Non-contact water flow meter?

[Roger Mills]

I think I know the answer(!) but there just might be something out there which I'm not aware of.

I'm trying to adjust the automatic by-pass valve on my heating system so that it opens when the pump is running with both zone valves closed (i.e. in pump over-run conditions) but stays shut when water is circulating normally.

Problem is that it's very difficult to tell when it's open. It's a Pegler USV 16/22-PE which has a knob to turn to set the opening pressure, but no external indication of what the innards are doing. It's in the airing cupboard, close to the pump and zone valves, and a long way from the boiler. Listening to its outlet pipe with a stethoscope or feeling how hot the pipe gets are both inconclusive because sound and heat are transmitted from pipe to pipe even when there's no flow.

What I really need is something akin to a clamp-on ammeter which will measure the flow in the by-pass circuit without having to be plumbed in. Anyone know if anything like this exists - at consumer-level prices, of course!?

[Bob Minchin]

Could you do something with a differential thermometer. If there is no flow through any of the radiators then the temperature difference between flow and return will be around zero.

Bob

[geoff]

Vaillant have tried this - it doesn't work very well

[John Rumm]

I know its possible because I used to have a water filter that monitored the cartridge use with a flow detector of some sort. It ran from a couple of AAs, and had a pair of small metal "pads" that poked into a recess in the plastic that had the water flowing past the other side. I often wondered how it worked, but never investigated much further. I presume it just timed how long water was flowing, rather than actually metering consumption as such.

A few ideas spring to mind:

Adding a flow switch in series with the valve would obviously tell you.

Perhaps some sort of system that relies on displacement of a magnet by the flow. which you could then detect with a a read switch or a hall effect transistor.

Maybe a pressure sensor on the feed side could detect the drop in static pressure when the valve opens.

Ideally it would be best if you could do it without assistance of anything introduced into the flow... but I am not sure what distinct property of flowing water you could detect. There may be an electrical characteristic that changes - I wonder if you could make a section of plastic pipe part of a capacitor and sense a change in value with water flowing?

[Roger Mills]

Many thanks for your comments. I was really hoping for something which I could fit round the existing pipework without disturbing the plumbing - but I guessed that that would be too much to hope for!

If I do have to break into the pipework, a flow-switch would indicate *some* flow, but wouldn't give any idea of how much the valve was open. Another option, I suppose, is to fit a water meter such as

[1] in the pipe. That has a twirly bit in the middle which rotates at a rate proportional to the flow, and the counter would actually give litres per minute if used in conjunction with a stop-watch.

[1] I already have one of these - with the plastic hose adaptors replaced with 3/4"MI to 15mm comp fittings - in my mains water supply to help me decide whether I would be better off having a water meter fitted by Severn Trent. The one I have is only suitable for cold water, but a hot water (up to 90 degC) version is also available.

[Rod]

No idea what you have looked at, but saw tyhis:

Describes quite a range of metering options!

[Doctor Drivel]

Best eliminate the by-pass valve. Insert a plate heat exchanger, about £90 and the size of a book. Have the boiler's flow and return pump into it right at the boiler. On the other side of the plate have the flow and return to the DHW and CH. Put a Smart pump on this side.

Then there is always full flow through the boiler and no sludge enters the boiler, as it is isolated, protecting it. The Smart pump winds down when the TRVs are all closed, and you can have TRVs on all rads. Simple, foolproof, better temp control in all rooms, total protection of the boiler in full flow through it and no sludge enters.

[Tim Lamb]

In message , Rod writes

Ah! Thermal mass.

I have long thought that the simple Thermistor could be used for non contact sensing applications.

Mind you, BG seem very anxious to change my electronic gas meter:-)

regards

>

[John Rumm]

Indeed it would be the nice option.

Any system that introduces a passive impeller into the flow should be able to give you flow and rate indication.

Perhaps if you do make changes to the pipework you could go for an alternative arrangement where you introduce a diversion valve ahead of the existing zone valves such that in its non energised state the flow is directed through the bypass (which would not even need the bypass valve in this case). You could then "wire or" each of the relay outputs from the existing two port valves such that they energise the new valve. Hence when any call for heat is made, the existing valves will obtain position, and then call for the flow to be diverted through them. When demand ceases and they all close, the diversion valve returns flow to the bypass. (you might get a very brief interruption to the flow in that form - however if you derive the permanent supply to the aux contacts on the zone vales from their activation inputs, the diversion valve would be de-energised when demand ceases and hence should be back in bypass mode before the zones all close).

[Roger Mills]

That's a very interesting thought! I had wondered about using another zone valve in the by-pass leg, which opened when the others were closed - plus some suitable relay logic to drive it. Using a valve which is open in the de-engerised state, as you suggest, would probably make life easier - although I may still need a relay unless I simply power it from the OR'd auxilliary contact outputs of the zone valves, which would result in a slight delay before my new valve opens. I may *still* need a by-pass valve in parallel with this new zone valve to provide enough flow through the boiler when all but one TRV are closed - but it could be set tighter than at present since it should never have to take *all* the flow.

[The Natural Philosopher]

Ther is SOMETHING I remember seeing about..it relies on some combination of electrical potential and magnets to make a sort of half a wind electric motor.

If the fluid is relatively conductive. Which CH or tap water is.

The principle being IIRC that te fluid can be considered to be a series of little 'wries/ that tarvel across a magnetic field, and induce a potential on either side of the tube at right angles to the magnetic field. Probaly a few millivolts at best, even with serous neodymium magnets, but measurable.

So something like a plastic pipe with a horseshoe magnet round it and a couple of electrodes inside it to measure potential into some sensitive meter. You could test it with a DVM and pins stuck through a garden hose..and any magnets that will go round it.

[John Rumm]

Yup, but that is ok, since we are only talking about a second or two, and you still have free flow through the bypass before it does divert.

Firstly have you not got a bypass rad (e.g. in the room with a zone stat)? Secondly would not your variable flow pump not take care of this situation anyway?

[Roger Mills]

Yes, it probably wouldn't long enough to matter. The contacts wouldn't open - and allow the by-pass motorised valve to move to the open position until the zone valve(s) have *already* started to close but, as you say, any time for which the zone valves and by-pass valve are all closed should be very short.

Yes, I *have* got a rad which is always open - but only just! It's in the hallway (where the room stat is) and is really a bit larger than is needed - so it's throttled back a hell of a lot on its lockshield - otherwise that space gets too hot and turns off the whole system too early.

The question is whether - with all but one rads shut - that one throttled rad would pass enough flow to keep the boiler happy. It would actually pass less still if the variable speed pump unloaded itself.

[At the moment, my Alpha+ pump is on a constant speed setting - which is the only way I can get it to open the automatic by-pass valve when the zone valves are shut without opening it all the time - see my other thread on "Automatic by-pass valves and variable speed pumps"].