's not the pump, will he instead be referring to the water heater itself turning on and off too quickly? I can see why that's something you would want to prevent, but I'm not clear on how an expansion tank would prevent it. Does the water heater somehow measure the water pressure?

Jeremy

[belfert]

Ok...so when the OP talks about trying to reduce 'short cycling' - if that's not the pump, will he instead be referring to the water heater itself turning on and off too quickly? I can see why that's something you would want to prevent, but I'm not clear on how an expansion tank would prevent it. Does the water heater somehow measure the water pressure?

I'm talking about short cycling of the boiler/heater itself.  The expansion tank is five gallons.  The expansion tank also acts as a buffer tank by holding 3 or 4 gallons of coolant.  The extra coolant stays warm for a period of time to reduce cycling.

[Sean]

... referring to the water heater itself turning on and off too quickly? ... I'm not clear on how an expansion tank would prevent it. Does the water heater somehow measure the water pressure?

Technically, it's not the expansion tank function, per se, that does this, but rather the thermal reservoir function.

You could get by with a very small tank at the top of the system for expansion purposes.  But with only enough coolant in the system to fill the pipes, the boiler will short-cycle, because it takes less time to heat that amount of coolant, and, of course, less time for it to give that heat back up in the fan units.

For this reason hydronic designers also include a coolant reservoir for "thermal mass".  Four or five gallons is a common figure for a motor coach.  This essentially doubles the amount of coolant in the system, which increases both ON and OFF time for the boiler, but reduces the number of cycles.  As the burner is least efficient when it is first igniting, this improves system efficiency, increases ignitor lifetime, and increases maintenance interval.

The thermal reservoir can be anywhere in the system -- it does not need to be at the highest point, nor does it need to be the expansion tank or the fill point.  In fact, there is a good argument to have it deep inside the coach where any heat it loses is productively used heating the coach.  However, in practice it is cheaper and easier to have a single large tank that fulfills both the thermal reservoir and expansion functions.  Our system has such a single tank, about five gallons, but it is mounted inside the living space for greater thermal efficiency.

-Sean
http://OurOdyssey.BlogSpot.com

[Jeremy]

Got it, thanks.


As it happens my own system must have a huge amount of "thermal mass" as it's based around a large, heavily-insulated calorifier - 50 litre capacity I think, or it may even be 75.

Jeremy

[belfert]

I talked to Sure Marine and they said the 5 gallon expension tank could also serve as the buffer/thermal reservoir tank.  They just said to be sure the inlet and outlet on the tank are always covered with coolant to avoid air entrance into the system.

[gumpy]

I talked to Sure Marine and they said the 5 gallon expension tank could also serve as the buffer/thermal reservoir tank.  They just said to be sure the inlet and outlet on the tank are always covered with coolant to avoid air entrance into the system.

Yes, that is correct. If you fill your tubes with coolant and the level of the reserve is above the level of the inlet and outlets, then no air can get sucked into the tubes, and there is
effectively no head on the pump. Plus, you get maximum corrosion protection by not allowing air into the system.

[Bill B /bus]

Having now installed 6 hydronic heating systems, two of my own, I can say with certainty that the higher the better for filling and venting the system. I mount the Webasto/Proheat unit on a bay floor. The expansion tank is located in a convenient location at least two feet above the floor. That puts some 5 feet of head on the suction side of the circ pump. On the last two installs the initial fill was exactly that. I filled the system. Manually started the circulating pump for system purge. I had to add about a gallon to the tank, five gallon tank, after 5 mins of pump run time the  system was filled and completely purged of air. Remember you have to get access to the tank for addition in case of leakage. Don't bury it.

Bill