Open vented system drawing air in

[gedska]

Hi, need advice on my CH system, it seems to be drawing small amount of air every time it pumps(pump set on 1), sorry about bad drawing:

would changing vent pipe to 1 inch including tank connection help? tank is 4 gallon and the way its plumbed there's les than 1/4 of water in it, also tank is ceiling height (not in the attic), have new tank here thinking of tackling it this weekend

 

[SJB060685]

Is your drawing your proposed suggestion or current set up?

 

[gedska]

Is your drawing your proposed suggestion or current set up?

its current, please note pipe sizes on went pipe 3/4 going up and only 1/2 tank connection, also tank connections are very low so its never more than 1/4 full, tank is 4 gallon

 

[SJB060685]

You're using imperial measurements, are you outside of the UK? not that that matters.
Your current installation is a combined cold feed and vent, quite common, however the position of the circulator is not ideal and is most likely the reason why you're drawing in air. To the left of the vent tee and followed round to the suction inlet of the pump will be under a negative pressure, this can draw air in from loose valves etc and also pull dissolved gases out of solution.
If you were to reposition the pump on the flow after the vent tee you should find your problem goes away.
The cold fill line in a CH header is usually only a couple inches above the tank outlet, this is normal.

 

[exedon2]

Agree re move pump to flow how much high between bottom of tank and top of highest radiator?

 

[gedska]

Agree re move pump to flow how much high between bottom of tank and top of highest radiator?

5feet, i was hoping i wont have to move the pump

 

[gedska]

You're using imperial measurements, are you outside of the UK? not that that matters.
Your current installation is a combined cold feed and vent, quite common, however the position of the circulator is not ideal and is most likely the reason why you're drawing in air. To the left of the vent tee and followed round to the suction inlet of the pump will be under a negative pressure, this can draw air in from loose valves etc and also pull dissolved gases out of solution.
If you were to reposition the pump on the flow after the vent tee you should find your problem goes away.
The cold fill line in a CH header is usually only a couple inches above the tank outlet, this is normal.

but pump would be pulling closer to the vent pipe, would that not mean more suction thru vent pipe?

 

[SJB060685]

but pump would be pulling closer to the vent pipe, would that not mean more suction thru vent pipe?

Not if its a combined cold feed and vent, which you have

 

[gedska]

Not if its a combined cold feed and vent, which you have

what about current setup? maybe its not vent pipe that's a problem, i mean I'm not sure if air getting in thru vent pipe

 

[SJB060685]

As I said above. To the left of the tee where the vent enters the system and through to the left, right to pump suction side is under negative pressure. That section can pull dissolved gases out of solution, as well as pull air into the system.

 

[gedska]

As I said above. To the left of the tee where the vent enters the system and through to the left, right to pump suction side is under negative pressure. That section can pull dissolved gases out of solution, as well as pull air into the system.

would this be correct position for pump?

 

[SJB060685]

would this be correct position for pump?
View attachment 47001

Yes my friend. That should solve everything.

 

[gedska]

Yes my friend. That should solve everything.

thanks, i will try that, and let you know if it sorts out the problem. Possibly not this weekend, as its bigger job than i thought, also just a thought, when i light the stove really heavy with coal, water is boiling in the rads and pipes, could that give me air in a system? and air always settles in same two smallest rads every time

 

[SJB060685]

When water boils it doesn't change chemically, only physically ie into water vapour, not air. But the solubility of gases decreases with increased temperature and lower pressure.
Where are these two small rads, at the highest point?

 

[John.g]

My system is almost exactly like yours (original drawing) with F&E tank in the attic, the attic is now (30 years) a bedroom with the top of the rad only about a foot or so below the tank water level, this rad always runs with a partial vacuum (suction if rad vent opened with circ pump on) but has never caused a problem.
the other difference is that the short cold feed make up is also 3/4" which may make a difference. I don't see any need to increase from 3/4" to 1" but would prefer to see the cold feed changed from 1/2" to 3/4". Are you sure its drawing air in through the vent?, I would fill a small plastic bag with water and tape it on to and with the vent pipe immersed in it then observe it while getting someone to start and stop the pump.

 

[John.g]

thanks, i will try that, and let you know if it sorts out the problem. Possibly not this weekend, as its bigger job than i thought, also just a thought, when i light the stove really heavy with coal, water is boiling in the rads and pipes, could that give me air in a system? and air always settles in same two smallest rads every time

A stove really shouldn't be piped up with a combined cold feed and vent , if it boils then its trying to release "steam" and at the same time feed cold water down the same pipe so while not unsafe IMO as its still a open vented system, is not ideal. My stove is a oiil fired boiler which will "never" boil.