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

belfert · August 19, 2010, 06:53:48 AM

I bought an expansion tank and other parts for my hydronic heating system from Sure Marine. Their instructions say to mount the expansion tank higher than anything else in the system. This would mean in my case putting the tank in the passenger area.

How high are you guys mounting the expansion tank? I've seen systems with the expansion tank in the cargo bay and that certainly isn't the highest point in the system. Also, doesn't an Aquahot or similiar have the expansion tank built-in?

Lee Bradley · August 19, 2010, 07:48:53 AM

You should have a spot near your engine expansion tank that you could mount it if you don't want it in the passenger area. I plan on mounting mine in the washer/dryer closet above the W/D.

bevans6 · August 19, 2010, 07:53:29 AM

The idea of an expansion tank is that it is half full of air, which creates a spring that the pressure from the water expansion can act against. Air rises, so you have the tank at the top to capture the air. If your system piping isn't self-bleeding you can have bleed valves to take out the air, or you can route lines from the high spots back to the expansion tank so the air in the piping has a place to go.

I don't know how a low mounted expansion tank would capture and retain the air that lets it work properly but I guess it could be done.

Brian

gumpy · August 19, 2010, 08:03:52 AM

Aquahot has a 15 gallon coolant reservoir with a pressure cap on it. The pressure cap overflow is connected to a reserve tank which is vented to atmosphere. When the coolant heats up, it expands and creates pressure. The expansion of the coolant volume causes it to flow out the pressure cap into the reserve tank. When the coolant cools down and contracts, it sucks the coolant in the reserve back into the aquahot reservoir. If the reserve is sized correctly there should never be any air in the reservoir tank, which promotes rust in the steel components within the system. This is exactly how an automobile cooling system works, and the pressure cap on the aquahot is the same as on an automobile.

FWIW

craig

bevans6 · August 19, 2010, 08:11:08 AM

Craig, wouldn't you call that a different type of system than an expansion tank system? I agree that a closed system with an un-pressurized reservoir tank would work exactly like that, but I thought that the OP was asking about the other kind of system like I described. Kind of like the old car rad where you have to leave an air space at the top, vs the current style where the rad has to be 100% full and the coolant tank handles the overflow...

Brian

Sean · August 19, 2010, 09:05:43 AM

Brian,

It only needs to be higher than the next highest component in the system, but I would also account for a certain amount of slope, say 6-8%.

Our tank is just above floor level, which makes the bottom even with the highest fan coil unit, and the top a good foot above that. The fluid level is somewhere in the middle, maybe 8" above everything.

We installed the tank against the rear wall of the bus, then cut an 8"x8" door through the wall, matching the identical door on the other side of the back wall for the radiator fill. This gives us fill/check access from outside the coach.

HTH,

-Sean
http://OurOdyssey.BlogSpot.com

belfert · August 19, 2010, 09:20:26 AM

I bought an expansion tank from Sure Marine that is 5 gallons. It has an inlet and outlet with a radiator cap on top. It also acts as a buffer tank to keep the system from short cycling. There is a seperate overflow tank that connects just below the radiator cap. Sure Marine told me that basically I have to have at least enough coolant in the tank to cover the inlet and outlet.

It looks like I will be putting this in the passenger area above the engine. This tank is essentially the size of a 5 gallon bucket so it really won't fit in the engine compartment.

David Anderson · August 19, 2010, 04:04:32 PM

Mine is similar to Sean's but maybe only about 4~5" above floor level. It is located in the engine compartment. I did get an air lock one time when I had my front end very high and back end low. That was 7 years ago. I was new and didn't know any better. Now, when I'm parked, I always make sure my rear is level or higher than front if I plan on using the Webasto. While driving, it won't matter because the pitch will change constantly and any air will burp to the top of the tank.

David

gumpy · August 19, 2010, 08:12:30 PM

Quote from: bevans6 on August 19, 2010, 08:11:08 AM
Craig, wouldn't you call that a different type of system than an expansion tank system? I agree that a closed system with an un-pressurized reservoir tank would work exactly like that, but I thought that the OP was asking about the other kind of system like I described. Kind of like the old car rad where you have to leave an air space at the top, vs the current style where the rad has to be 100% full and the coolant tank handles the overflow...

Brian

I was just clarifying how the Aquahot worked, which was in response to the final question of the original post.

bottomacher · August 20, 2010, 05:15:40 AM

If one is using a bladder type expansion tank, why should it matter where it is located? There should not be any air in the system after bleeding- the bladder only allows expansion for the increased volume of heated water, doesn't it?

mikelutestanski · August 20, 2010, 07:12:01 AM

Hello; The tank needs to be higher than the rest of the system. Higher can be 1 inch but higher. In our system it is in the closet in the bathroom. It is a piece of 3 inch copper pipe made into a tank with the pressure cap at the top. It is also the fill line for the system.
I found the hardest part of the installation to be the getting rid of the air in the system. It takes a while. Ours is a suremarine system but we bought it 10 years ago. Webasto 2010.
If you use fan heaters to heat the air in the coach pay attention to the return air for each system. It is a requirement. air in equals air out.... you must provide return air to each heater .
regards and happy bussin
mike

Jeremy · August 20, 2010, 07:31:27 AM

Quote from: bottomacher on August 20, 2010, 05:15:40 AM
If one is using a bladder type expansion tank, why should it matter where it is located? There should not be any air in the system after bleeding- the bladder only allows expansion for the increased volume of heated water, doesn't it?

And for the pressurization of the water system by the pump I think; the larger the tank the more air there is to compress on the other side of the bladder / diaphragm, so the less the pump should cycle - which I gather is the purpose for which the OP bought it. But I am also puzzled about the instruction to mount it high as I cannot think why that would matter with this type of device, which is a sealed vessel with no venting to the outside air. (Unless it also happens to have a bleed valve built into it, rather than having a separate bleed valve somewhere else in the system).

Jeremy

bevans6 · August 20, 2010, 07:45:19 AM

I'm getting the distinct impression there are a number of different types of tank and system... And different requirements for mounting and bleeding air. One thing I do on tricky water systems is build in a valve to allow bleeding air at local high points. Hot water house heating systems used to have a bleed valve at every radiator. My bus has a bleed valve at the driver's heater core.

Brian

Sean · August 20, 2010, 07:46:04 AM

Quote from: Jeremy on August 20, 2010, 07:31:27 AM
And for the pressurization of the water system by the pump I think; the larger the tank the more air there is to compress on the other side of the bladder / diaphragm, so the less the pump should cycle -

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

Sean · August 20, 2010, 07:50:34 AM

Quote from: bevans6 on August 20, 2010, 07:45:19 AM
... Hot water house heating systems used to have a bleed valve at every radiator. ...

The MSR fan-coil heaters that Sure Marine sells for hydronic systems all have individual bleed valves. In addition to our high-mounted expansion tank, which is also the system fill, we also have six bleed valves throughout the system, five of which are on fan heaters. You need to use all of them when filling the system; even so, we managed to burn out two pumps before getting it right.

-Sean
http://OurOdyssey.BlogSpot.com