Do i have a positive or negative head?

[Masters02]

Hello All,

I should already know the answer to this after doing some reading online, however for one reason or another im still not entirely convinced i fully understand.

I just replaced my shower pump in the loft with a positive head pump, there is a ~1m drop between the water tank and the shower head. However, the pump is actually at the same level as the tank itself...

What is happening, is that the pump is engaging for a second, stoping, engaging, stoping, rinse and repeat. I assume this is because there is no drop between the water tank and shower pump, so the inlet is not getting a steady stream of water at the required pressure?

In this scenario, would a negative pump be needed? Or is there more to it...

*the pump fitted is a brand new Salamander CT50

Many Thanks in advance

 

[Millsy 82]

Have you managed to get all the air out of the pipes? This would cause the issue that you have.

In theory you need a positive head pump but because I'm guessing your pipe is running up and over I would personally opted for a negative head pump. Sometimes the flow rate you have will struggle to get the pump running but once it is running it should carry on. Which is why I think air.

If you have valves on the flexis then isolate them remove flexi from pump then run loads through into a bucket or 2 until no air is coming out either side. Re attach and then try again. If that fails turn pump off and open shower and suck water through the shower, ideally a wet vac would be best but if done it by literally just sucking. Remember to turn the shower off though otherwise you get a huge mouthful of water then very wet.

 

[Aire]

You've got negative head. You need a minimum of 600mm between the base of the tank and the highest part of the shower. In your case, your pump, or the pipework coming off the top of it would be classed as the highest point of your shower. I would advise you move the pump though. Having your pump sited above your cylinder is not an ideal location for it, unless of course it's piped correctly for the situation.

 

[Masters02]

Thanks for reply, i hadnt actually considered there could be air in the system. I had fully bled all of the other outlets (taps etc) but it didnt occur to me to do the same with the shower.

I'll give this a go this afteroon and see if i get any improvement.

 

[Masters02]

You've got negative head. You need a minimum of 600mm between the base of the tank and the highest part of the shower. In your case, your pump, or the pipework coming off the top of it would be classed as the highest point of your shower. I would advise you move the pump though. Having your pump sited above your cylinder is not an ideal location for it, unless of course it's piped correctly for the situation.

Thanks for your input Craig, that is what i feared was the case. I'll attempt to get all of the air out as Millsy suggested first, as i may get away with it and not need to spend more money.

I could move the pump into the airing cupboard, but i would need to get a plumber in for that as i think that is beyond my ability. Im happy to replace like for like but that is where i draw the line

Is it an expensive job to re-pipe a shower pump?

 

[Ben-gee]

Just depends on the situation, obviously - I have moved a 3bar monsoon recently at a cost of £160 all in.

 

[Anakin]

You can buy better quality pumps, that actually serve the purpose for both positive and negative head. This is an automatic feature and you don’t need to do anything. Personally I’d move the pump out of the loft and downstairs, the inlet flow rate is probably very poor.

 

[Masters02]

My plan is move the pump and relocate it into the airing cupboard. Looking at the existing pipe work, the hot water feed for the shower pump comes straight out of the hot water cyclinder and up into the loft.

Initially my plan had been to simply extend all of the pipework from the loft, back into the airing cupboard. However, this seems non-optimal for the hot water feed as it will be doing a pointless loop into the loft and back down. So i intend to cut/re-route the pipe from the hot water tank instead.