Can anyone suggest a reason why when I open my hot tap fully I get air bubbles in the hot pipe, yet if it is run slowly there are none.
I've already tried looking at the hot output from the top of the hot water cyclinder (which is in the loft beside the cold water tank) to see if there was any air getting trapped in a malformed pipe, but it looks like it should in most diagrams. I've also tried opening the cold water source valve fully, but it is already fully opened.
I'm hoping to install a power shower, so I need to get rid of the air.
Yes. I had this problem previously in a 4 storey house ( hot water cylinder on 3rd floor, bath on 1st floor).
Causes? Assume the cold water storage tank isn't running dry. With no water flow, the water pressure (static pressure) at each point in the system is proportional to the height of water above it. P = density x G x H. Hydro-statics, easy stuff.
When you open the hot tap, water flows down the cold feed, up through the hot water storage cylinder and then off through the hot distribution pipes, passing the open vent pipe connection. The moving water possesses a kinetic energy and there is a hydraulic resistance to the flow of water, which absorbs some more energy. All this energy is taken out of the water pressure, so that when the water is flowing the pressure at every point along the pipe will drop. The pressure at any point now depends on the water height as previously, minus the water velocity pressure and minus pressure due to the upstream hydraulic resistance. The pressure at the branch where the open vent pipe connects will drop, so the water level in the open vent pipe would normally drop when there is a draw-off.
So what has this got to do with your problem?
In your system, the flow rate and/or the upstream hydraulic resistance is so great that the pressure at the open vent branch has dropped below atmospheric. The water in the open vent pipe drains out and air is sucked down the open vent pipe into the flowing water. You can confirm this by putting your hand over the open vent and getting someone to turn on the affected bath taps. You should feel your hand being sucked onto the pipe. Air won't get entrained in the water until you remove your hand. You'll hear the air hissing in if you restrict the opening. Alternatively, immerse the end of the open vent in a glass of water. The water should get sucked out of the glass.
Solutions? Don't put a non-return valve in the open vent, as someone is bound to suggest.
You have to reduce the hydraulic resistance of the pipe from the storage tank to the open vent branch, AND/OR reduce the flow rate at the affected taps.
Check the cold feed pipe for restrictions ( partially open valve, blockages, etc). Check the cold-feed pipe size is adequate. Check there are two separate cold feed pipes, one to the hot cylinder and one serving the cold water down service. Consider fitting flow limiters to the affected taps. In my case, I got the storage cylinder moved to the second floor.
Beyond this you'd need to start doing some calculations for which I don't have time.
It's usually as a result of an inadequate flow between the roof tank and hot water cylinder.
Since both are on the same floor, there is very little head between them and wneh you turn the tap on full, the water level in the vent pipe drops and air is sucked into the flow.
The only improvement that you can make without radical movement of the tank and cylinder is to improve the flow between them. This could be done by upgrading the pipe from 22mm to 28mm, or possibly even higher. 28mm pipe needs a larger bending machine normally and you need to have slowish bends rather than sharp elbows. I did this on my system by using pairs of 135 degree bends with a straight length of pipe in between so that the flow is not disturbed as much. It works well.
Other than this, the only other real option is to convert to some sort of mains pressure system like a heatbank as long as the mains flow is adequate.
As far as I remember, and I'll check tonight, there is NO non-return valve on the vent pipe, so maybe this is the solution.
So, if I fit the non-return valve, does this mean that when running the tap hard the vacuume created by the downward water to my tap will effectively provide a suction force on my hot water cyclinder, thus providing sufficient flow to the tap.
Did I miss something here? So its not a good idea to fit a non-return valve on the vent? When you said explode, did you mean implode? I don't like the bit about killing people and stuff.
The input pipe from cold tank to water cyclinder is already 28mm, but the cold water tank is only slightly above (couple of foot) above cylinder. But it's not possible to move the cylinder down a floor (no room!).
1) Increase the size of pipe between tank and cylinder. Will improve but still may not be enough
2) Move the cylinder down one floor.
3) Change to an alternative system involving mains fed hot water. Either a heatbank, a pressurised cylinder or even a combi boiler. Check that the cold water mains is adequate before contemplating this.
It might do, but shouldn't be done because you are risking an explosion and a shower of boiling water coming through your ceiling.
No. I meant explode. With maximum prejudice. Enough to entirely demolish sizable portions of a house. Easily enough to blow through solid 9" masonry walls. Don't even think about it. It might work until there was a fault on the system, possibly even for a few years. Then, boom. The purpose of the vent is to allow steam to escape when a fault leads to overheating. If it can't get out because some nutter has put a valve on the vent, the amount of energy in the resultant explosion is immense.
Of course, it might implode as well during that time, but that is less likely to kill anyone, although someone was killed when a hot water cylinder ruptured and dumped 120l of scalding water on top of their bed through the ceiling.
Rather than get a power shower, consider upgrading to a heat bank. This will provide excellent power shower performance without the air, or the debris. You can buy new ones, or convert your old cylinder as a DIY project.
I'd misread your post and missed the bit about the cylinder in the loft, which aggravates the problem. If the flow out of the tank exceeds the flow rate in (e.g., if you put in a shower pump, the water suppliers reduce the pressure, etc.) the problem will return as the water level in the tank drops, unless you put the cylinder down a floor.
The high level space in the stairwell, over the stairs at ground floor level, is usually unusable. I've seen cupboards constructed in this space, to house a cylinder. It avoids putting it in the (unheated) loft & the associated risk of freezing.
If you do nothing else, you need to ensure that the existing installation cannot freeze, particularly the cold feed and open vent pipes. Ensure the pipes are very well insulated.
If the steam generated by a small coal fire in a steam locomotive contains enough energy to move the pistons on said locomotive and drive said locomotive and many ton(ne)s of train when used in a controlled way, what do you think said steam energy will do to your house when it's been trapped by force unable to expand and then explodes violently in an uncontrolled way?
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