Hot water heat pressure drops when circulator turns on

DanielC

I have a one-room addition on the 2nd floor of my house that is heated with hot-water baseboard radiators. The water pipe is heated via a coil in my oil-fired steam boiler. When the circulator pump on the hot-water piping turns on, the water pressure in the line drops. Also when the circulator turns on, the water in the radiator burbles and then it becomes quiet.

Here are the numbers:
--Cool, about 100 degrees, 8 psi, circulator off
--Hot, about 215 degrees, 10 psi, circulator off
--Hot, about 213 degrees, 4 psi, circulator on

In the thought that there may be air in the system, I tried bleeding the radiators to get air out. I got a little air out of the last radiator in the series and nothing out of the other radiators.

Are these numbers OK? What do you advise doing?

Thanks in advance.

Jamie Hall

Where is that pressure measured? And where is the expansion tank located?

DanielC

The pressure gauge is on the hot water output pipe. It is closer to the boiler than is the circulator pump. The expansion tank is on the cold water input pipe. It is closer to the boiler than is the pressure relief valve.

Thanks
Daniel

Jamie Hall

There's the reason for the pressure drop. When the circulator turns on, it has to pull water all the way through the boiler from the expansion tank. The pressure at the tank won't change. That's why we like pumping away...

I'm a little concerned that that low a pressure at the inlet to the circulator, with hot water, may cause cavitation of the pump under certain conditions -- which will shorten the life of the pump.

DanielC

How would I increase the pressure?
Thanks,
Daniel

hot_rod

So the boilers on the main floor, one level is above it? If so 8-10 psi cold is fine.

As Jamie mentioned, here is how the relationship of the expansion tank connection into the system changes the pressure dynamics when the pump is running.

When the pump is placed so it pumps away from the tank connection, the additional [pressure that the pump develops is added to the fill pressure.

If the tank is on the outlet side of the pump, the pressure difference is subtracted from the fill pressure.

Removing and keeping air out is easier when you are pumping away.


In this example the system is filled to 10 psi, the pump adds 8 when pumping away.

A pic of the boiler piping or a sketch would help us "see" your system piping.

DanielC

Here's a photo and a sketch of the piping. Thanks for your help.

bob_46

connect the x-tank where the gage is and put the gage where the x-tank is. Buy some new B&G flange gaskets the square o-ring ones and don't over tighten them like it is now. Make the nuts finger tight plus one turn.

EBEBRATT-Ed

Personally I would prefer to have the restriction of the tankless coil on the discharge side of the pump. I would install the circulator between the expansion tank and the coil inlet.

Then everything else could stay the same.

@bob, I think if he swaps the expansion tank and the gage will put the expansion tank on one side of the coil and the city water m u on the other side of the coil. I think that could cause a problem.

Tankless coil has a ton of 1/2" tubing...a lot of restriction

bob_46

@ed I think you are right, that would be better. I was thinking he could make the swap without repiping and benefit from pumping away.

Gordy

Technically that is pumping away no. However not as optimal as having the xtank closer to the circ. We see pumping away with xtank farther than that.

Looking at the circ, and piping I suspect high resistance of a fouled coil......couple it with their developed length.

Solid_Fuel_Man

The idea with pumping away is that the expansion tank is almost directly behind the circulator. And pumping toward (adding pressure) toward the restriction. The restriction in this case is the tank less coil, and then the zone.

Just as with high head HX mod/cons, expansion tank on return then circulator into return of boiler.

Gordy

You mean x tank directly behind (inlet) of the circulator. I knew what you meant though

6 psi drop that's quite a bit of resistance. There are some other ways to accomplish this.

https://heatinghelp.com/systems-help-center/category/steam#hot-water-zone-off-a-steam-boiler

Solid_Fuel_Man

Haha, edited it.

DanielC

Thanks for all your comments. What exactly do pumping away and pumping towards mean?

Thanks

Jamie Hall

The terms are in reference to the point of no pressure change for the overall system -- which is the expansion tank, the whole purpose of which is to hold a reasonably constant pressure.

Gordy

Circulators,in a closed hydronic system move water by pressure differential.

The x tank sets system pressure, and provides room for expansion.

Hot rods PDF is a great example of how pressure diffemtial acts around the hydronic system when the circulator is running.

The short. If the xtank is piped on the discharge side of the circulator the pressure diffential is taken away on the inlet side. If you pipe the x tank on the inlet side the pressure differential is added to the discharge side. So

hot_rod

I agree with Gordy in that it is pumping away and some other high pressure drop is in play.

Gordy

I look at two ways. I see what Taylor is saying. If you take the developed length of the tankless coil into consideration. It theoretically throws the circ out in the middle of the tankless coil, and the emitters with the xtank back at the tankless coil.

However I also look at a primary secondary piping arrangements with the x tank out in the secondary, but with larger piping arrangements.

I still say though that 6 psi drop in differential is large for the coil unless its fouled. Looking at the corrosion at the circ flanges, and tankless piping plus the high temps makes me say hmmm.

EBEBRATT-Ed

Had a relative who had the exact same set up and it wouldn't heat. They had every plumber in town look at it. It was a Taco 007 circ. It couldn't overcome the resistance of the coil. This was 20 years ago

I forget what I put in but I bumped the pump up a couple of sizes. It heated fine then but the noise was awful. I had to take that pump out and drop down one size and it was ok.

Solid_Fuel_Man

I bet the coil has a lage pressure drop. For all we know it was used for DHW at one time. If the circ is placed pumping into that coil I bet it would work adequately.

DanielC

Yes, @Solid_Fuel_Man the coil was indeed used for DHW some 15+ years ago.

Steve Thompson (Taco)

Agree coil pressure drop might be the issue. Could insufficient air charge in the expansion tank be part of the problem? If there was no air in the diaphragm the expansion tank might as not even be there (removing the point of zero pressure change).