Hot Water Circulation Problems

[peterchavasse]

My plumber is stuck, and don't have much clue, so I am after any ideas that I could pass his way!

We have a hot water loop, with a Grundfos UPS 15-50 pump circulating the hot water, fitted on the return just above the hot water tank (which is downstairs). We know it is working because the hot water comes out hot in the kitchen almost immediately (this is the end of the loop), and the return pipe is hot.

The problem is that when the pump is on, we get almost nothing out from the upstairs hot taps. When the pump is switched off however, and we just have gravity on our side, we get a reasonable flow from these upstairs taps.

Has anyone seen anything like this before, or got any clues as to what may cause these symptoms?

 

[waterworks]

Hi when you say you have a hot water loop, what exactly do you mean?.
Do you have a conventional gravity hot water system ie open vented cylinder?.
Where is the return entering the cylinder?.
Is the pump fitted as a secondary return aid to stop a long dead leg?.

 

[peterchavasse]

Hi - thanks for replying.

I'll try to answer as best I can, but I'm not sure if I will use all the correct terms.

It is gravity fed - there is a venting pipe going up into the attic tee'd off the pipe feeding hot water into the tank from the boiler (which is right next to the tank) - is this open vented?

There is a 'secondary circulation loop' (I think this is what the plumber called it) to give us hot water straight away - the kitchen is some distance from the hot tank. The return comes back into the side of the hot tank on its own - no other pipework is connected at that point. The pump is fitted just before it enters the tank. Does that make sense and hopefully help?

 

[waterworks]

Open vented hot water system should be piped this way.

Cold feed to the hot water cylinder enters the bottom of the cylinder, hot water comes off the top of the cylinder which is also the open vent.

Boiler flow and returns enter cylinder at the side through a coil, this water is seperate from domestic water.

Is this what you mean you have?.

 

[peterchavasse]

That looks like what I have.

 

[trevw]

isn't he talking about a hot water supply loop with a bronze pump? Where your hot water supply is taken from the top of the tank, around the house and back to the hot tank (ideally about 1/3 down?) with 'spurs' off it for the various supply points and the pump ensures that the contents of the loop are hot at all times so you get quick hot water..

(DISCLAIMER: I am not a plumber, but have just installed one of these in my house)

there ishould be a check valve which stops the water going the wrong way round the loop.

Your situation sounds odd, unfortunately I have no idea what might cause it, sounds like the upstairs taps are drawing against the flow of the pump for some reason? (which they shouldn't be able to do if there is a check valve)

Has it ever worked properly? (has the problem just started)
Does the problem get better if the pump is on low speed?
Are your upstairs taps on the run that goes to the kitchen or the return leg from it?
Is the pump working in the right direction (towards the tank on the return)
What are the pipe sizes at the various points of the loop - is it all in 22mm? ( a little diagram might be useful..)
Where on the circuit is the check valve (I think some pumps have integral ones)

just trying to think of things that might suggest an answer..

hope you get it fixed

Trev

 

[rssteve]

sounds to me like the pump is sucking too hard on the return, and your upstairs taps which may be high pressure? when open are too restrictive, so the gravity fed hot water is just passing the open tap and returning to the cylindewr via the pump. either you need to slow the pump down, or possible fit valve on the returns to limit the flow and hence more pressure on the feed out of the cylinder.

 

[peterchavasse]

Thanks for the replies so far. Here are some answers to your questions...

trevw:
Q. Has it ever worked?
A. It did once, but the loop was extended to go on to the kitchen. They then found a problem with a loop going to the shower - this was changed to just be another spur. It still didn't work (with a very old pump) so it was changed to this new Grundfos, but fitting this has made things even worse.

Q. Does pump speed improve things?
A. No!

Q. Are the upstairs taps on the run to the kitchen?
A. Yes, I believe they are on the flow. I don't think there are any spurs to taps on the return.

Q. Is the pump working in the right direction?
A. Yes - the arrow is pointing to the tank.

Q. Pipe size?
A. 22mm

Q. Where on the circuit is the check valve?
A. I am not aware of one.

rssteve
Interesting point about reducing the flow - the plumber did try this - he said to about 1/4, and we did start getting a little bit of water out of the upstairs taps, although it was only a very small flow, and not really usable. Turning on a downstairs tap resulted in this flow disappearing.

I really appreciate your time and thoughts.

 

[trevw]

is the pipework still accessible? Trying to think of a way you could test rssteve's idea..

What kind of connections have you got under the bath and sink? is there anywhere near the trouble area that you could break in before a tap (i.e. an existing compression fitting) and stick on a bit of temporary plumbing with a full flow shutoff valve on the end so you can see what kind of pressure you've got with the pump on and off?

A check valve looks a bit like an extended brass straight through compression fitting with an arrow on it (sometimes with a screw as well)
e.g. http://www.screwfix.com/prods/86023/Plumbing/Brassware/C-x-C-V-H-Check-Valve-22mm
but maybe they are not actually required, I thought they were though.

 

[peterchavasse]

We can get to the pipes to the upstairs basin. On the hot spur to the tap there is a shut off value (compression fit) - about half way between the main hot loop pipe and the tap.

I guess it would be interesting to see if we see anything come out of this when the pump is on.

If there is too much 'pull' from the pump, do you know what we could do? What about a smaller pump?

 

[waterworks]

Hi i don't know how your pipes are laid out but if you could feed your kitchen tap from upstairs pipework and then return to side of cylinder from downstairs without the pipework dropping below the connection level to the cylinder you would be able to get gravity circulation without the need for a pump.