Anti-return or check valves on secondary circuit

[crosbie]

I've only a single check valve before my circulating pump (for secondary DHW circuit).

However, I've noticed a problem. When I have my pump turned off (to save heat loss), I find that for a given tap (especially the shower), when the hot water arrives, it is warm rather than hot, and only later reaches the hotter temperature.

I've realised that there is still cold water coming via the secondary circuit which is mixing with the hot, and thus it is only when the secondary circuit is hot (including the primary filling it), that both primary and secondary pipes are hot.

Thus I need a check valve on every spur of the secondary circuit, not just one at the pump, to make sure only the primary supplies each tap (when the pump is off).

I'm wondering what kind of check valve has the lowest cracking pressure? I need to not only consider the low head of the circulation pump, but also the problem that occurs if only one check valve opens thus preventing every part of the secondary circuit from flowing.

For inexpensive valves, I think I have a choice: a) A swing check valve, or b) A universal check valve with the spring removed.

If I have five spurs of 10mm secondary pipe work, I'm thinking a 3/8" swing check valve on each spur should be fine.

What do you reckon?

 

[chris watkins]

I reckon you have a different problem crosbie.

It sounds more like you have a mixer valve some where that is feeding cold water into the hot water system, it may well be using the return to travel to the fitting in use but it should not require fitting a check valve to each secondary connection.

Check all the shower (& other) mixer valves if you can to find the one which either has not been fitted with the single check on the H&C supplies or where it is stuck open.

 

[jonnyswamp]

http://i.stack.*********/Cq17A.gif
Look at this image, this is how your secondary circuit should look like, albeit you'll have a cylinder instead of a water heater, the principal is the same
You don't need any extra check valves in the circuit, but you should have them before any mixing taps or shower valves as these can allow cold water into your secondary return cicuit and cause the problems you're explaining

 

[oz-plumber]

Or the simple solution would be to change the existing check valve and see if that fixes the problem.

You would not draw much, if any, water from the return line when you have the pump switched off if the existing check valve is working correctly.

 

[crosbie]

In my configuration, it's more like a star topology than a ring topology, and all the secondary pipes are joined together not far from the check valve at the pump.

So, given that all secondaries can flow in both directions, by being fed from their respective primaries, I'm fairly confident it's the cold secondary flow that's mixing in with the hot primary flow that's creating the triple phase of cold->warm->hot (20/35/50C).

I know that the source of the primary is at high temperature, so it can't be a case of a mixing valve taking time to react.

Anyway, I'm pretty much set on installing check valves on each secondary, where they fan-in to the pump inlet (removing the redundant check valve at the pump).

So, my primary question concerns the choice of check valve, and whether there are likely to be balancing issues of 'one open, others closed'. If no-one has ever installed check valves on each secondary spur then that would be interesting to know - and any reason why not.

The normal case, when the secondary circuit is being pumped, is of course working fine. I'm just interested to be able to revert to primary-only during certain low-use periods.

 

[jonnyswamp]

Draw your pipe work layout as best you can and post it up

 

[chris watkins]

So, given that all secondaries can flow in both directions, by being fed from their respective primaries, I'm fairly confident it's the cold secondary flow that's mixing in with the hot primary flow that's creating the triple phase of cold->warm->hot (20/35/50C). Using your language, both the primary & secondary hot water pipework is fed from the same source (mains or cistern), so are under the same head pressure, for water to move between them (HWS F&R) there must be a difference in that pressure.

I know that the source of the primary is at high temperature, so it can't be a case of a mixing valve taking time to react. It could be back feeding higher pressure cold back through the valve somewhere else & into the lower pressure hot, causing the drop in temp.

Anyway, I'm pretty much set on installing check valves on each secondary, where they fan-in to the pump inlet (removing the redundant check valve at the pump). You will be wasting your time & money IMHO, if you must, swing checks are normally used but of course they can only be installed in the horizontal or vertically with the flow going up, otherwise gate just falls open. What you will need to do in install balancing valves to adjust the circulation from each.

With the secondary pump off & return cold, get someone to run the fitting that is causing the problem, at the same time hold the secondary return pipe close to the pump if you are correct the pipe will get hotter as the water is drawn out of the cylinder & up the the return, if there is no real increase I am right & it is a back feed.

Good luck.

 

[chris watkins]

Just a silly thought, you have checked that the check valve you have installed at the moment is not stuck open? (non-return valves are normally installed between the pump & the cylinder)

 

[crosbie]

With the secondary pump off & return cold, get someone to run the fitting that is causing the problem, at the same time hold the secondary return pipe close to the pump if you are correct the pipe will get hotter as the water is drawn out of the cylinder & up the the return, if there is no real increase I am right & it is a back feed.

Good luck.

That's a good idea.

Moreover, there is also a shut-off valve for the secondary from the shower, so I can close it, and see if it makes a difference.

I may get round to posting a diagram of my water circuit in due course. Incidentally, it is a mains fed DHW supply, indirectly heated via a coil within a heat store.

Thanks for the tip re swing checks.