Expansion Tank and Air Vent Question

J.C.A._3

Great minds( O.K., mediocre and slushed minds at least)think alike.

From the decription....I'm thinking that the boiler side is fine. Do you have a fill and expansion tank on the system side of the radiant zone(s)? And YES...the boiler side WILL eventually waterlog itself without a careful eye and maintainance...

My best advice would be to start thinking about ways to raise the floor level....abandon the black pipe and find a way to make an "above floor" system fit into the space. JMHO. Chris

cattledog

Expansion Tank and Air Vent Question

I have read previous threads about problems of automatic air elimination in hydronic systems with air cushion(non bladder)expansion tanks. I understand how disolved air can be drawn from the system with a loss in pressure or water logging if automatically filled with makeup.

In my system I have an automatic fill valve (open), a sealed expansion tank (without bladder) and an automatic air vent (spirovent), all of which appear to be operating normally without water logging the system. I monitor the expansion tank pressure with a gage and the level with a sight glass.
The tank and gage are about 2 feet above the pump and join, via 1/2 inch line, with the fill line near the pump inlet. The spirovent is several feet farther away upstream from the pump inlet. When I filled the system after some work in the fall, I could hear the air hiss out of the automatic vent, so it was working then.

There are several unusual factors with my system. It is a vintage 1950 in-slab multi loop black iron pipe monster with a small but consistent leak somewhere in the slab. I lose about a gallon a month with about 110 gallons calculated to be in all the in-floor piping when I work through all the loop lengths and pipe sizes. When the auto fill is closed, I loose both tank pressure and water level on the sight glass in a way which is consistent with the air/water volumes in the tank. Its pretty consistent whether or not the heat is running, and there appears to be black magnetite corrosion products in the piping. To minimize the water loss, I run the infloor loops at 2.5 psi gage pressure and it is separated from the boiler loop with a heat exchanger. Circulation is about 30gpm through the spirovent.

Why does my system appear to function OK? I wonder if its the small diffusion path from the tank to the circulation loop? Perhaps is the low absolute pressure in the loop and the pressure drop bubble formation doesn't occur in the automatic separator? Perhaps I'm fooling myself and it will waterlog?

Ken_40

A bit more info please...

regarding the HX? It sounds as if you're running completely isolated sub-system called radiant, via a boiler HX?

I am not sure whether the expansion tank is isolated from the radiant part, the boiler or neither. Could you do a fast schematic showing what's connected to what so we can answer?

cattledog

isolated boiler loop/floor loop

Yes, the boiler and floor loops are isolated from each other through the heat exchanger. The boiler loop (new mod/con boiler) has its own bladder expansion tank and auto fill valve which is typically left closed. The boiler loop runs at 15psi, has a simple auto air bleeder, and is leak free.

I separated the old, floor loops from the boiler, because I knew of the small leak, constant water make up situation, and presence of corrosion products. Because they are separated, and it is a ranch house on a single level, I could run the floor loops at low pressure to minimize the water loss.

Because I wanted to monitor the leak, I used an expansion tank with sight glass on the floor side, rather than a bladder tank.

I don't know why it is working without loss of air. I'm pretty sure the auto air eliminator is working, but the expansion tank is not water logging with the auto fill left open.

cattledog

new floor?

Chris,

Yes I considered going with a new radiant floor above the old slab, but there were several financial, architectural and aesthetic issues which led me to my current solution of minimizing and managing the leak in the existing floor piping. If the small leak turns large, or the floor side of the HX fouls badly, I will have made the wrong choice. Time will tell.