Pumping over - on hot water only

[Rory]

Got an open vented system with two 2 port valves and a fairly recent ‘electronic’ pump, set to fixed speed 2.

In the recent colder weather, I turned the boiler up a bit and then became aware the boiler was singing so turned the pump to 3.

By chance noticed an icicle hanging from the expansion tank overflow. Got up In the loft and with a bit of trial and error determined that it’s pumping over, a steady trickle, when the HW is on – makes no difference if the CH side is running or not.

I’ve put everything back as it was and thankfully it’s all fine again.

Any ideas why it would only pump over when HW is on?

 

[John.g]

Presume it was the boiler flow temperature you increased?.

Can you post the pump make and exact model and may be able to suggest a mode with a setting in between 2 and 3.

You possibly have a partial blockage somewhere as well depending on where the cold feed is teed into the system

 

[Rory]

Thanks, yes, I upped the boiler flow temp.

This is the pump: WILO YONOS PICO 4169842 CENTRAL HEATING PUMP - Harworth Heating - https://www.harworthheating.co.uk/product/wilo-yonos-pico-4169842-cantral-heating-pump/ (I got it from Screwfix for about £100 but they don't list the same one now).

I have it set on II.

I just realised that the power display on CH is 18W, When it HW is on, regardless of CH status, it changes to 24W. At 25 ish, it pumps over.

I tried it on max proportional setting but on CH only it drops to 11W and the boiler cycles like mad.

 

[John.g]

I have a model very like that, a Wilo Yonos Pico 1-6 (a 6M pump) but it differs in that fixed speed i,ii &iii are in the middle and PP mode is on the left, like yours and CP (constant pressure) mode is to the right of the three fixed speeds, you are showing the 3 fixed speeds on the right but it is also showing a rectangle with the CP symbol (a straight horizontal line) so wonder how do you get CP mode??.selected
I am also surprised that on full PP setting that the power is only 11W as on mine you can incrementally change the PP (or CP) settings from 0.5M to I think 5.5M, in 0.1M steps, you should see the selected head flashing when you change the setting, after a few seconds it then reverts to watts W. So a PP setting of 5/5.5M should IMO pump as much as a fixed speed setting of II,
I have mine set to a setting of PP4.5M which = 3.5M on CP and ~ 18/22 W on CH and ~ 22/24w on HW only or HW&CH.
You might first try to run in CP mode and see does it behave better, put the boiler to CH only and increase CP setting (note the "flashing head" to see can you get say 16/18W then put the HW on as well and note the watts W, then
switch off the CH and note the watts, W.

Constant Pressure (CP) Mode

Looking at my own pump curves......on max PP setting then 11W indicates that it is pumping nothing.

Proportional (PP) Mode

 

[Rory]

I have a model very like that, a Wilo Yonos Pico 1-6 (a 6M pump) but it differs in that fixed speed i,ii &iii are in the middle and PP mode is on the left, like yours and CP (constant pressure) mode is to the right of the three fixed speeds, you are showing the 3 fixed speeds on the right but it is also showing a rectangle with the CP symbol (a straight horizontal line) so wonder how do you get CP mode??.selected

I think it's nothing more complicated than proportional on the left and fixed on the right. Top centre is vent.

To be honest, I was thinking it was fixed speed, but if it's fixed pressure then it makes sense that the pump goes faster when the HW is on as its flow is relatively unrestricted.

I also hadn't realised the display showed head - I thought it was just power. Pump is near the bottom of the airing cupboard so I suppose it's only going to be ~3 metres to the water level in the expansion tank (and therefore the vent).

 

[John.g]

3M to the water level has nothing to do with the (a) pump head, the pump head is the differential head (pressure) developed by the pump between the suction and discharge and depends on pipe losses, boiler Hx losses, valve losses etc, you would seem to have restrictions somewhere in your CH system hence the boiler cycling.

Did you note the head settings as you are/were changing the CP settings??

 

[Rory]

3M to the water level has nothing to do with the (a) pump head, the pump head is the differential head (pressure) developed by the pump between the suction and discharge and depends on pipe losses, boiler Hx losses, valve losses etc, you would seem to have restrictions somewhere in your CH system hence the boiler cycling.

Did you note the head settings as you are/were changing the CP settings??

Ah, thanks - I'm very out of my depth here!

At the moment 18W is 3.8M - the house is pretty warm now and a fair few of the TRVs almost closed to reduce their radiator temps. If I wind the cylinder stat round to bring the HW on, the power is going to 33W now and the head looks the same 3.8M.

I think the system cycling is just that it's an old fixed output boiler and the house is a little under-radiatored as well as being a not very well built 60's house. It's generally OK but struggles when the temp is around 0C.

 

[John.g]

Ok, thanks.

You would seem to be circulating 6.2 LPM on CH only and ~ 23 LPM with CH&HW.

Are those readings on speed II?,

Now, if you feel up to it can you change to PP 5.2M and note the watts on both CH and CH+HW.
You should get around the same readings as you did on speed II, you can then change back to speed II if you like.

 

[Rory]

Ok, thanks.

You would seem to be circulating 6.2 LPM on CH only and ~ 23 LPM with CH&HW.

Are those readings on speed II?,

Now, if you feel up to it can you change to PP 5.2M and note the watts on both CH and CH+HW.
You should get around the same readings as you did on speed II, you can then change back to speed II if you like.

Yes, those readings were on II.

5M is the max in PP at 11W on CH. Add HW it goes to 33W.

What confuses me here is the pumping over on HW seems counter-intuitive. It would be understandable (to me) on CH but on HW you'd think it would be busy enough circulating the water to bother with pumping it out of the vent.

 

[John.g]

Its the way PP mode works, even though its set to 5M, on CH only when it sees a flowrate of 6.2 LPM at 3.8M it will ramp down the speed slightly until it sees ~ 5.9 LPM at 3.5M, practically the same, but when it sees a flowrate of 23 LPM at 3.8M then it ramps up to max at ~ 5m and that's why the pump over happens, the proof is if you change to CP3 at around 5M? then the pump over starts again.

Of course it shouldn't pump over at any head but that depends on the pipework and the way the cold feed tees into the system with relation to the vent, mine is a combined cold feed and vent where the cold feed is teed straight into the vent at the header tank with the circ pump on the return to the boiler and can't pump over, another method is where the pump is mounted on the boiler flow side and you have from the boiler, vent, cold feed and pump.

 

[Rory]

Well it was pumping over on III before during multiple trys so no reason to thing it won't do it again.

The pumps max rating are 5M and 33W, by the way so it's flat out on max PP setting. So I'm suspicious about the head reading when it was on II with both CH & HW, I'll recheck that tomorrow.

Feed and expansion are taken from an air seperator in the airing cupboard, just before the pump.

 

[John.g]

That's correct, I misread the flow rate on speed II on HW+CH, its actually ~ 13 LPM (not 23) but that doesn't change the PP flat out output, its looking for 0 flow at 2.5M and ~ 14 LPM at 5M so it just ramps up. CP2 will never be higher than 3.8M (if that's its CP setting) but could be lower if running flat out but you won't be able to see the actual head and CP3 is a constant 5M so you can't run on it.
The air separators are notorious for sludge build up, some say they should not be fitted and should actually be removed, you are getting pump over for quite awhile I would think judging by the icicle?, you might post a photo of the vent & cold feed set up sometime.

 

[Rory]

The air seperator looks just like this one: Air Separator - 22mm with Cold Feed - https://www.bes.co.uk/air-separator-22-mm-with-cold-feed-18639/ - it's on the floor of the airing cupboard and with the expansion in 22mm over the expansion tank and a separate 15mm feed.

With the pump back on II everything seems normal with just CH or both on. However I'd really like to have more flow through the boiler on CH, because as the system gets hot it's cycling pretty frequently and the boiler does a thing where it stops and restarts the pump every time it fires. This was the reason I stopped using the PP settings as it starts at full speed and backs off.

I didn't realise it does the same on CP (as I thought it was speed that was fixed, not pressure) and I think if both CH + HW are on then starting at full speed it might be momentarily pumping over - although I haven't caught it doing that yet.

The bathroom radiator is plumbed as a bypass - might it help to open that more? It's only minimally open to stop it short-circuiting the rest to the system and when the kids were small we didn't want it too hot.

 

[John.g]

The cold feed should be after the vent/separator but no more than 150MM from it otherwise you can get pump over particularly if that separator is partially blocked.
These pumps certainly don't start up at full speed, they take 3/5 secs, I've often watched mine with a energy monitor, each time it starts/restarts the set head will flash for ~ 3/5 secs, during this time the pump is ramping up in ~ 2W stages to its final position and sits there then until the flow changes due to TRVs throttling in or whatever.
Even with only one rad fully open (all the rest shut off) you should get a flowrate of ~ 8 LPM at that 3.8M head so even if all your rads are throttling in on their TRVs I would expect 10 to 12 LPM flow like I get with my 8 TRVd rads, you are only getting ~ 6 LPM so I would just open up one or two fully and note the watts on CPII, also when you start up in the morning when all the TRVs are open then you should get ~ 12 LPM or ~ 23/25 watts on CPII.
Don't know what the output of your boiler is but if you assume 20kw then at a flowrate of 6 LPM the deltaT through the boiler is, theoretically 47C, and it will cycle on/off very rapidly, if the flowrate was 10 LPM then the deltaT is 29C which helps.
Pump overrun after boiler cut out would help as long as the overrun time is ~ 3 to 5 minutes through that by pass rad. If you have zone valves then that rad should be piped with the flow upstream of the valve.
,

 

[Rory]

The cold feed is just as shown on that air separator – so it’s 50mm from the vent. Pump is just after it.

You’re right about the pump – in fact when the boiler is re-firing and doing its pump wiring check, the pump does little more than blip, and it comes back showing the same watts as before, no head reading. It’s only from a ‘full’ start that it shows the head then switches to watts. I think I was remembering how the previous pump, a Grundfoss Alpha, worked – that used to start with a “whoosh”.

We have quite a small system – just 8 rads plus the bathroom as a bypass (it is before the 2 port valves). All the rads are medium size single panels. The boiler is the infamous Potterton Suprima – I think it’s something like 12kW. Really, once the house warms up, the bedroom TRVs pretty well shutdown so that takes 3 rads out.

To my mind, the lack of flow through the boiler is making the boiler too readily hit its ‘stat, so the pump runs for 3 mins and, in colder weather, it’s amazing how much heat the system loses in that time.

The graphs baffle me – are you working out the LPM from m/s ? The graphs for my pump are a little more squashed than yours: https://atacsolutions.com/eshop_uploads/wilo/wilo_yonos_pico_25_5_130.pdf