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Active Contributor

Join Date: Sep 2007
Location: Saint-Louis-De-Blandford, Qc.
Posts: 10

How to purge hot water heating system?

02/06/2008 10:48 PM

Air keeps building up into my hot water heating system even if 3 automatic vents were installed, expension tank, and safety valves replaced.

The heat is produced by a combo : mazout furnace/wood burner ,heating water and circulated by electric pump throughout the house.

Two companies came to fix it with no result but a 1000$ bill.

Can someone HELP ME !

overhead Mario

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Join Date: Dec 2005
Posts: 96
Good Answers: 6
#1

Re: how to purge hot water heating system

02/06/2008 11:37 PM

If the pump seal is leaking you may suck air in the system. Also when you first fill the system. It's dissolved in solution, and cold water holds a lot more air that hot water. When you heat the water, the air comes out of solution. If you vent the air, the system pressure will drop. You'll have to add more cold water to bring the pressure back up to it's normal level, and when you do, you'll be letting even more air into the system. you might want to install a Bell & Gossett's IAS air separator which has no moving parts. IAS stands for Inline Air Separator. It has two chambers, and it's a bit wider than the pipe it serves. They separated the two chambers with an orifice, and therein lies the secret to the IAS's great performance. Remember, automatic air vents installed at the high points of a system can't effectively remove entrained air bubbles from the high-speed flow we see in modern hydronic systems. You have to snatch those bubbles out of the flow. And that's exactly what the IAS does

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#8
In reply to #1

Re: how to purge hot water heating system

01/01/2009 3:19 PM

I'm going to look into the IAS unit for my system. My original system was fairly easy to purge (see caveat below), but they made a design change when I got a new furnace and I've had problems ever since. My house has 3 zones, each with it's own thermostat and thermostat-controlled valve near the furnace. The original system had a separate circulating pump for each loop, but the new system uses one pump (single speed) to handle it all. I suspect that the flow is too high when only one or two thermostats are calling for heat. Even on my original system, I rarely could get any air from the valves at the radiators (even when there was obvious air in the system) but the so-called "auto vents" located near the pump did a pretty good job. Now, those vents don't release much air, even though there's quite a bit of air in the system. I suspect that there is a leak somewhere to cause this air in the first place, but it sure would be nice to be able to remove it for now. Finding the leak will be real challenge; other than the radiator's themselves, much of the copper piping is buried under concrete slab. Any thoughts on a good way to find a leak, other than a pump seal leak?

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Active Contributor

Join Date: Jan 2008
Posts: 23
Good Answers: 1
#2

Re: how to purge hot water heating system

02/06/2008 11:47 PM

My guess is you are creating suction with your pump and there is a small hole in the plumbing that is acting like a valve and only leaking in air. This can happen on seals on some pumps as they are often designed to creat a seal due to the internal water pressure.. I'd look there first. Your pump maybe too large for your supply so it is creating the suction.

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Join Date: Jul 2005
Location: Stoke-on-Trent, UK
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#3

Re: how to purge hot water heating system

02/07/2008 6:25 AM

Are you sure it's air? When rads need venting it's usually build-up of hydrogen due to reaction between water and steel rads (eventually eating through the rad of course). You can check by venting and trying to light the gas. If it burns you need to add corrosion inhibitor, there's plenty on the market.

If not, worth checking that the plumbing is arranged to keep pressure as high as possible everywhere, to avoid any spots where it dips below atmosphere, with risk of inward leaks. Feed pipe from the expansion tank best to enter the circuit close to the pump suction. So you want, in flow direction, boiler - open vent - cold feed - pump. Years ago it was common to have the cold feed on the boiler return, but as well as giving lower pressure at the pump suction (due to boiler headloss) it causes "see-sawing" in the vent and feed pipes when the pump starts and stops, drawing aerated water into the system and adding to corrosion.

Cheers.....Codey

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#6
In reply to #3

Re: how to purge hot water heating system

02/08/2008 10:12 AM

Yours was the best post I have seen so far reading from the top. I rated it so!!

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Commentator

Join Date: Jan 2008
Location: Illinois
Posts: 70
Good Answers: 2
#4

Re: How to purge hot water heating system?

02/07/2008 10:56 PM

Mario,

I had this happen to one of my Water Treatment customers. I checked the temperature on the make up water line. It was 53 degrees. I told him that he had a leak. He said impossible. Six months later he discovered a leak in the system in a crawl space. He fixed the leak and the problems stopped.

1. Do you have chemical treatment in the system, and if so does the chemical residual keep dropping?

2. What is the temperature of the make up line? It should be at room temperature if you are not making up water to it. If it is cold (or at city water temperature), then you are adding water.

If one or both of the above is occurring, then the problem is caused by the release of gas from the make up water as you heat it. The air fills the Expansion Tank. Hot water holds less dissolved air than cold makeup water. Usually the closed systems are also at a lower pressure than make up water as well which causes an additional release of air. Someone else has already stated that dissolved air could be the cause. I concur, but think it is highly likely that the water is coming in to replace the water lost by a leak.

Dick Hourigan

www.RichardHouriganInc.com

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Power-User

Join Date: Dec 2005
Posts: 125
#5

Re: How to purge hot water heating system?

02/08/2008 1:42 AM

I would suggest a couple of things. 1) No mention is made of the static pressure you are carrying in your system. If the st. press. is not high enough, the vents at your high points may allow air to be introduced into your system. You should carry a few pounds extra pressure above the high point of your system. 2) A bladder type expansion tank will keep the air and water separated, whereas the "old" conventional expansion tank allowed the air to intermix with the water. Even with an airtrol (tank mounted air separator), the water would still absorb the air and eventually the tank would become water logged..........and you still would have air in the system.

Install a compound pressure gauge on the suction side of the pump to determine the suction pressure when the pump is running. If you are pulling a vaccumn (below 0 on the guage) you might consider adding static pressure to the system to prevent pulling a vaccumn. Static pressure is set by the pressure regulator in the make-up water line which is connected to the system usually close to the pump suction.

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#7

Re: How to purge hot water heating system?

02/10/2008 3:47 AM

There is another possibility. If your system uses plastic piping, poly butelene, it will allow oxygen to pass into the water through the pipe, but the water in the plastic pipe will not leak out.

The new hydronic heating systems use pex plastic piping.

Cheers :-) Gary

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Participant

Join Date: Mar 2011
Posts: 1
#9

Re: How to purge hot water heating system?

03/19/2011 12:57 AM

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