Air in Sealed Central Heating System

[AmateurRob]

Hello, This is my first post - and I'm looking for some help on a Central Heating System that's sending me mad!

I've got the following system:

- Sealed CH system in 3 storey house + Cellar (i.e. 4 floors)

- Expansion vessel in Cellar with Pressure Gauge

- Boiler on Ground Floor

- 20 radiators across all floors

Problem is I get air in the (top) 2nd floor radiator and hence a cold radiator eventually. I'm having to bleed it about every 2 days to keep it from going cold. I understand this is air getting into the system somewhere, so I've increased the pressure to 2.0bar to try to identify leaks. I've found a few dodgy radiator tails and so sorted those out. There's no sign of water in the air side of the expansion vessel. Since I have the pressure gauge in the cellar and then 3 floors above that, then I also believe I need a higher intial cold pressure - so I've ended up with about 1.8bar. When warm the pressure only increases to about

1.9bar, so I've assumed the pressure vessel is doing its job. I've thought about adding some leak sealer but was trying to avoid that by fixing the leaks 'properly'.

I can't see how the air is getting in if the system is pressurised (i.e. if I put a needle in a water balloon to make a leak then surely only water would come out - and no air would get in).

So I'm out of ideas and wondered if anyone could help before I got a plumber in.

I've read the (excellent) Sealed CH FAQs at

still none the wiser.

Thanks in advance, Rob.

[Vic Smith]

If it's a steam system, ignore this. Never had steam radiators. I've had a couple hot water systems. You don't say if the system was recently emptied, then refilled If so, when refilling the radiators should be vented from the bottom up as they fill. If this wasn't done, there may be air trapped in them that migrates upward. With 29 psi on the system you would be aware of leaks. From what you've said, the only possibility is the radiators weren't properly vented upon refilling. If not, it's new to me.

--Vic

[Evan]

It sounds a lot more like there were pockets of air in some of the radiators which were never properly bled out that are now migrating on their own to new places in the system...

~~ Evan

[Limp Arbor]

Vic has it right. Any air in the system will be picked up by passing water. The water will then release this air at the point of least pressure. This is usually at the highest/farthest radiator. Rich Trethewey gave a great explanation of this on one episode of TOH. A simple explanation is that water will boil more vigorously in a pot with the lid removed than a pot with a lid on.

You need to bleed every radiator in the system until all air is gone. I usually do this first with the system off then run the pump(s) for a while then shut down and bleed again. In a big system like you have it can take a long time but you will eventually get all of the air out. Then you just have to keep the pressure high enough to prevent the new water from releasing air again.

[Stormin Mormon]

Many boilers have an air eliminator on top of the boiler. Years ago, I saw a demo of a "Spirovent" which I thought was totally fascinating. Perhaps the OP air eliminator isn't working properly.

Christopher A. Young Learn more about Jesus

.

Vic has it right. Any air in the system will be picked up by passing water. The water will then release this air at the point of least pressure. This is usually at the highest/farthest radiator. Rich Trethewey gave a great explanation of this on one episode of TOH. A simple explanation is that water will boil more vigorously in a pot with the lid removed than a pot with a lid on.

You need to bleed every radiator in the system until all air is gone. I usually do this first with the system off then run the pump(s) for a while then shut down and bleed again. In a big system like you have it can take a long time but you will eventually get all of the air out. Then you just have to keep the pressure high enough to prevent the new water from releasing air again.

[Jules Richardson]

It might be a problem with the fill loop or the pressure relief valve - have you checked for leaks there? Your setup might also have an automatic air bleed valve as part of the system, and apparently these can sometimes fail and start letting air in.

cheers

Jules

[Mark]

question....is the system ALWAYS under pressure?...even when idle?

it's hard to understand how new air could enter the system if all of it is under positive pressure all the time?

Mark

[Vic Smith]

You may need a local expert's help if thorough bleeding doesn't work. A pressurized system can't let air in unless it comes from a make-up water supply. Never saw that in a water supply. I've had expansion tanks on top floor and basement, and they were never an issue. They had no "air inlet." Their purpose is to absorb system hydraulic shock. They can get water-bound, but I think that's from years of oxygen dissolving into the water. The one I had in the basement could be isolated and drained. I never got significant water from it when I drained it as a check.

As far as pressure to maintain, that's something that's specced for the system. One of mine had a straight city water connection. The other had a reducing regulator between city water and system. But no gauges at all, so I never knew if it worked. I drained that system a couple times, and had no problem refilling. City water pressure was about 40 psi in both cases. I expect any hot water system can easily handle that. I always shut off the water supply after completely bleeding. No need for make-up water in these systems. And if you did spring a leak, you'd have a continuous one. Once full, the only pressure of meaning is head pressure. And pump capacity as measured by pressure if not naturally circulating. Mine had no gauges, but the circ pump working was evident by a slight flow noise. That circ pump system is still running fine after maybe 50 years.

The other system I had was even older, and entirely natural convection. Both were 2-story houses. These are really simple systems, and I suspect your problem is incomplete bleeding. Could be wrong though. Please let us know how it gets resolved.

--Vic

[Limp Arbor]

Never been to HVAC school but I've heard "10 pounds per floor" is standard. So in a slab house 10 pounds is enough, 20 pounds in a ranch with the boiler in the basement...

Using that 'rule' you would need 40lbs. My mother's house is two floors (10 feet) with a basement and has always been set to 35lbs.

If it were me I would check the specs on the boiler and go just shy (5lbs) of the max when cold. Run it up to temp and make sure the pressure doesn't get too high.

[Stormin Mormon]

Well, at .434 psi per foot of height, that 10 psi per floor might be a bit much.

Christopher A. Young Learn more about Jesus

.

Never been to HVAC school but I've heard "10 pounds per floor" is standard. So in a slab house 10 pounds is enough, 20 pounds in a ranch with the boiler in the basement...

Using that 'rule' you would need 40lbs. My mother's house is two floors (10 feet) with a basement and has always been set to 35lbs.

If it were me I would check the specs on the boiler and go just shy (5lbs) of the max when cold. Run it up to temp and make sure the pressure doesn't get too high.