Why loft vents for boiler and immersion cylinders?

[Dave Baker]

My heating system is a traditional boiler, loft tank, vented cylinder type affair. The lower outlet to the coil inside the immersion cylinder tees off to a vent into the small tank in the loft which therefore vents the boiler side of things.

The heated water in the cylinder fills from the bottom and the outlet at the top to the taps also tees off to a vent into the large loft tank. All fairly normal and so far so good.

A few years ago I was up in the loft for the first time in ages and it was dripping wet in there. Everything stored had gone musty and water was running off the roof felt. Turned out the boiler vent was pumping over into the small tank which was consequently full of hot water and producing clouds of condensation. I tried the pump on its lowest setting and it still happened. I examined the pipe runs for the whole system and concluded that there was no point in a vent being there. The boiler fills up under gravity feed from the small tank and any excess pressure can just go straight back up there if it wants. The boiler certainly isn't going to explode and anyway there's a blow-off valve on the back of it.

So I dug out an old radiator valve, slapped it on the end of the vent in the loft and turned it off. It's been that way for years and hasn't made a scrap of difference to anything other than the loft is now dry. So what purpose, if any, was the vent really serving?

The vent to the main part of the cylinder doesn't overflow of course because it isn't pumped but again I see no point to it other than maybe to let any air in the cylinder get out. However wouldn't the air be immediately displaced out of a hot tap somewhere the first time the system was filled if there was no vent?

-- Dave Baker

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[Doctor Drivel]

You have converted the system to a combined heat and vent:

1. The pipe should be 22mm
2. The boiler should suitable for sealed systems (high limit stat)

Firstly, find out if the boiler is suitable. If yes, then have a 22mm pipe from F&E tank to system an deemove the vent pipe with the valve on. . If not, converts it back to a two pipe system and fit an air ejector. Screwfix No.90641

Also fit a Magnaclean filter on the return after dosing with desludger and flushing out. This will elimiante sludge which may accumulate in the ejector and cause problems.........and protect the whole system

[Aidan]

With all due respect, you're daft. Take it off.

In normal operation it releases any air that gets into the system. Typically dissoled oxygen in the boiler return wil be released from solution when the water is haeted in the boiler and is discharged from the open vent.

In the event of a control failure, it is a safety device; it releases steam, prevents the system becoming over-pressurized and bursting. In your system, such a failure would probably now cause the contents of the heating system to be discharged into the loft and the boiler would dry-fire to destruction.

It was pumping over probably because it was piped incorrectly. Get someone competent to fix that problem. Conversion to a sealed system would be an improvement, if the boiler is suitable; if you'd like that done, get someone competent to do that.

I find your faith in a cheap safety valve to be touching.

They don't have to do anything in normal operation. If called upon to operate in anger, a disturbing number fail to work. Many have been dripping inconspicuously for years, the water evaporates from the hot safety valve and the accumulation of limescale deposits concretes the valve immovably shut. Have you tested the valve?

Kin 'ell! Is this a wind up? Leave it alone.

Immersion heater contacts frequently weld themselves together, so the heater fails on. The open vent would then discharge steam. If you block it, the steam will blow the contents of the cylinder into the storage tank. If that discharge route is blocked, this happens;

is a small water heater (12 US gallons). Drivel loves posting that link to promote his thermal stores, but someone proposes plugging the vent on a domestic hot water storage system and he makes no comment; he is a dangerous idiot.

Unvented water heaters are good, but they have at least 3 sequential safety devices for each heating system. They are costly. A vented cylinder cannot be converted.

[Doctor Drivel]

I disagree. Heat banks give better performance as they can take higher pressures becaus eof the plate heat exchangers, open vented and a minute to zero chance of an explosion. Unvented cylinders require an annual service charge. That alone is enough to discount them totally...especially when a safe, service free, better performing unit is available....the heat bank.

[Andy Hall]

An interesting story is the sinking of the Fleur de Lys, a fishing vessel.

This one went down, fortunately not killing anybody, when its 120 litre HW cylinder, a class 4 type (weakest rating normally used for open vented systems) was being used in a pressurised system.

A firm of plumbers who didn't know what they were doing had replaced the cylinder and fitted the wrong type.

cause of the explosion was listed as follows:

"The hot water storage cylinder ruptured because the water in it overheated and steam generated, causing overpressure. The rupture of the cylinder probably occurred at about 3.4bar, four times working pressure of the cylinder. The pressure relief valve must have been set at this pressure or above, or it was seized in the closed position.

The cylinder probably overheated when the immersion heater thermostat failed to shut off the electrical supply to the heater. There were no safety devices fitted to the system to prevent overheating and overpressure as a result of thermostat failure."

Returning to the original point of blocking the vent - it is quite common to have an isolating gate valve etc. on the cold feed to the cylinder. If this is closed when the heat source is still on and the vent is blocked, a potential bomb will have been created.

We can see from the fishing boat example that there does not need to be a great deal of pressure at all to achieve an unfortunate situation.

[Dave Baker]

I'm actually a rather competent engineer who was hoping for reasoned opinion on the merits of this vent.

No thanks. As I say everything works very well as is.

Firstly, the water in the boiler system always has an open vent path via the fill point at the bottom of the tank in the loft. If water can get down that I see no reason why dissolved oxygen can't get back up it.

Secondly, The vast bulk of the water in the system is in the radiators and that's where the vast bulk of any dissolved oxygen will end up. There is no way for bottom fed radiators to have this oxygen removed than by bleeding from the valves at the top. A bleed valve at the top of the boiler system would surely do the same rather than having to have a permanently open vent.

Nonsense. The system isn't pressurised other than by the head of water from the loft tank. The contents of the system can't be discharged into the loft other than by the same pipe from the bottom of the loft tank which is continuously filling it up anyway.

Possibly but it was certainly ok for many years and the only change made to the system in 19 years was a new pump which was fitted by a Corgi plumber. Now it's possible this was fitted to pump in the opposite direction from the original pump but I can't instantly see how this would make a deal of difference anyway. I'm open to suggestions as how it might. At present the vent is on the return side of the pump so if anything I would think this would give the least chance of pumping over.

Get

It's been checked, cleaned and the washer replaced a couple of times and is fine. In any case the system is always vented to the bottom of the loft tank as I have said.

I have every intention of leaving it alone because it cause no problems as I have made clear. Please try to confine yourself to the physics and hydraulics of the issue rather than pontificating.

There is no electric heater in the tank.

-- Dave Baker

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[Doctor Drivel]

If the boiler has a blow-off valve the boiler is most probably suitable for sealed systems ..... and a combined feed and expansion pipe. If so, then he has a one pipe F&E system, except the pipe is probably 15mm not 22mm. Then it will not dry fire itself to destruction as a high limit is on the boiler.

[Dave Baker]

Sorted :)

I've just been up into the loft and airing cupboard and mapped out all the pipe runs. If you look at this basic diagram.

my pump would be on the blue line immediately adjacent to the boiler, pumping into it and then back out into the line that feeds the vent and then the rads and cylinder. The vent is therefore in the wrong place, before the rads rather than after them and hence seeing all the pump pressure. Turning the pump round wouldn't help because then the feed from the bottom of the loft tank would see this pressure instead thus similarly reducing flow through the rads.

So at present I only get normal flow because the vent is blocked and the expansion/feed pipe does any venting required. When I get round to fixing all the niggles in the system I'll reorganise everything to correct this.

As I say though, whether vented, for the first 15 years here, or unvented for the last 4 I've always had piping hot rads and hot water and I guess just the pressure head from the loft tank has sufficed to keep the system running ok. Black mark to the guy who fitted all this before I moved in I suppose.

-- Dave Baker

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[Doctor Drivel]

The older it is, the near the time to when it boils over. Get it sorted ASAP.

[Andy Hall]

If your pump is on the blue line adjacent to the boiler, it's the pump that's in the wrong place relative to the other pipes.

It's not so much an issue of whether the pump is before the heating circuit or after it, but rather that there is a pressure differential between the feed/expansion pipe and the vent.

If your set up is as shown on the web site except that the pump is adjacent to the boiler, you have a classic pumping over situation because the two pipes to the tank are on opposite sides of the pump.

Actually, the setup in the diagram is not a lot better because the F/E pipe and the vent have the boiler between them. Since there is a level of flow resistance through the boiler there will be a pressure differential so in the case illustrated there is a possibility of air being sucked down the vent if the boiler flow resistance is high - e.g. with pump on high speed. If you imagine the flow through the boiler stopped (for the sake of illustration) you can see that the effect would be that the water level in the tank would rise and air would enter through the vent. In normal operation it would depend on the pump setting, the height of the tank and the flow resistance of the boiler and heating/HW circuits.

A better design would have the flow from the boiler going to an air separator - e.g.

part 11334 - and the F/E and vent pipes separately joining it. The pump goes after that followed by motorised valve(s) and the heating and HW circuits. The returns of those would be commoned and go directly to the boiler.

This achieves two key objectives:

- The F/E and vent pipes are close together. This means that there is very little pressure differential and no chance of sucking down or pumping over. Where a separator isn't used, but the pipes join the circuit, it is recommended that there is not more than a 150mm space between the two connection points.

- There is a clear path from the boiler flow to the vent. This helps with venting but also the safety purpose of an easy path for the escape of steam in the event of control or other failure.

An additional benefit of the air separator is that the system becomes very easy to vent and little or no air should collect in the radiators after the initial filling and expulsion of dissolved air.

[fred]

[Aidan]

Stick to racing engines. I'm an HVAC Engineer. I suggest you take it off.

So far.

It's often on the return and it's commonly lower. The oxygen won't get to it, it will corrode the radiators,

It would be discharged into the F&E tank through the cold feed pipe.

You may think so. I do not. You didn't notice anything amiss, this doesn't mean it was OK.

Ye're doomed, doomed I tell ye.

I have done. You're convinced you know everything worth knowing about it and can't believe that someone else may know more.

Take it off.

[Tony Williams]

That's where our pump is.... in the return line, and immediately next to the boiler. It is the original 1970 plumbing and works ok.

ISTM that if the radiator loops are free-flowing, then the pump will pull a low pressure on the pipe feeding the loops, lower than the head required by the vent pipe. Radiator circulation is tickety-boo.

But if the radiator loops get blocked, (by sludge, or later fitted valves?) then the pump will preferentially do a local circulation, in just the loop between the header tank and the vent pipe. Any chance that that is what you are seeing?

BTW: My old plumbing book has a gruesome tale about a plumber who blocked a vent pipe with a cork, to try and reduce the amount of water lost during a drain down. The resultant vaccuum caused parts of the plumbing to collapse inwards. Blocking a vent pipe may not be good news.

[David Hansen]

On Sun, 5 Mar 2006 00:17:08 -0000 someone who may be "Dave Baker" wrote this:-

So you claim. However, in my view a competent engineer would have found out what was going on and then sorted it out at source, rather then dealing with the symptoms without knowing what they were doing.

Finding out what was going on is fairly simple, as it is an elementary part of building services engineering. For example in a library one can find, "Faber and Kell's Heating and Air-conditioning of Buildings", the book on the subject. In there one should find some examples of piping layouts.

(keyword+INCLUDES+'heating')&SS=(Finding out would also involve tracing out the pipes, something you say you have only just done.

From my 1984 edition of the above book, page 190, "In no circumstances should the feed and expansion pipe be combined with the system vent pipe as case D. This practice was shown to be dangerous more than 50 years ago."

The reason why it is dangerous should be obvious. It is to do with what happens if this one pipe is restricted or blocked, especially if it is only 15mm. You are living on borrowed time and should sort it out immediately.

BTW, your one safety valve may have been cleaned and the washer replaced a few times. What pressure is it set at? How often is the operation of the valve tested? Can it pass the necessary volume of hot water/steam?

[Dave Plowman (News)]

Is there a bypass fitted? Commonly just a link pipe between feed and expansion with perhaps a gate valve in it?

[Andy Hall]

All of this can be very easily avoided and the risk of pumping over eliminated by positioning the FE and vent pipes on the same side of the boiler.

There should be a clear path from the boiler flow side to the vent with nothing in the way in terms of pumps or valves.

The FE pipe can be connected to a point on the circuit within 150mm of where the vent pipe connects. The pump is located after that, followed by the motorised valve(s).

An even better solution is to use an Aerjec air separator which will do the correct plumbing connections in one place as well as helping to purge the system of air as it is circulated following refilling and in general.

The flow through the radiators then becomes irrelevant and if inhibitor is used, no significant corrosion or sludgng either.

Another issue to consider is that if TRVs are used then the flow resistance through the CH circuit will increase anyway as they start to close, so it having an arrangement where avoiding pumping over/sucking down is on the basis of clear path through the CH circuit is never going to be clear of potential problems.

[Doctor Drivel]

(keyword+INCLUDES+'heating')&SS=(>> Finding out would also involve tracing out the pipes, something you

I don't know what context this passage was written. Combined feed and expansion pipes are common and inceasingly copmmon as 99% of boioer has a high limit stat If a boiler has a high limit stat (suitabkle for sealed system) it can have a combined feed & expansion pipe that must be 22mm minimum.

A a normal sealed system in millions of homes:- If the blow-off valve is seized and the main boiler stat fails as well, the only protection is the high limit device. It is exactly the same protection level with a combined feed and expansion pipe, except that to block a 22mm pipe will a hell of a lot more difficult on the flow at high level, where they are supposed to be, and are specifically for this reason (no crud at high level), than a blow-off valve failing. Which means a combined feed and expansion pipe setup is safer. The reason why 22mm pipe is used as a minum (some require that size because of system size), is so that the probability of being blocked is minimum. A blow-off pipe on a sealed system in most domestic homes is only

15mm.

He says he has a blow-off valve. If so, the explosion danger is minimum. If the boiler is suitable for a sealed systemn, which it appears to be with a blow off valve on it (I am assuming the valve is integral with the boiler, not an add on), then it will have a high limit stat which improve boiler protection. If not suitable for sealed systems then he has no boiler protection.

[Tony Williams]

Followup to myself.

We don't have the vent pipe taking off from the feed to the radiators loop. Ours is a 4-pipe boiler, one pair feeding the radiators (pumped in the return), and the other pair doing the gravity feed to the h/w storage tank. That second pair does an upslope to the top of the h/w tank, with a dropdown into the h/w tank, and the upslope continuing up to become the vent pipe.

[Aidan]

You and Drivel have misunderstood what I have written. It is not nonsense, it is common sense.

If the boiler thermostat fails in the ON position, the temperature of the water will quite likely exceed 100 degC. The water in the boiler will boil, producing steam. This happens. Your heating system will THEN be pressurized by the steam and the steam pressure will become greater than the static pressure provided by the head of water in the F&E tank.

The water in the heating system will be pushed up the cold feed pipe into the F&E tank. It will be pushed out rapidly, probably faster than the F&E overflow pipe can remove it. There will be no water in the boiler and it will continue firing.

This assumes that the cold feed pipe is clear and that there is not a stopcock (acting as a non return valve) fitted to it. The cold feed on my heating system was totally blocked with limescale when I bought the house. This is a common fault. I had to cut out the tee and replace it. There had been a longstanding leak from the heating system through the coil in the indirect cylinder.

With a non-fubarred system, such a failure would cause steam to be blown out of the open vent, but cold make-up water would enter the system through the cold feed. This could continue indefinitely.

You can't get 2-way flow (steam up & water down) in one pipe, which is probably what Faber & Kell were referring to. Their book (20 years since I had a copy) deals with commercial installations on which such a combined CF &OV would be inadvisable.

[Aidan]

Good point sir! Probably the copper hot water storage cylinder, which will be crushed by atmospheric pressure if you were to drain it without relieving the vacuum (hose out bathroom window, no hot taps open).

On unvented hot water storage cylinders the mandatory T&P relief valve also acts as a vacuum breaker valve (I think, goes off to get book to check).