Negative head of water

[Brian Drury]

The bathroom hand basin in my new loft conversion is 300mm higher than the head of water in the open vented heating system. No problem I thought just fit a small pump.

Ignoring the problem of 'kick starting' I was convinced that once air was expelled from the system it would not be a problem.

Unfortunately being a second bathroom the hand basin is not used very often and develops an air lock which is very difficult to remove.

The pump is a twin impellor (which curiously is cheaper than a single impellor) with the two inputs and two outputs coupled together. The feed is 22mm plastic as can be seen in this image:

output is 15mm as can be seen here: the pump is started after a few days rest there is obviously air around the impellor and a surprisingly large amount of air is expelled before the thing primes properly. This would be ok but it takes several gallons of hot water to prime.

I guess the main problem is that the initial output path is downhill but this is determined by the pump design rather than the installation. I could fit a vent in the output side but although it may make 'bleeding' easier it will not stop the problem occurring.

So the obvious questions:

Why does the air reappear? What can I do to fix it?

Suggestions involving a combi boiler would be sensible but not appreciated :)

Brian

[cynic]

If the tap is higher than the supply tank why didn't you buy a negative head pump in the first place instead of the bodge you have ended up with?

[briandotdrury]

here:

>>> When the pump is started after a few days rest there is obviously air

Salamander suggested that this:

work but because the negative head is so small it probably would not.

I thought that my installation looked ok but I guess your standards must be a lot higher than mine.

It would still be interesting to know where the air comes from.

Brian

[Tabby]

Looks like the pump is in the wrong place. Move it close to the hot water cylinder, then air wont get into the pump to begin with, and you get virtually instant flow.

NT

[briandotdrury]

here:

>>> When the pump is started after a few days rest there is obviously air

Thanks for that.

I am sure you are correct. The problem then is how to kick start the pump initially. The small negative head is not enough for flow switches to detect as there is no movement in the pipe when the tap is turned on. Also in this instance there is nowhere for the pump to go on the lower floor.

Incidentally, the bath next to the basin is slightly lower and it starts ok so as a last resort this could be used to start the basin but that really is a bodge.

The other obvious suggestion would be to raise the header tank but there simply is not the room. Lowering the header tank might be possible but not enough to be sure that a negative head pump will start.

I can see why every other loft conversion I have looked at has a combi boiler!

The reason for choosing an open vented system is that the power shower requirement is far more than a combi could cope with. Also having recently spent ~ £1000 on the new boiler I am not keen to change it.

Brian

[Man at B&Q]

So what about a small "instant" water heater?

MBQ

[Brian Drury]

Now thats a good idea!

A quick browse:

suggests that the instant type are rated at about 3.5KW so a separate feed from the CU will be required.

Best idea so far. Thanks.

Brian

[John Rumm]

picture suggests the pump is high up in the eves... I suspect this is part of the problem. These things are much better at pushing water uphill than they are at sucking it.

> When the pump is started after a few days rest there is obviously air

I would be inclined to mount a pump (to feed all the top floor outlets in one hit) somewhere lower in the house. Close to the hot water cylinder is common choice, although it does not have to be there. That would solve the priming problem. As to starting issues, you may have to resort to some form of pull switch that can be used to give it a kick.

How about an unvented (i.e. mains pressure) hot water cylinder then? ;-)

[Owain]

If the tap is above the system water level then you could use a vented tap or use an ordinary tap and fix it to be permanently open. Then use a push switch / PIR switch to control the pump when water is required.

A room occupancy PIR might work to kick start the pump with an ordinary tap.

Owain

[Steve Firth]

Actually the obvious question is "where did you fit the non-return valves?"

Along with "Are you going for bodge of the week?"

[Tabby]

You could just fit a little press switch.

it only needs to go below the water level in the header tank, so floor level in loft should work.

counterproductive

There's no way the boiler's the problem.

An instant electric heater on the cold feed is another option, but just moving the pump sounds like all thats needed.

NT

[Onetap]

Maybe dissolved oxygen in the water which comes out of solution when the pump stops and the pressure drops (although I wouldn't have thought you'd get enough in the pipes). There's a lot of dissolved oxygen, most of which appears as bubbles when the water is heated in the cylinder. For that reason, the cylinder should have a Surrey/Warix/Essex whatever flange to avoid drawing the bubbles into the pump.

Do you have a Surrey/Warix/Essex whatever flange?

[funkyoldcortina]

is a Salamander CT50+, a positive head pump. It's not designed for use in a negative head situation.

You should've installed a negative head pump.

[funkyoldcortina]

Yes, I'd have thought that would have been a better bet as it is designed to work in a negative head scenario. The one you did get (CT50+) is a shower pump and is not designed to work in a negative head scenario at all. You should have got that one. The SPV pump has a pressure vessel to maintain a positive pressure to the outlets.

See p5 of

[briandotdrury]

Thanks to all the respondents.

The suggestion that downstairs is stealing water from upstairs led me to fit a 22mm one way valve (check valve) as this is easy to do. The result is no more air in the system.

Incidentally there is indeed a Surrey flange fitted.

So a satisfactory solution to the problem - thank you all.

Brian