Vacuum in hot water loop

abellia

Hi,

I have a strange system. It's a single pipe steam system, but at some time in the past someone wanted radiant floor heat, so a loop to a heat exchanger was added. When the radiant system demands heat, a pump in the loop comes on to circulate boiler water through the heat exchanger. In addition, if the water temp in the boiler is low, it will turn on the burner. Both the intake and outflow from this loop are at the bottom of the boiler.

Some mornings when it's cold and I suspect the system has been running a lot, a vacuum gets created in the hot water loop. I can tell because sometimes the pump will make bad noises because it's dry, and if you open the vent spigot that is attached, it will suck air.

Why does this happen and what, if anything, can I do to fix it? And why doesn't the boiler water fill the vacuum?

Daveinscranton

Pump should be rotated 90 degrees for one thing I think.  Not wild about how it appears to be piped.  Water level has to be above the intake at all times.  

Jamie Hall

To be clear, the air lock is occurring in the boiler water loop, and not in the hot water radiant loop?

It is not clear from the phot which way the pump for the boiler water loop is pumping. Is it from the boiler towards the heat exchanger, or the other way? If the Y strainer is installed correctly, it would appear that frow is from a pipe connected to the boiler (at the wet return below the Hartford Loop, maybe? where?), then through the Y strainer, then through the heat exchanger, then to the pump and back to the boiler.somewhere. If that is so, the pressure at the suction side of the pump will be too low, and it will vapour lock if the boiler water is hot. Once it has vapour locked, it can't pump any further water. Which valve is the vent spigot you open to get it going again?

As a short term measure, if this has only recently begun to be a problem, or is getting worse, clean the Y strainer and flush the boiler side of the heat exchanger to reduce the head loss as much as possible. In the long term some rethinking of the piping is in order.

abellia

Yes, the air lock (vacuum) is in the boiler water loop.

I don't know what a Y strainer is. Is this the blue Y on the top pipe going into the heat exchanger? If so, what purpose does it serve? I'm supposed to clean it? My system didn't come with a user guide

it would appear that frow is from a pipe connected to the boiler (at the wet return below the Hartford Loop, maybe? where?), then through the Y strainer, then through the heat exchanger, then to the pump and back to the boiler.somewhere. If that is so, the pressure at the suction side of the pump will be too low, and it will vapour lock if the boiler water is hot.

This sounds correct. I'm adding another picture that may help. It appears that the return from the loop through the heat exchanger is via another pipe into the bottom of the boiler. This has been a problem for as long as I have owned the house. People wince at the piping, but nobody has really wanted to do anything. "When you have to replace the boiler..." is the usual comment.

Sometimes (I suspect when things cool), the problem takes care of itself. The spigot at the top of the heat exchanger was added as a way to deal with what I thought was air in the system (rather than vacuum), but opening it or the spigot that's past the pump toward the return to the boiler seems to help, but I don't really understand what's happening so can't really deal with it thoughtfully at 4AM when the pump starts screaming.

Are there standard ways of piping such things in books that I should try to locate?

thanks,

abellia

I had the pump replaced a couple of years ago and that helped with the noise coming from the pump, but I suspect this just covered up what is an ongoing problem.

Jamie Hall

That Y thing is a filter -- and I would be willing to bet that it is almost completely plugged. It should be serviced and cleaned at least yearly, if not more often. That may be most of the problem right there.

But... the way that's piped, that may be a lot easier said than done, because I don't see any handy dandy valves to isolate that filter from the boiler... and you have to take it apart to clean it... and if you can't isolate, you're going to have a flood.

EBEBRATT-Ed

The pump is cavitating because it does not have enough static head on it. I would remove the Y strainer and reinstall it in the pipe between the pump and the boiler. I would turn the pump so the motor is not straight up and down (it must be horizontal) and I would turn the pump 180 degrees so you are pumping from the boiler and into the heat exchanger.

In addition that pump should be a bronze two piece pump if you want it to last.

Furthermore when you water gets to near 200 degrees while steaming it will flash and cause cavitation. There should be a bypass on the boiler side of the heat exchanger to keep the water at a reasonable temperature.


By the looks of things this has never been right from day 1. How do I know this? Just look at the pictures and count how many capped off copper stubs you see. This thing has been changed and reworked and changed again.

In reality this thing is probably more costly to fix and repair than it's worth.

If your hot water load is large enough I would suggest a seperate how water boiler

abellia

Thanks for all the information. I'd ask more questions, but I'm afraid I'd wear out my welcome.

Can you recommend a way I can find someone competent in such systems locally? From what I'm reading, the people who have looked at the system perhaps don't have the necessary expertise.

Jamie Hall

Check find a contractor or let us know where you are.

abellia

I'm in southern NH. Thanks.

KC_Jones

abellia said:

Thanks for all the information. I'd ask more questions, but I'm afraid I'd wear out my welcome.

Can you recommend a way I can find someone competent in such systems locally? From what I'm reading, the people who have looked at the system perhaps don't have the necessary expertise.

Don't be, we are here to help people as best we can. Ask away.

abellia

OK. Can someone explain how I'm getting a vacuum in the hot water loop on either side of the pump that pulls water through the heat exchanger? If the main vent is working, why wouldn't any vacuum be instantly filled with water from the boiler up to the water line or air from vent in the main steam loop? I think I understand how you'd get a momentary vacuum when the steam condenses, but I don't understand how it could be held, assuming the system is open to the atmosphere by the air vents.

EBEBRATT-Ed

@abellia

I can tell by looking at your pictures that your pumping away from the blue strainer into the heat exchanger through the pump and into the boiler (as long as the pump isn't installed backwards).

I think it would be much better if the pump was turned around. Then you would also have to move the blue strainer.

In my opinion if you measure from the floor to the center of your water gauge glass and then from the floor to the center of the pipe the blue strainer is on and subtract the two measurements the blue strainer pipe is not submerged below the water line enough. When the boiler water is making steam for heat the water is at least 212 degrees. When the pump starts it is cavitating

What I would do is turn the pump around, move the blue strainer and install a bypass loop to reduce the water temperature in the boiler loop.

Buy a copy of Dan Holohans book "How Come" on this site and look on page 157

DanHolohan

I’m with Ed. 

Jamie Hall

abellia said:

OK. Can someone explain how I'm getting a vacuum in the hot water loop on either side of the pump that pulls water through the heat exchanger? If the main vent is working, why wouldn't any vacuum be instantly filled with water from the boiler up to the water line or air from vent in the main steam loop? I think I understand how you'd get a momentary vacuum when the steam condenses, but I don't understand how it could be held, assuming the system is open to the atmosphere by the air vents.

It's very important to realise that the pressure at the inlet to a pump such as a circulator is going to be lower -- perhaps very much lower -- than the pressure even a diameter or two upstream of the inlet. When pumping hot water, you may get serious cavitation even at seemingly adequate pressure, as what happens is that that the water boils at the lower pressure -- and it is that boiling water, at whatever temperature it may be, that causes the trouble.

abellia

Thanks so much. Buying the book

I understand this (I think) -- as the water turns to steam at the pump loop due to the lower pressure, a vaccuum is formed, leading to pump cavitation. The bit that I still don't get is why that vacuum isn't quickly filled with water from the boiler. Why wouldn't the negative pressure suck water in from the boiler?

Jamie Hall

It's not a negative pressure. It's not a vacuum! It's steam at a lower pressure, yes, but it's steam. And it does suck water in from the boiler -- as fast as the pump pushes it out the other side of the pump. But if the local dynamic pressure is below the boiler point of water at that pressure, steam will form and you will have cavitation.

abellia

Well, when I open the spigot on either side of the pump, it sucks in air, so that seems like a vacuum. Steam doesn't come out, which I would expect to happen from your description. I guess I'm still confused.

KC_Jones

abellia said:

Well, when I open the spigot on either side of the pump, it sucks in air, so that seems like a vacuum. Steam doesn't come out, which I would expect to happen from your description. I guess I'm still confused.

The hot water loop is like a straw in a glass of water. You put your finger over the straw and lift it up, water gets pushed up into the straw. Now, if you slowly move your finger, you will get a vacuum as the water is trying to pull air in as it goes down. This is why it's critical for those hot water loops to be sealed as tight as possible, one tiny leak and air will get in and you will lose that head of water.

The hot water loops are capable of going up about 3 stories and still having normal water level on the steam side. Again, it must be perfectly tight.

The spigot you installed and then open is like your finger on the straw and could be causing you to lose the prime every time you open it, possibly making things worse.

Here is a great article on the subject:

https://heatinghelp.com/systems-help-center/how-to-run-a-hot-water-zone-off-a-steam-boiler/

"It pays to wander off the wall", right @DanHolohan ?

abellia

I think we're talking about different things. Perhaps my description was bad. The low pressure isn't in the hot water loop. I have no issue at all with the hot water loop. It's in the loop that runs from the boiler through the heat exchanger to the pump and back to the boiler.

Jamie Hall

abellia said:

Well, when I open the spigot on either side of the pump, it sucks in air, so that seems like a vacuum. Steam doesn't come out, which I would expect to happen from your description. I guess I'm still confused.

The low temperature steam will condense immediately when the pressure in the system rises or the pump turns off. Water from the boiler may not be able to get there as fast as air from wherever it's being sucked in; it just depends on configuration. I probably shouldn't have used the term "steam"; perhaps water vapour would be better, but the gas in question is 100% water, no air.

abellia

Thank you.

KC_Jones

abellia said:

I think we're talking about different things. Perhaps my description was bad. The low pressure isn't in the hot water loop. I have no issue at all with the hot water loop. It's in the loop that runs from the boiler through the heat exchanger to the pump and back to the boiler.

The line from the boiler to the heat exchanger is still a hot water loop off the boiler, it's not steam, hence the pump for moving water.

abellia

I was trying to understand why I might have below ambient pressure in this loop I've tried to describe. This was the case today when the boiler was essentially cold. Just to see what would happen, I removed the main vent for the steam loop and it sucked in air and water filled the low spots in the piping off the boiler.

Am I correct in thinking that the main vent should open all the way to let the main steam loop fill with air at room pressure when things cool and the steam condenses? The vent is a Ventrite 75.

Thanks,

Jamie Hall

Let's go back a step. I am now hopelessly confused. This problem -- low pressure -- is occurring in a loop of pipe which as I understand it starts with a tapping on the boiler itself, then goes through a pipe to one side of a heat exchanger, then through a pump and back to the boiler -- I hope with a connection to a wet return.

Is that correct?

If so, what are the relative elevations of the various parts of this arrangement? Critically, are any parts of it -- anywhere along its length -- above the water line of the boiler?

On the other hand, your most recent post describes a steam loop with a steam vent on it. Although it isn't common, is this a system where the input side of the heat exchanger is steam? And you have a steam feed from a steam main which goes to the input of the heat exchanger and then the condensate from the heat exchanger is pumped back to the boiler? This is not a common arrangement, but it is possible.

And, if that is the arrangement, again what are the relative elevations of the various parts of the system in relation to the boiler water line?

abellia

All of the portions of the "hot water loop" on the boiler-side of the heat exchanger are below the boiler water line, but it is connected to the condensate return/hartford loop at the low point of the boiler.

The "steam loop" (maybe steam main is the proper term?) I'm referring to runs all the way around the basement just below the level of the floor joists. The main vent is tapped into that piping as far away from the boiler as possible on a 6" stub. Is that clear? Should I take more pics?

Jamie Hall

What does the steam main have to do with the hot water loop? If it is the steam loop -- which is called a steam main -- to which you are referring, yes, the main vent should open shortly after the boiler stops to let air in. If it doesn't, it may be not operating, which would also affect your heating in the rest of the steam system.

The steam main should have nothing to do with the hot water system -- or the other way around.