Pumping over - on hot water only

[John.g]

Should have seen the link to the tower separator, if clean then there should be, theoretically, no pump over at that vent/feed distance of only 50MM, of course it is also a perfect dirt trap but unfortunately cannot be cleaned out easily as there are no compression fittings, if mine, I would consider removing it altogether and just have the vent....cold feed.....pump, or else replace it with a new one if old one cannot be cleaned out.
I would suggest there is definitely a partial blockage there as the pump over only happens with a high flowrate which means a higher pipe velocity and leads to venting.

Re pump readings, LPM = M3/hrX16.666 or if you prefer LPM = l/secX60.
My pump curves are meaningless now as you have a 5M pump (not a 6M) and the settings are a bit confusing, especially the constant pressure CP ones, your attachment seem to indicate that CPi is 0.8M, CPii is 2M and CPiii is 4M, can you just confirm that on your pump that setting CPii is flashing 3.8M on startup? because if its only 2M then there is a big problem with blockage(s).

 

[Rory]

Thanks again.

If you look at the picture of the pump in the link earlier, it's got 4 segments with I, II and III between the segments. I've got it midway between II & III. On CH only, from a start (turn room stat up to open the valve) it flashes 3.8M and settles quickly to 18W.

 

[John.g]

Ok, on CH only, the flow rate is ~ 6 LPM
on HW+CH, the flow rate is = or is > 17 LPM.
At 6 LPM the deltaT through the boiler is 29C and at 17 LPM is 10C so on CH only the pump cycle time or overrun time will have to be sufficient to allow the return temp to be at least 30C and preferably 35C or more below the boiler SP to give it any chance of a few minutes run, if the boiler SP is say 70C then the return temp should be ~ 35C or lower on boiler re firing. If the room stat (if installed) isn't satisfied then the pump will continue to run so the cycle time is important and if room stat is satisfied then the pump overrun time is.

 

[John.g]

You said in post #15 that the pump runs on for 3 minutes when the boiler hits it's stat, I take it you mean by this that the pump continues to run (as it should) and the boiler doesn't fire up for 3 minutes, the anti cycle time?, the pump may stop/start briefly for some boiler check?. When the house/bedrooms etc are up to temp what is the normal boiler cycle time? (room stat still calling for heat) and what is the boiler SP temperature?.

Another point although not hugely important since you run on CP mode is that when you ran on max PP mode, it controlled exactly as it should with HW+CH on but the flow fell to ~ 3 LPM with CH only on, it should have controlled to almost exactly (very slightly higher) at 4.2M and 6.3 LPM the same (almost) as on CP mode,

 

[Rory]

Does SP = set point? There’s just a dial with blobs and it’s backed off a little from max. Have an old analogue pipe thermometer on the flow above the boiler and it gets to just under 70C, then drops to 55C in the 3 mins the anti-cycle time (pump keeps running). When the boiler re-fires the flow temp on ther thermometer continues to drop to 50C before climbing again. Running at the moment for about 10 mins, but it is 9C outside here now (21C in the house). One thing noticed is the return is all but cold to touch.


I watched for the water reheating as wife used a load of hot water this morning. Was dismayed that when the HW valve opened water trickled out of the vent. Had to back the head off to 3.3 (26W) to stop it. Noted the return at the boiler was very hot when HW being reheated.

Now it’s reheated, and back on CH only, the pump is down to 16W (still 3.3 head).

I’m surprised by the low LPM figure for CH – what’s ‘normal’? 6LPM (I guess a bit less now I've turned it down) doesn’t sound much and I’m surprised at that flow rate that I can open up a TRV and hear water rushing in.

 

[John.g]

Yes, SP = setpoint, 70C seems OK, the return will get very cool when all the TRVs close in, theoretically, if no permanent by pass then the flow can go to practically zero, when I installed my Wilo Pico two years ago I took a good few readings with the circ pump on but (20 kw oil fired) boiler off and with just any one rad only full open I was getting a minimum of 5/6 LPM and with 2 or more full open ~ 7/10 LPM. so I would suggest making a note of say 3 rad TRV settings and then opening them fully for a few minutes and monitor the watts/flowrate. The boiler seems to be behaving OK though even with that small flowrate, your main problem IMO is probably a partial blockage in that Tower, I have a combined Vent&cold feed in my system which I consider to be the very best arrangement in a open vented system, my system is spotlessly clean with some of the original rads after 40+ years with only the odd drop of inhibitor added.

From your boiler spec, here are the recommended flow rates.

"Systems with Two Port Valves Where a pair of two port zone valves are used, the total length of the by-pass circuit taken from the boiler connections should be greater than 4 metres of 22 mm pipe. The bypass should be capable of maintaining a minimum flow through the boiler of 4.5 litres/min (1 gal/min). Systems with Thermostatic Radiator Valves (TRV’s) Where mechanically operated thermostatic control valves are used, the total length of the by-pass circuit taken from the boiler connections should be greater than 2 metres of 22 mm pipe. The bypass should be capable of maintaining a minimum flow through the boiler of 9.0 litres/min (2 gal/min)."

 

[Rory]

Only the upstairs rads close their TRVs - the downstairs ones, apart from the hall, are wide open. The biggest rad is a long single panel under the living room bay window, which is piped in a long and tortuous run of 10mm Hep2O, and that radiator gets very hot (the room needs it) even at its return end.

Anyhow, I now understand the pump isn't constant speed so I understand how it's able to pump over when HW in on. Bit of a pain as I'd like more flow through the boiler on CH to stop the boiler singing but I can't have that as it messes up when HW on.

The air separator is almost behind the hot water cylinder and that would have to come out to get at it. We do have plans in to extend and refurb the house and certainly the Suprima boiler will be getting ditched at that point and I have a space earmarked for a Megaflo (or similar).

 

[John.g]

I would choose a boiler that can modulate down to 2kw, the majority of boilers, even 15kw ones will only modulate down to 4 or 5kw including the Baxi Megaflo? and some make a awful meal of cycling on/off, they just don't like it, at least your old Suprima can cycle away happily even though its a non modulating boiler.

 

[Rory]

Sorry for confusion - I was using Megflo as shorthand for 'unvented hot water cylinder'.