Pressure drops to zero during cycling

Budd

Hi there. This is my first post. I hope its ok. I've got a 600 sq.ft. garage with in floor heat. All insulated and I have it at 63 degrees. works great I heat it with a 4500 watt water heater. Three 200 ft loops. My pressure goes to zero ( or near zero) when it is in cycling mode. I've tried to pump it up but can only get to 15 psi. It stays there until it cycles. Goes to zero and when it stops it goes back to around 15 psi. Any thoughts? Thanks.
 

EdTheHeaterMan

What pump are you using? Is the gauge on the inlet of the pump or the discharge of the pump? Where is the expansion tank in relation to the pump and the heat source?

A picture or a diagram would help

Budd

Here is a pic of the board. My pump is a submersible. Maybe they don't go past 15 psi??? Thanks for the help Edward.

EBEBRATT-Ed

Sounds like an expansion tank issue. Take another picture from farther back. Is the pressure gauge on the pump suction or discharge?

Budd

Its on the suction side. I'm trying to pressurize itwith a 1/4 horse utility pump. Is it the correct one to use?  I was thinking about fabricating a small tank that I could fill with fluid and pressurize to  40 psi  and hook it into the system, open the valve and it will pressurize the system a bit. Sounds crazy right?

EBEBRATT-Ed

We need to see more piping

delta T

Do the pipes coming out of the bottom of the pump/exp tank assembly go to the manifolds for the in floor tubing?

Budd

Ok, more cowbell haha. This is the hot water tank also.

hot_rod

If the pump is flowing down, look for an arrow on the body, then the expansion tank should connect to the red handle valve near the pump. In my mind. 

The exp tank precharge pressure could be set at 12 psi, then fill the system to 12 psi

Budd

They go to the manifolds.

Budd

You can see that I have 16 psi right now.

Budd

Thanks Bob, the tank says its pre charged to 12 psi. It also says working.pressure 75 psi. And I have 14 psi but the gauge could be out.

Budd

Also, the expansion tank is the last thing before the fluid goes into the hot water tank.

hot_rod

16 psi static, drops to0 when the pump runs?
Which pump setting are you on? Try speed 1 see how it changes pressure. As I see it you are pumping at the expansion tank, should be pumping away,
Was the panel built that way?

Budd

Yes it was. Came from the company that way. I'm on an auto setting. I can try the number 1 setting and see what that does.
Thanks again.

Youngplumber

The pressure is different in all parts of the sytem, if it wasn't it would not flow. If your sytem works well the pressure shouldn't matter much to you unless you think it's hurting something. If you choose to record the pressure at the pumps inlet your on the suction side of that circulator. What pressure would you think you should be at, on the suction side? Remember it's "sucking".

(I realize this is over simplified but it's illustrating my point) 

Budd

Yes, I see your point and never thought of that. I may rig up a pressure gauge and put it in different places in the system to see the change when its active versus static. it'll be a good learning experience. It doesnt appear to be hurting anything. Thanks, great forum

hot_rod

yeah, you really do not want the pressure at the pump inlet to drop to 0 psi. All pumps, even small residential have a NPSHr, minimum psi requirement at the inlet. I think something like 1.5 psi at 140F for that pump, no harm in seeing more pressure. Pulling to 0 or below invites cavitation, not a good condition.
Personally I would move the expansion tank, pressures will be better and air removal improved.

Or you could remove the tank, pump it to 20 psi, reinstall and pressurize the system to 20 psi, then you may see 5 psi or more at the gauge when it is running.

Not sure if you are looking for a work around, or the best solution?

Static pressure is the pressure you fill to and observe when the pump is off. Dynamic is the pressures you see when the circulator is running. You'd like to see positive pressure in all parts of the system when it is running. The pump head added should show up in the gauges.

Here is a graphic showing how the expansion tank connection affects the pressure in the system.

EBEBRATT-Ed

You need to repipe the expansion tank to where the ball valve is on the pump suction.

A system with the restriction you have has to pump away from the expansion tank. You do not have that now.

You want the system restriction to be on the discharge side of the pump not the suction side

Youngplumber

Listen to @hotrodand @EBEBRATT-Ed

Budd

Thanks a lot for the posts. Very educational. I will try that setup.