Boiler feed valves/System pressure

[timberbuilt]

If I ever let my boiler burn out or die I loose all the pressure in my system. My pumps and system seem to run best when I have about 20 psi on it, which I usually do when everything is hot. Once the temperature drops I loose all of the pressure (make sense although I didn't expect it to be this drastic) and my pumps tend to vapor lock or something where the water does not circulate. I wasn't aware of boiler feed valves until I started doing a little googling but apparently they will maintain a constant pressure on the system. Is there any advice as to what I should be looking for on these? I would think I would want an adjustable one but is this necessary? What is the rule of thumb or criteria that I should know about system pressure?

On a similar subject I have two zones in my loft that tend to get air in them. It makes sense that air rises but where is is coming from? I've purged the zones several times and everything will be running smooth for a week or two but eventually the zones quit circulating and seem to have air in them. I've got an air eliminator on my expansion tank in the boiler house but maybe I should also have one in the house or at least on these upper zones?

Any advice or suggestions?

Thanks guys.

 

[rkusek]

Sounds like you have a leak somewhere. If your system is 12-15 psi cold then 20 psi when hot it should still be that same 12-15 psi when boiler is cold again. Do you have a bleeder on those upper zones? My basement is probably 14 feet higher than were boiler is located 350' away. I had a hell of a time purging all the air out of that underground pex. Bleeding when the water was really hot helped. I fixed a small leak on my black iron piping last week. I think when the pipe cool is when it leaked. I would have to re-pressure ever week or so until I fixed it. I used Megaloc on those fittings this time instead of dope or teflon tape.

 

[Singed Eyebrows]

I run a boiler feed valve & always have through the years. Never had a situation where there was a leak & it kept feeding water. These aren't expensive & I would recommend an adjustable one. Mine is set for about 8 psi cold. My system runs 12.5 to 13 psi at 180 degrees. I don't use any air eliminators as I have radiator bleeds at the top of the system & my tank traps the rest. This is what works well for me, Randy/ ps, What is your total system gallons & the expansion tank size? Storage/non storage? boiler? etc.

 

[in hot water]

You might just have some rouge air that has not been vented out. I'd suggest an AutoFill valve like the one pictured below. You want a few- 5 psi at the highest point in your system. Generally 12- 15 psi is good for a two story home. Be sure the expansion tank is sized correctly and the pre charge is adjusted to the fill pressure BEFORE you pressurize the system.

If you have inslab radiant, a small pinhole would cause a slow loss of pressure and could go undetected. With pex they may only seep when the tube warms up. Same for a fitting that was left un-crimped or mis crimped.

try adding a fill valve for a few days, if the problem does not go away you could add a small water meter before the fill valve to see how much you are losing. Watch the meter for a few days or week.

Also be sure your pump is installed near, and downstream of the expansion tank location. Pumping into an expansion tank, especially with high head circs can present problems with pressure and air removal.

hr

 

[timberbuilt]

Thanks guys, lots of useful information. I think I am going to order an Autofill valve and install to see what happens. The talk about my expansion tank brings up some interesting points though, which I may have missed during install.

This is what works well for me, Randy/ ps, What is your total system gallons & the expansion tank size? Storage/non storage? boiler? etc.

To answer Randy's questions I would estimate my total system volume around 80 gallons. I have a Wood Gun 140 in an outbuilding approximately 125' from the house, I do not have storage yet. My expansion tank is on the boilerside (outbuilding) and is probably 5 gal. I've got my only air eliminator on the top of the expansion tank, I believe I left the factory charge of 15 psi on the tank.

I did nothing more than plumb the expansion tank into the system during install, should I have filled the system then charged the tank when it was cold to 5-10 psi? When I have no heat the pressure on the system is 0. I haven't tried maintaining a pressure when cold although it always seems that when the fire goes out I loose pressure and vapor lock pumps. The pinhole theory may still be valid although due to the amateur nature of my installation I may need to look at the system configuration and practices as well.

Also be sure your pump is installed near, and downstream of the expansion tank location. Pumping into an expansion tank, especially with high head circs can present problems with pressure and air removal.

I'm not sure where you're going with this but my main circ pump is on return side of manifold in the house so it pumps 125' to the boiler then through the boiler to the expansion tank...

Thanks again guys,

 

[Singed Eyebrows]

Start with the Auto fill as you mention & see what the pressure rise is in your system. As HR says charge the tank first to about 12 psi & then set your Autofill. Everything I have ever read says not to pump into an expansion tank, yet that is how mine is set up. I followed the Atmos diagrams & they want the tank right after the Laddomat on the pressure side. The Lado pump is wimpy & nothing like the high head pump HR mentions, so maybe thats why it works. Randy

 

[in hot water]

These drawings from the next edition of Modern Hydronics, thanks John Siegenthaler This edition is in color which helps see what this drawing is explaining, better.

In both examples the system is filled to 10 psi (static pressure) Notice on the one "pumping away" all the head the pump adds when it starts (green dotted lines)shows in the system as a pressure increase. That's a good thing it keeps the air in solution and moving back to the eliminator, and maintains pressure at the high points in the system.

When you pump towards an expansion tank notice what happens to the pressure throughout the piping system. So if you need to maintain a few lbs pressure on your upper floor, guess what... this is why the pumping away is so important and why air and noise problems, as well as better heat transfer all benefit. Water with air entrained is noisy, but also a poor conductor. Small bubbles become large bubbles, they rise to the high point and you have an air lock. Remember the post about pump cavitation and how they ned some positive pressure to prevent cavitating, and creating air bubbles. Pumping towards the tank increases the potential. Someone posted a pressure chart at various temperatures for a circ recently, something like 7- 10 psi maybe on the pump shown to provide enough NPSH at the circ.

Burp the air out up on the high points, it starts to circulate, and you end up with the re-occuring air problem. A similar thing happens when you tie expansion tanks into the piping at different locations, the system tries to establish a PONPC point of no pressure change, which is where the expansion tank is tied into the system.

Check that tank pre-charge first, they are not always what the label indicates. Too high or too low creates problems, with not enough expansion "room" you could be tripping a relief valve and causing the pressure to drop to 0psi?? Watch the relief valve line as the boiler heats or put a can under it to see if it trips when you turn your back. No sense in adding an autofill if the system keeps dumping pressure and water out the relief.

hr

 

[timberbuilt]

OK, I think I've found a leaking valve, which I fixed and tested today. I let the fire die out and the system maintained pressure (20 psi hot to about 10 psi cold). I think I'm still going to install the autofill since I ordered it. I also ordered a couple of airscoops or actually labeled 1" Air Purger which I was going to install inline on my loft zones to try and eliminate any air on the higher zones. I'm a little confused though, I had thought they were a standalone item but they have what looks like a 1/4" threaded hole on the top and a 1/2" threaded hole on the bottom. What are these for? Must be something else I'm missing.

PS I'm still trying to digest the pumping into and away from the pressure tank...

 

[Singed Eyebrows]

OK, I think I've found a leaking valve, which I fixed and tested today. I let the fire die out and the system maintained pressure (20 psi hot to about 10 psi cold). I think I'm still going to install the autofill since I ordered it. I also ordered a couple of airscoops or actually labeled 1" Air Purger which I was going to install inline on my loft zones to try and eliminate any air on the higher zones. I'm a little confused though, I had thought they were a standalone item but they have what looks like a 1/4" threaded hole on the top and a 1/2" threaded hole on the bottom. What are these for? Must be something else I'm missing.

PS I'm still trying to digest the pumping into and away from the pressure tank...

Bottom hole for expansion tank, top for automatic bleeder, Randy

 

[ewdudley]

OK, I think I've found a leaking valve, which I fixed and tested today. I let the fire die out and the system maintained pressure (20 psi hot to about 10 psi cold). I think I'm still going to install the autofill since I ordered it. I also ordered a couple of airscoops or actually labeled 1" Air Purger which I was going to install inline on my loft zones to try and eliminate any air on the higher zones. I'm a little confused though, I had thought they were a standalone item but they have what looks like a 1/4" threaded hole on the top and a 1/2" threaded hole on the bottom. What are these for? Must be something else I'm missing.

PS I'm still trying to digest the pumping into and away from the pressure tank...

Air scoops are normally for hot water leaving the boiler:

http://www.pexuniverse.com/docs/pdf/taco-air-scoop-catalogs.pdf

For high points in the system coin vents should do the trick. Once air has been bled from the high points it should not re-accumulate continuously unless something is wrong with the system design or some component is malfunctioning, in which case the solution is not more and better air venting but rather prevention of air ingression or water loss in the first place.

--ewd

 

[in hot water]

Hre is an assortment of vents that are used on the piping at manifolds or high points.

Consider that small hydroscopic valve. It can be used as a manual valve to bleed at first start up, then you tighten it down to use it as an automatic bleeder. Typically they mount in a manifold or a fitting with an 1/8" thread. They do make bleeder tee or ell fittings to screw these into.

hr