air in system ...

by **andymort** » Tue Dec 04, 2007 9:39 am

hoping you may be able to help me with a problem on my vented heating system ...

to cut a long story short

i have relocated my hot water cylinder and pump in the attic (to liberate space on the landing ... airing cupboard)

in order to do this I have lifted both the storage tank and the central heating expansion tank about 1.5m in order to provide the necessary head on the hot water cylinder and keep the expansion tank above the pump ... maintaining a positive presuure throughout all the system.

the problem i have is that my system is continuously filling up with air, rads require bleeding and the system is noisy (assume it is the air)

i think i am drawing air in through the vent to the expansion tank when the pump starts ... the length of the vent pipe is less than it was originally and i do get overflow from the expanion tank which seems to coincide with the pump starting

i have overcome the problem by selecting a lower speed on the pump, unfortunately at a lower speed i do not seem to be able to balance the system such that all the rads heat up ... no matter how much i try.

currently in the 22mm vent line there is a simple air separator, vessel

i was wondering if anybody knows of anything on the market which i can put in the vent which will prevent air being drawn in.

i was looking at mechanical air vents/separators and backflow prevention products ...

by **marrtin** » Tue Dec 04, 2007 11:42 pm

This problem can occur wherever the tank is and is due in part to a poorly designed system. Ok, so by moving the tank it has made it worse, but the root cause is the same.

If the heating filling pipe is enters the loop at a different place to the expansion pipe, particularly if they are on either side of the pump, air can be sucked into the system, or in the opposite case, water forced out of the expansion.

Re-position either pipe so there is no opportunity for this pressure differential to cause a problem. Put the expansion on the highest part of the system, and the feed a foot or two lower in the same pipe. Consider when redesigning, that the expansion must allow for steam to escape if the water was ever to boil.

You may find a similar problem when filling a bath say, that air is drawn down the expansion on the DHW side too!

by **andymort** » Wed Dec 05, 2007 11:13 pm

the system is configured as you describe ...

would it be better if the vent pipe was on the discharge side of the pump ?

or would this result in water being pumped through the vent line ... with it being a "circulating pump" i guess anywhere on the circuit is on the discharge/suction side of the pump

by **marrtin** » Sun Dec 09, 2007 5:51 pm

It is best to put the expansion pipe as close to the top of the boiler feed as possible so if the water were to boil, the steam would find an easy way out. Then put the feed pipe close by and lower down if possible, but not more than a foot or two away.

This would guarantee regardless of whether the expansion is under vacuum or pressure, the feed is going to be the same.

See the topic "Central Heating Problem" (9th Dec)

by **andymort** » Thu Jan 10, 2008 6:56 am

think i've done it but not in a conventional way ...

firstly i dropped the vent into the expansion tank, below the water level. This solved the air problem, pump speed increased ... all the rads nice and hot.

but ... water was drawn out of the tank, through the vent, and replenished up from the system, causing it to heat up like a rad

i have now fitted a non return check valve in the vent ... which seems to have done the trick

i'm not a pro so not really sure if this "solution" will introduce any other problems ... ?

comments/advice much appreciated

by **marrtin** » Sat Jan 12, 2008 10:05 am

According to my copy of the Hot and Cold water supply regulations, The system should have an......."open safety vent to permit escape of steam in the event of system overheating (and) should not be fitted with check valve or other obstruction".

Frankly, I think you are on dangerous ground here. You may recall someone was killed last year by an exploding backboiler where steam was trapped and unable to escape. If the one way valve was ever to seize shut................

Really the only way to resolve this properly is to move the feed pipe to a better location.

by **marrtin** » Sun Jan 13, 2008 10:39 pm

You could try raising the height of the expansion pipe so the pump has to lift the water higher before it overflows back into the header tank.

by **dcc** » Mon Jan 14, 2008 8:37 pm

before i start, what you have done to the vent is VERY VERY dangerous! remove the check valve immediately!

height of the tank isnt your problem, its the way its been plumbed up.
the correct way is as follows . . .

(VENT)~~(FEED)~~(PUMP) and its recommended that this set up is within 150mm

with this set up the vent will be in the 'neutral' zone and will not draw in air

by **andymort** » Tue Jan 15, 2008 12:19 pm

thanks guys ... i will put it straight back to normal

would i be right in thinking the risk is associated with the fact that a mechanical valve could sieze up ? (causing the system to pressurise)

anyway ... just so i don't do anything stupid again, would i be correct in saying that i should introduce the feed from the header tank into the system between the vent and the pump suction ?

at the moment the vent is on the pump suction but the header tank feed to the system is where it was originally, under what was the airing cupboard

(wish i knew how to insert a picture)

reasonably straight forward to change but just want to make sure

thanks again guys

am[/img]