Loft shower negative head pump unexpected shut offs...

[srmans]

Hi,I have a conventional vented central heating and vented hot water cylinder setup and I'm finding the negative head pump (Techflow QT45-2) that supplies the loft conversion shower shuts off after a couple of minutes of running in the morning during the colder months. The pump has been installed for a couple of years now and we've had this problem almost every winter.I've been trying to debug this for a few weeks now and it seems to be the build up of air in the hot water supply to the pump. The draw from the side of the hot water tank is via a techflow flange which routes down to the pump next to the cylinder. The flange has a T connection to support a manual air vent and tests show this section of vertical pipe always contains air and bleeding it helps with pump reliability but it seems this setup is not enough to avoid shut offs.It looks like I need to change the setup to reduce the amount of air that remains in the cylinder or increase the air volume of the bleed pipe to enable a 5 minute shower. Anyone had experience with this kind of setup? Advice?Cheers.

 

[Best]

Had to look up Techflow Flange to be honest! Nothing special about it, so I was wondering is the flange fitted properly, & not upside down, where it would draw more air in?
In my opinion, to have a longer bleed pipe to hold more air is not solving the problem of air getting into the hot pipe in the first place.
A Surrey or Warrix flange on top of cylinder work fine also.
If air free supply is decent, then negative head pump could be also faulty perhaps?Maybe ask manufacturer if no solution.

 

[srmans]

Thanks for the suggestions.
I plan to drain down the hot water cylinder this weekend and remove the immersion heater to check the condition of the inside and the position of the flange.
Last weekend I checked the inlet filters of the pump and they were clear so I will also consider rotating the pump 180 to switch ends so the current cold end will be pumping the hot and vice-a-versa to check the pump.
I also wonder if it would be best to heat the water well before it is needed in the morning to give any air created during the heating a chance to exit via the vent pipe.

 

[srmans]

Well I've checked out the flange, changed heating water heating times and flipped the pump but things have not changed. :sad_smile:

I'm beginning to think that perhaps the pump is operating near it's maximum 5 meter head and that I should just replace it with a more powerful pump.

I also noticed there has always been a pressure difference between the hot and cold sides. The cold supply from the pump seems to have a faster flow than the hot supply. Both supply pipes are effectively the same length but the hot goes via the hot water cylinder, maybe an extra 2m 'ish. I wonder if changing the down pipe from the cold tank to the cylinder from 22mm to 28mm would improve the hot flow rate. Anyone know?

 

[Ged Simpson]

22mm should be more than adequate for a single shower. Not sure why there would be a pressure difference between hot and cold as they are supplied from the same head source. You say your pump has a max. head of 5m which is only 0.5bar so not a very powerful pump. If your pump is close to this distance lower than the shower head, then this would appear to be your problem.