Is there a future for vented systems?

[ReJect]

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I'm a little confused ...... an open vented system won't absorb nor release air, nor will the water within the actual heating circuit evaporate. There will be a little at the header tank as they tend to be open topped but that's replenished by the valve. The actual system water, if there are no leaks (less likely long term due to it's low pressure), will require very little new water.

The water absorbs air in an open vent system and releases the air when the water heats, unfortunately often into a towel rail or a radiator on upper circuits in many systems, and repeats the cycle. Sealed system is possible to not have that problem. Got to be careful of the problem of auto vents leaking though and I avoid using them if possible.

The f&e tank will allow some evaporation, especially if it has no lid on it. But it admittedly would take many months to be of any significant amount. Still a water loss though that would be impossible in a sealed system.
I have some sealed systems, (installed with all compression joints fully sound and valves high quality) that hold water pressure without requiring top up in several years, which I think is unbeatable compared with open vent.

 

[ReJect]

I think you'll find that all those owners that have had their unvented cyclinders fail and not be covered by the warranty would disagree with you there
There's been a slow but increasing no. of Megaflo owners with failed baffles...new cylinder time.
The cost of a pump on an open vented cylinder is easily matched by routine costs of maintainence requirements...failed vessels, safety valves etc on an unvented.
You also run the risk that the local water undertaker will not maintain the higher pressure required and refuse to sort out flowrate issues...I've had customers with new cylinders with great flowrates/pressures down to 9litres/min after mains work in the road and their boundary stopcocks blocked. Only until you get to 8 litres/min will they investigate.

Megaflo cylinders can have a suitable sized potable expansion vessel retro fitted if the internal baffle fails. I believe Megaflo allow this.
But I personally think Megaflo units are overpriced.

I fully agree, - unvented can be a costly and problematic choice, but not always.
They can also be a very simple and cost effective solution to balanced high pressure supplies, particularly in a full plumbing refit or new build house, if comparing to traditional hot and cold tanks with all the extra pipework and the usual power showers or twin head pump.

 

[Madrab]

The water absorbs air in an open vent system and releases the air when the water heats,

The only place I am aware of that an open vented system would absorb air is in the F&E tank and if the system is watertight, that water won't find it's way down into the system. There will be some movement through expansion in the tank but not enough to draw any new water into the system.

I certainly don't disagree that a well installed sealed system can be just as clean and I'm certainly not suggesting open vented is any better but they have stood the test of time and I've yet to find a sealed system that's ten years plus that is as clean as some of the open vented systems I look after that are twice as old.

Lets be honest, an unvented HW cylinder is really only installed to give the user the ability to supply multiple mains fed showers and to give good flow and pressure from some of the newer mix in the body designer taps and fittings. If you are happy with individual full flow taps and power showers then it is a much cheaper to keep a gravity system rather than converting to a whole unvented system IMO.

Don't get me wrong, I love my unvented system, I can run multiple shower and outlets at the same time without issue but on the other hand if I had had a gravity system from the start then I could easily configure that to do the same.

 

[ReJect]

The only place I am aware of that an open vented system would absorb air is in the F&E tank and if the system is watertight, that water won't find it's way down into the system. There will be some movement through expansion in the tank but not enough to draw any new water into the system.

I certainly don't disagree that a well installed sealed system can be just as clean and I'm certainly not suggesting open vented is any better but they have stood the test of time and I've yet to find a sealed system that's ten years plus that is as clean as some of the open vented systems I look after that are twice as old.

Lets be honest, an unvented HW cylinder is really only installed to give the user the ability to supply multiple mains fed showers and to give good flow and pressure from some of the newer mix in the body designer taps and fittings. If you are happy with individual full flow taps and power showers then it is a much cheaper to keep a gravity system rather than converting to a whole unvented system IMO.

Don't get me wrong, I love my unvented system, I can run multiple shower and outlets at the same time without issue but on the other hand if I had had a gravity system from the start then I could easily configure that to do the same.

I find sealed systems usually very clean if they are good systems and not needing topped up often.
Strangely some open heating systems have little corrosion after many years which I assume is because of the design of the system, accidental or otherwise. Old systems that have pumped rads and gravity pipes to cylinder seem to have an advantage.
The vent pipes can draw air in and the feed pipes can mix water if they are too short a run, which is common I think.

As to domestic water supplies, my own system is gravity and I am very happy with it. Shower pressure poor though.
Pity there wasn’t a simple solution that we can have gravity supplies and good functioning mains to showers.

 

[fixitflav]

I would have though that the radiators would rust, unless they are the old cast iron type.

I don't know about Canada, but in UK until about the early 1970s vented systems had the cold fill on the boiler return pipe. Piped cold feed - boiler - open vent - pump. With that arrangement, each time the pump starts the water in the vent pipe falls by something like 1m, depending on boiler headloss and system details. And same volume flows from the cold fill pipe into the F/E tank. You can see it if you draw a sketch. Opposite when the pump stops. Known as see-sawing. That brings a fair slug of possibly aerated water into the system every cycle.
Then it was changed to boiler - open vent - cold feed - pump (open vent and cold feed not more than 150mm apart). With that arrangement there's a negligible exchange of water between the system and the F/E tank. If inhibitor is kept to correct level no corrosion problem.
One reason for installing a sealed system is it's less trouble for the installer!

 

[Madrab]

Known as see-sawing.

Gosh .... That brings back memories, our old master plumber used to call it a 3T setup, due to the number of connections on the flow or on the return, when we came across them, more used with the old low resistance cast iron HEX's. He always pointed out that there must be a significant height from the boiler/pump to the F&E tank to ensure there wasn't any air draw due to the draw down but as the water was always contained within the pipe and much higher than the subsequent draw, there shouldn't be any worry about air unless the the system got dirty and the resistance across the HEX rose.

He always always pointed out how important to check for it when an old boiler was being removed and replaced with a new one as that setup could only be used when there was a low resistance boiler. It was something you still have to watch for as when converting to a higher resistance boiler otherwise it could cause major issues with air drawdown or pumping over due to the larger pressure differences.

 

[vulcancontinental]

If you have the head over a shower rose vented cold water is fine with correctly sized pipework. No line strainer, pressure limiting valve, non-return valve pressure relief valve, temperature and pressure relief valve, expansion vessel or expansion space, no quals to fit and additional safety cut outs to boiler primaries. Might need 28 CF to cylinder if bathroom in on the ground floor but that's not a problem. Open cold feed gate valve fully then back a quarter of a turn and that will not stick either. Tempting fate but my old house, now my son's is 35 years old and nothing touched except replace bv.

Vented central heating problems mostly were 15 mm cold feed and high pump speeds; trouble. 22mm cold feed and vent, generous pipework, low speed on pump moving water slowly and it'll last 30 years. Might have to jog the BV every couple of years in a hard water area. Have only one left now like that, 29 years old, 25 years on the MV's 26 on the pump and control system. The other two remaining customers like that have moved to bungalows due to age.

 

[sirocosm]

I don't know about Canada, but in UK until about the early 1970s vented systems had the cold fill on the boiler return pipe. Piped cold feed - boiler - open vent - pump. With that arrangement, each time the pump starts the water in the vent pipe falls by something like 1m, depending on boiler headloss and system details. And same volume flows from the cold fill pipe into the F/E tank. You can see it if you draw a sketch. Opposite when the pump stops. Known as see-sawing. That brings a fair slug of possibly aerated water into the system every cycle.
Then it was changed to boiler - open vent - cold feed - pump (open vent and cold feed not more than 150mm apart). With that arrangement there's a negligible exchange of water between the system and the F/E tank. If inhibitor is kept to correct level no corrosion problem.
One reason for installing a sealed system is it's less trouble for the installer!

It makes sense to avoid pipework that allows the pump to push/pull water up into the F&E tank.

There is also the expansion of the water itself to consider. If one checks the level in the tank cold, and then looks again once the system has heated up and the pump is off, there will also be a significant level difference due to thermal expansion of the system water. This will depend on the difference between the hot/cold temperatures and the total volume of water. With a 30C differential for a system with 100 litres of water, 30 x .0002 * 100 = .6 litres will be pushed/pulled from the F&E tank with each cycle without considering the effect from the pump.

Vented central heating problems mostly were 15 mm cold feed and high pump speeds; trouble. 22mm cold feed and vent, generous pipework, low speed on pump moving water slowly and it'll last 30 years. Might have to jog the BV every couple of years in a hard water area. Have only one left now like that, 29 years old, 25 years on the MV's 26 on the pump and control system. The other two remaining customers like that have moved to bungalows due to age.

I wonder if "generous pipework" might also help to stop corrosion, at least in the F&E pipe. If the F&E feed pipe's volume totals more than that (i.e. 9m or so of 22mm pipe, or 7m of 28mm), then water that has been "in" the F&E tank will not mix with the system water. Of course there will always be diffusion, but I guess that is orders of magnitude less than convection.

My current vented system's F&E pipe is way less than that (2.5m), and the system is full of sludge, and one of the radiators has rusted through. It is around 25 years old.

 

[vulcancontinental]

That's how my layout evolved. Slow water flow, generous pipework minimum turbulence.

 

[fixitflav]

I find sealed systems usually very clean if they are good systems and not needing topped up often.
Strangely some open heating systems have little corrosion after many years which I assume is because of the design of the system, accidental or otherwise. Old systems that have pumped rads and gravity pipes to cylinder seem to have an advantage.
The vent pipes can draw air in and the feed pipes can mix water if they are too short a run, which is common I think.

As to domestic water supplies, my own system is gravity and I am very happy with it. Shower pressure poor though.
Pity there wasn’t a simple solution that we can have gravity supplies and good functioning mains to showers.

You can have mains pressure hot water cylinder, giving good shower performance, with either open vented or sealed system on the primary side, whichever you prefer.

 

[fixitflav]

There is also the expansion of the water itself to consider. If one checks the level in the tank cold, and then looks again once the system has heated up and the pump is off, there will also be a significant level difference due to thermal expansion of the system water. This will depend on the difference between the hot/cold temperatures and the total volume of water. With a 30C differential for a system with 100 litres of water, 30 x .0002 * 100 = .6 litres will be pushed/pulled from the F&E tank with each cycle without considering the effect from the pump.

That's a good point. But as others have said in practice there's not much to choose. There are pros and cons with sealed and vented systems. Starting from scratch it might be 50:50, but I wouldn't think it's often a good idea to remove an existing F/E tank and replace with an expansion vessel.

 

[sirocosm]

That's a good point. But as others have said in practice there's not much to choose. There are pros and cons with sealed and vented systems. Starting from scratch it might be 50:50, but I wouldn't think it's often a good idea to remove an existing F/E tank and replace with an expansion vessel.

The advantage would be to limit corrosion. For a heating system, I can't really see any downside to sealing it, replacing the F&E tank with an expansion vessel, and running it at 1-1.5 bar, with a 2 bar relief valve.

 

[fixitflav]

The advantage would be to limit corrosion. For a heating system, I can't really see any downside to sealing it, replacing the F&E tank with an expansion vessel, and running it at 1-1.5 bar, with a 2 bar relief valve.

But experience (as others have posted) seems to show that if the inhibitor is kept up to scratch (with either system) corrosion isn't a problem. With sealed system you have to check the pressure periodically, and top up when required, while a vented system looks after itself. Then there's the question of higher pressure in a sealed system, which might give a leakage problem if retrofitted to an older system.

 

[sirocosm]

But experience (as others have posted) seems to show that if the inhibitor is kept up to scratch (with either system) corrosion isn't a problem. With sealed system you have to check the pressure periodically, and top up when required, while a vented system looks after itself. Then there's the question of higher pressure in a sealed system, which might give a leakage problem if retrofitted to an older system.

I am not so sure. Some vented systems are clean, whereas others (like mine) are horribly corroded.

If your sealed system loses pressure, you need to find and fix the leak. Just replacing the water is not really a solution.

 

[sirocosm]

I am not so sure. Some vented systems are clean, whereas others (like mine) are horribly corroded.

If your sealed system loses pressure, you need to find and fix the leak. Just replacing the water is not really a solution.

It is all about oxygen intrusion.