Calling All Plumbers

[Transit]

I have a problem with my base board heating system that has driven me nuts for years. During the heating season I have to bleed air out of the system three or four times or the banging and grueling will have me go Postal. The system is a closed loop, boiler > pipes > radiators > pipes > circulation pump > boiler. I have pressure checked the system several times, no leaks. At the highest point in the system there is positive pressure above atmospheric, hence no way air can leak in. So where is the air coming from?

My thinking has evolved to the point that air is being drawn in at the suction side of the pump. The water is not in motion, weighing maybe 100 -150 pounds. When the circulator pump starts, the water is at rest and its at-rest inertia will not allow it to accelerate instantly causing the suction side of the pump to briefly fall below atmospheric pressure.
For this brief time, a wee amount of air is drawn in around the pump drive shaft seals and eventually gets larger. Now you would expect that the expansion tank would compensate for the pressure drop, but the expansion tank is on the output side of the boiler under pressure. I would think that a second tank on the input/suction side of the water pump would compensate, acting as a shock absorber for the sudden pressure drop in the suction line? Or only one expansion tank is needed at the suction side of the pump?

Every hydronic heating system I have seen has the expansion tank on the output side of the boiler, why?

 

[Mace Canute]

Have a read through this. It might answer some of your questions.

 

[tglass]

I watched this on one of the home fixit tv programs and how it was repaired. An fairly easy repair if i remember correctly, had 4 different people on the show that explained alot of small repairs, but darned, I can't think of the name of it. Maybe someone else on here saw it too. Ended up not really being air in the system from what I remember.

 

[Ken_CT]

Nice article Mace. The expansion tank location isn't important, but what makes a big difference in air in the system & air bound systems is where the circulator is located. Most older systems (and too many new) have the pump on the return line, sucking water through the radiators. It should be on the supply line, pushing the water through. Why was this done? A long time ago the pump was placed there because the pump was kept out of the hottest water and the seals would last longer. It stayed that way for a long time because "that's the way we always did it", and it was easier to package the boiler for shipping. A great book on this subject is Pumping Away, by Dan Holohan. It should be a required read for everyone in the trade. It's better to know why something works, than how to put it together. If you put a pressure gauge upstream from the pump, you won't see a drop in pressure when it starts, so you are probably not sucking in air anywhere. As Mace's article implies,the air will probably clear up after a couple of bleedings. If your system uses a purge setup to get the air out, you are adding fresh water to the system when it is purged, bringing in more air, adding to the problem.
These things can be a real pain to clear up. I have been away from the trade for many years, there may be additives out there that can help you.

 

[crazyal]

Have you thought about adding auto bleaders on each radiator? I know they are cheap and work for years.

 

[Chilly807]

If it IS air in the system, it may be caused by cavitation at the pump impeller. Essentially what happens is the dissolved oxygen in the water comes out of solution as it passes by the end of the impeller blades. Same thing with submarine props turning at high speed in shallow depth (low pressure), air bubbles are formed at the trailing edge of the blade because the water pressure isn't high enough to keep them in solution.

You may want to install a de-aerating valve, called a Spirovent microbubble resorber

Mine is installed at the top of the system, just after the pressure reducing valve and before the expansion tank as in the second pic.

Sean

 

[E/S]

There is probably a very simple fix for this. If your system pressure is 10 psi or more below the relief valve setting, turn the regulator for the make up water up by 2 psi.

E/S

 

[Transit]

Reading through all the replies, I have done 99% of what has been suggested. That 1% undone is to install an inline air-water purge valve.

 

[mjncad]

If it IS air in the system, it may be caused by cavitation at the pump impeller. Essentially what happens is the dissolved oxygen in the water comes out of solution as it passes by the end of the impeller blades.

Sean

Another possible solution is a vortex breaker installed on the pump inlet side if one is available for the OP's pump. It's essentially an "X" in the line to break up the swirling motion.

 

[kidr]

This is a common problem because in fin type baseboards there is no space for air to accumulate and be vented in one shot. You have a tube with slugs of air every few feet. You bleed one slug, but ten ,feet down the line there's another and another etc. I installed these systems by running continueous loops .In one rad out the rad and into another.I put shut offs and boiler drains at each end of the loop. Now I could shut the valves, connect a hose from the house water supply to one drain, connect the other boiler drain to the sewer or run it outside, and flush ,purge the system. Zero problems this way. If your system has a main feed and branches off the main ,you will never solve this problem. Air valves on the baseboard will do nothing but frustrate you. An auto vent at the boiler will help but not solve this problem