positive head pump

Merch

Is there a rule of thumb calculation for determining if a negative head pump is really needed. Im trying to find information as the minimum inlet pressure for a positive head pumps and see if it is actually a suitable replacement for a negative head pump.

Friend has apartment, pump seems knackered, I havent looked yet, but Im trying to actually determine if they really need a negative head pump.
I dont actually want to do the work myself, just try calculate if a neg head pump is really needed.

I keep getting told I need a negative head pump, even though they are unaware of the vertical seperation involved between the tank and pump. A negative head pump, from what I read is to draw up water from a lower height (Or where the height above is insufficient to provide adequete pressure at the inlet).

Ie they have a combi cylinder and all the taps off are below this, except the shower. The Combi tank is above the level of the pump, by about a metre (thats what Im told), this would seem to suggest a positive pressure at the inlet of any pump,I want to try find out what is the inlet pressure required by a positive head pump and compare that by calculating the inlet pressure by measuring the difference in height of the pump centreline and the top of the combi.

Assuming the correct inlet pressure, either a neg or pos head pump will pump up to a certain height above itself (at sea level?)

I am aware of cavitation/that the water can boil off at low pressure and this damages positive head pumps, where a neg head pump is actually needed.

Carlow52

if the shower head is above the top of the cold water in the tank then it wont flow. This is got over by either have a + head pump with an air-switch or a - head pump.

search this forum, issued discussed recently enough..
What happens if the shower head is dropped to bottom of shower-tray,[unless its a fixed head:) ]
- head v expensive vs + pump

Merch

I mean if there was a positive head pump in that system and it had enough inlet pressure then surely it would pump up to a height above itself, in this case a very small height (or is it only to the actual height equivalent of the head?)
not sure what you mean by the last bit
are you suggesting fit a +head pump and anytime the person showers drop the shower head into the bath before turning it on?? and comparing that to the price of a neg head pump?

Carlow52

for a + head pump, if the natural flow is not enough the flow switch wont click in.

To answer ur question yes, if all the other stuff works.

After all the shower head will prolly be taken down to wait for the hot water unless ur pal likes the cold shot to start with:D

Can u get the exact model of current pump?

This may run better over in plumbing

Merch

I was told it is a techflow QT45? I'm not 100% sure it is neg head pump as i need to go over and have a look, she contacted me as she has had a few people charge her to fix it but with out success over the last few months, last guy quoted her a figure she thought was too much so she asked me to look into it, I havent even got over to look at the pump, but it does appear that neg head pupms aint cheap.

I was wondering is there a quick way to calculate if the pressure presented at the inlet is suitable for a + head pump.
Does a pump (either pos or neg head) tell you in its specs what the inlet pressure needs to be or be between.

I was thinking something along the lines of P= density x height x gravity? is that calc used?

I did ask in plumbing, it ended up as definitely need a neg head pump, but after further reading and your info (re + head pump with an air switch), Im trying to find out if it is absolutely essential, maybe this air switched + head pump is cheaper?? and will work, I'd like to try find a calculation for the pressure at the inlet, measuring the height difference between the pump and the top of the tank supply, I think P= Density x gravity x height should do and can convert the answer from Pascals to PSI (using online calculator )

Is the air switched positive head pump automatic operation? I assume so

Carlow52

Air switch is manual, like a bell push, fitted in shower, u press it and it fires the pump, its air as its in a wet enviro..

re head see here:
http://www.plumbworld.co.uk/link/1/pwte1002_16649_t.pdf

Flow is the key, sufficient differential pressure will cause enough flow: as to the 'math' have had 3 glasses wine with dinner so pass...:)

Joey the lips

Brilliant site this

http://www.salamander-pumps.com/how-to/install-negative-head-pumps.html

There is a how to section explaining positive and negative head pumps. The rule of thumb i always when with is there must be at least 3 ft from the top of the shower head to the bottom of the water tank.

This means combnation cylinders will ALWAYS be negative head because the shower head will ALWAYS be above it or a combination tank is not needed.

Where the distance really comes into play is in attic conversions if the water tank is being moved in under the rafters.

Merch

ok, thanks for the replies.
I'm thinking the idea of them pushing any switch before showering is out, so they will have to get the cash together and get a negative head pump and pay someone to install it.

and thanks for the links, they explain it in a way that is (visual) easy to understand (for me ).