Negative head sensing


Postby johnheritage » Fri Jun 12, 2009 2:36 pm

Okay, in my rush to update the house, and finally rid myself of the evil low flow rate combi boiler issues, I picked up a used booster pump so I can hook up the old cylinders again and be blasted onto the floor by the shower - which, in my humble opinion, is THE ONLY WAY to shower.

The problem, I bought a positive head pump by mistake, knowing I needed a negative head version. No idea why, enthusiasm maybe.

I can fit the positive, but we have a loft conversion that I plan to stick the new TV in for people to watch films on. I don't watch much myself, but thought it'd be annoying to hear the cold cistern filling up as someone uses the water. And, I don't really want the effort of running the plumbing back up there.

Can anyone think of a smart way I could sense the pressure drop on the output lines of the pump and have it switch on? It's a Stuart Turner Monsoon pump, and the negative head versions I've seen seem to feature an expansion vessel with what looks like some form of pressure switch attached to the wet side.

Thanks for any ideas!
John
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Postby plumbbob » Fri Jun 12, 2009 8:45 pm

I was really folloing this post up to the third paragraph where you said

"but thought it'd be annoying to hear the cold cistern filling up as someone uses the water. And, I don't really want the effort of running the plumbing back up there"

then I lost the plot. Thinking I had missed something, I re-read it. Now I am thoroughly confused.

Ok, firstly, a positive head pump has to have all the pipework and shower below the level of the header tank because it needs a flow to trigger the pump. A negative head pump works on pressure drop as a trigger. The expansion tank is to maintain an even pressure to prevent the pump hunting if the shower head were to drip when it is off.

Now for the rest. How were you thinking of connecting the pump to the system. Specifically the combi? Pumps will only work with header tanks and a store of hot water. They cannot be connected to a combi.
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Postby rosebery » Fri Jun 12, 2009 9:54 pm

Your post is confusing. You say you have a combi boiler but you still have the old cylinder (and roof tanks?) in place but you don't want to run the plumbing back into the roof but you still want to use the cylinder.

Sorry but none of that makes total sense to me. For example where does the cylinder get its cold feed from?

Please describe your system comprehensively in terms even an idiot like me can easily understand. Ta.

BTW what makes you think you so certain you need a neg head pump? If the one you bought is used I guess it was cheap - fleabay? If so put it back on again and buy a neg head one rather than bodge it - assuming you need a neg head one of course which I can't tell atm.

Cheers
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Postby johnheritage » Sat Jun 13, 2009 2:29 pm

We used to have a standard cold cistern / hot cylinder to shower setup.

Then they pulled the tanks and put a combi in about ten years ago. The flow rate is terrible. There was now no hot tank and no cold cistern.

Since then I've bought some hot water cylinders to reinstate them. They're going to be down in the garage.

I don't want to have to plumb the cold cistern back into the loft, I want it down with the hot cylinders in the garage. That means I have no positive head, since the shower will be upstairs.

But I have a positive head pump. My question is, can I modify the positive head pump to work in a negative head setup? The negative head versions have something that looks like an expansion tank on them (the positives don't).
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Postby johnheritage » Sat Jun 13, 2009 2:33 pm

Nevermind, I'll just pick up a negative head version. :wink:
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Postby htg engineer » Sat Jun 13, 2009 7:08 pm

'Since then I've bought some hot water cylinders to reinstate them. They're going to be down in the garage'

So what type of boiler are you installing ? or are they electric ?

Why not renew the combi for one with a better flow rate (hot water flow rate - can only be as good as the cold water though) ?? seems to be a much simpler option. And for a 10 year old boiler, you've had your monies worth.


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Postby plumbbob » Sat Jun 13, 2009 9:10 pm

I think you can buy conversion kits to fit on the pump so it will work as a negative head.

But you know, you really are making this hard work for yourself. It's going to be difficult to plumb. How do you heat the water? Not easy from a combi.
Another thing, you will have backflow problems if the header tank is below the shower valve and most of the pumps strength is going to be used up just by lifting the water up from the garage to the shower head. You may just end up with a trickle!

As Htg says, why not just cure the problem of low pressure???? Mains showers off a combi should be excellent - far better than even a 3 bar pump, and quieter too!
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