Automatic air vents - central heating...

[Adrian]

HI All

I've not used float-type auto air vents before - so I may be doing something wrong - expert advice welcomed !

Regular readers may recall the saga of my heatpump system - which was causing the CH header tank to regularly overflow. This was finally sorted by bleeding trapped air out of the heat store.

Systems looks like a conventional CH boiler - except that the 'boiler' is a heatpump, and the return from radiator and DHW circuits passes through a 200 litre, heavily insulated copper 'heat-store' before it goes back to the heatpump. When the heat pump switches off the system circulates water from the store through the DHW / CH until it's cooled sufficiently.

Anyway....

The whole thing's piped up in copper / pex - the circulating pump has a little brass float-type auto air vent on the highest point of the circuit - but the feed to the top of the heat store had no such vent.

The only way to bleed the air from the heat store was to slacken off the compression fitting at the top of the store and wait while the air finds its way out - necessitates leaning over the store and applying lateral pressure to the pex pipe to slightly unseat the pipe - a slow and uncomfortable process!

So - I bought another auto air vent and fitted it as close as possible to the feed into the heat store - confidently expecting it to vent air .. automatically ....

Trouble is - it doesn't It's still necessary to do the 'lean over the heat store for a hour at a time to let the air out'....

So - what don;t I understand about using these auto air vents ? It's fitted pretty much at the high point of the circuit - certainly on a level with the union into the heat store. Fitted into a 3/4" x 3/4" x female threaded 'tee' - with the air valve fitted vertical and into the threaded part of the tee...

Any ideas please ??

TIA Adrian

[Doctor Drivel]

Questions: You have a thermal store/heat bank heated via a heat pump. It serves CH and DHW via a plate heat exchanger. Is that so?

Is it a pressurised thermal store? Or is it open vented - uses an F&E tank to top it up?

[Adrian]

Not quite.

The heat store is simply a large tank containing water that's circulated through the radiators / dhw tank. The circulating water is heated by a heat pump (ground source)

No - not pressurised - it's part of the open f&e-tank-fed heating circuit.

Adrian

>

[Spamlet]

Perhaps the air is getting into the tank (ie, as bubbles) faster than it can go up the vent tube. Only when the water stops circulating will the bubbles rise to the top. So really you need the vent in the high point of the tank itself. You can also put it on an extended tube to maximise the volume of air in the tube instead of in the tank: but obviously you can't go higher than the header tank, or the float won't float.

We have a float bleed on our ch and it can be tricky to set it up so that the float doesn't get stuck. Ours has a tyre valve like nipple at the top which you can depress from the outside to jiggle the float and bleed the air manually from time to time to make sure it is topped right up. It's very like an old carburettor float chamber; that one used to have to tickle to get it to fill right up.

S

[Adrian]

Hmm - possibly

That would be ideal - but it seems that the manufacturers of the tank didn't think of that

I'd thought of that - maybe fabricate a nice little 'J-shaped' piece of copper tube that could connect to the tank inlet, then have a 'tee' so that the valve can sit a few inches above the tank inlet - and the return from the heating can come in the short leg of the tee...

Ah - that takes me back - BSA 350 single - a right pig to start!

Both of these valves have a removable cap on the top - but the odd ting is that when you remove the cap from the one that's on the heat store and slide a piece of tin wire down to depress the float there's only a tiny amount of air that escapes - while loosening the union on the tank itself allows air out, and you can hear the water trickling inside the tank.

When I got it sorted the last time (before I drained everything down to add a proper drain c*ck and this air-valve) I managed to get it to the point where the circulating water was silent in the tank - but it took a couple of hours with the union slackened off and 'jiggling' the pex pipe to get the air to come out - I was hoping to 'automate' the process!

Before anybody asks - yes - the air is coming _out_ rather than 'in'

- soap solution blows bubbles when wiped around the loosened union.

Thanks Adrian

[Doctor Drivel]

Adrian, where is the thermal store vented from, on the store and where is the cold feed connected to it?

[Adrian]

The easiest way for me to describe it is....

Imagine a conventional, non-pressurised, boiler system. Replace the boiler with the Heatpump. On the return circuit from the radiators / hot-water tank to the 'boiler' (and immediate 'before' the boiler), insert a big water tank - with the return from the heating circuit piped to the top of the tank and the return to the boiler piped from the bottom of the tank.

No separate vents or feeds connected to the store.

Heatpump and store are at ground-level, f&e tank is in the loft...

Hope that makes it clearer?

Adrian

[dennis]

The float vent I have in my hand has a screw on the top that appears to close the vent even if the float is not floating. I believe it is really for fast filling a pressurised system and not actually an auto vent to release trapped air. It looks like you open it to let the air out while filling and then close it to ensure air can't get drawn in by the circulating water. Maybe yours is the same and you have it closed?

[Doctor Drivel]

As clear as mud. Firstly, is this right? "the return from the heating circuit piped to the top of the tank and the return to the boiler piped from the bottom of the tank".

The return from the DHW cylinder and CH to the "top" of the thermal store? Is that right?

Where is the CH flow connected to the system? Where is the heat pump's flow connected to the system?

The vent pipe from the F&E tank. Where is it connected to the system? The cold feed pipe from the F&E tank. Where is it connected to the system?

[Adrian]

Absolutely right.

Same as above...

Flow goes from the heatpump, through the rads or dhw cylinder coil, then back as the return.....

See above

Top end of the dhw cylinder coil...

Same place as the vent pipe.

Look - I'm clearly not explaining this well - I'll see if I can find time to sketch the system tomorrow or early next week.

The original question was 'is there something that I don't know about using automatic air vents on circulating water systems?' - - and it seems that I maybe need to create a point on the return circuit that is deliberately higher than the store - but very close to the store, and fit the air vent there....

Thanks Adrian

[Doctor Drivel]

You have a "buffer" only on the return pipe to the heat pump to ensure cool water is fed back to the heat pump. This give the large delta T needed for heat pump operation. This is not a thermal store.

A large tall heat bank/thermal store using a plate heat exchanger for DHW probaly would have been better and using off-peak electricty could be heat up during the night.

If the auto air vent is not working it is duff.

[Andrew Gabriel]

The closing is because they don't last long if left open. The ball has to stay wet to seal, and this means there's moisture exposed. This dries and eventually leaves deposits behind, and these build up and stop the seal working, so the valve starts leaking water.

There isn't normally negative pressure anywhere in a system -- the header tank is usually higher than the pump by more than the pressure differential the pump can achieve, although there are some less well designed systems which do manage to suck air in the expansion pipe.

[Adrian]

Yes - mine have those - like a little threaded cap arrangment..

I've tried all combinations . There is a tiny amount of air let out if you unscrew the cap after it's first been tightened down & left - but at the same time, the loosened union on the tank can be 'bubbling' out trapped air...

It must be a thing with levels. It's not that easy to see, behind the big tank / buffer / store - I think the only way forwards is to make up a sort of 'swept' entry that drops gracefully down into the inlet on the tanks and allows the air back up to the air vent.... Life's way to short to spend hours hanging over the tank, leaning on the loosened pipework to encourage the air out..

Shame the manufacturer didn't think to build in an air vent before they wrapped the tank in yelow foam !

Thanks for the comments Adrian

[Adrian]

HI Andrew

Andrew Gabriel wrote:

Ah - so they're not so much an auto-air-vent - more of an 'auto-air-seperator-with-manual-vent' - I didn't realise that.

So in normal operation you'd leave them with the cap screwed down tight

- and release any trapped air by unscrewing the valve from time to time...?

Don;t think we've got problems in that department. As I said - once the air in the big tank is removed then everything runs very happily - but excessive amount of air in the tank causes the header tank to overflow as the system warms up and the air expands...

Thanks Adrian

[Dave Plowman (News)]

The purpose of the tickler was to flood the carb thus giving an over rich mixture for cold starting. Not to allow it to 'fill up'. In other words it disabled the cut off valve by lowering the float.

[Dave Plowman (News)]

The reason for that valve it is the device will leak past the float valve when it gets older. So you shut it off after the venting is complete.

[dennis]

Venturii effects say that may not be true, I wouldn't take bets if it were in a 22x22x15 reducing tee on the main drag in a none pressurised system.

That is easy to do, just put the expansion pipe on the other side of the pump to the fill pipe.

[Doctor Drivel]

Auto air vents are for filling purposes sonly. Once filled, the screw should be tightened.

You should look at why air is being drawn in - or is it corrosion? Look at where the cold feed & open vent pipes from the F&E tank is. The relationship of these and the pump may solve matters.

Having an auto air vent permanently open will solve nothing as all it will do is advance corrosion as air is being drawn in.

[Doctor Drivel]

If this buffer cylinder had a large coil in it and the cold feed to the DHW cylinder run through it, it would pre-heat the DHW and ensure a cooler return to the heat pump. It is enhances condensing boiler operation and creates a larger temperature differential if the heat pump requires it. I am surprised this was not done.

A heat bank/thermal store can ensure a large flow and return temperature differential for a heat pump or condensing boiler, act as a buffer for the heat pump and CH system and provide DHW - all off the same cylinder. Wonderful things.

[Dave Plowman (News)]

I'd hope they have EFI. ;-)