John Carroll

The photo shows either a mid position valve or a diverter valve (with the adjustable spanner), why/how is the F&E cistern only feeding the CH circuit, its teed into the pump inlet?, and whats the other pipe for?

Are the vent and cold feed dedicated only as a vent and cold feed?, ie, there are no returns (or flows!) teed in anywhere to either of them. My Vent from the boiler on the ground floor right up to the HW cylinder (on the landing) have all the flows teed off from it.

There should be a arrow on the filter body as well, check this is also pointing UP. Is the filter clean?

So, the magnaclean, cold feed & vent are all very close to the pump inlet (suction end) and the arrow on the pump body is definitely pointing UP??

Yes, something like that. Re your existing set up as per your line drawing, can you give a estimate of the distances starting at the vent.... vent to cold feed, cold feed to filter and filter to pump inlet.

Yes and thats why the distsnce between the cold feed and the vent should be kept as little as possible, < 150mm so no pressure loss between the two. Its illogical allright but I have seen systems like yours that display the same problems "cured" by converting to my type, with the two combined as close to the F&E cistern as possible, needs a bit of plumbing though.

That looks fine, flow from the boiler outlet (hot) with Vent then Cold feed then Pump?. Sometimes the cold feed where its teed into the flow can get partially blocked and cause problems, can you watch the vent for pump over while someone statrs/stops the boiler/circ pump, also try holding that glass full of water with the vent end immersed in it to see if the level falls. What type of filter is installed before the pump?.

I don't get that, you said, above that, "I've got a pump head of 3.0 m but the actual height of the F&E tank water level to the pump is closer to 2.0 m" ?? can you provide a rough sketch also showing where the cold feed is teed in and position of the vent.

It depends on where the cold feed is teed into the system, if its just before the pump (suction) then the pump discharge head is the "height"+the pump head, in your case, 2M+3M=5M. If its after the pump on the discharge side then the pump suction head is the height-the pump head, 2M-3M=-1M, pump running with a negative head of 1M. The rad in my converted attic runs with a very slight negative pressure, if you open the rad vent with the pump running then no water comes out but as soon as you stop the pump, water will start bleeding out, its run that way for ~ 30 years with no problems and heats up fully, as stated above, I require a pump head of 3.6M to fully heat all my rads.

The only reason I can give for noisy operation now vs with the stove is that the stove probably had a much higher resistance to flow because probably a build up of sludge etc over the years compared with the new cylinder resulting in the pump over on start up & (probably) shut down, is there any possibility of raising the vent pipe higher before it turns down?. I am very surprised still at this pump over with a pump developing only 1.0M head, normal pump head would be ~ 3/3.5M, like mine at 3.6M, same set up as yours, pumping into the boiler return but with a "combined" vent & cold feed, they are combined at the F&E cistern. (but I still retained the vent), I have a oil fired boiler with little or no resistance to flow.

If, which it is, the pump is pumping over on start up, then its more than likely that its doing the same on stopping so will eventually draw in air from the vent, your pump (I have the pump curves) will only develop a head of 1M on speed1, a bit surprising that that head is sufficient to heat all your rads, but shouldn't in itself cause that pump over except it starts up at full speed (6M) and then ramps down which I wouldn't expect it to, you said it uses a bit more power than the Grundfos, does it display this power (W), what is the power wherever you are getting it. The pump over often points to a blocked or partially blocked cold feed where it joins the flow pipe, in one of your posts above you show the cold feed & the vent where they are teed into the flow pipe, is the flow from left to right or right to left?. What was the pump power and where did you get that from?.

If you remove the vent then the now combined feed & vent must be 22mm with no isol. valve. What is the height roughly from the F&E cistern water level to the pump and what is the pump set to, speed 1,2or3?. If that height is greater than the pump head, then the pump suction could possibly be running at a negative head and eventually pulling air in, for example if the distance is say 3M and the pump head is 4M, then the pump might be running with a negative suction head of 1M. Anyway try the glass test and give a rough idea of the height and pump speed. Did you have a gas or oil fired boiler previous to the change?

With the pump running, hold a "glass" of water with the end of the vent immersed in it and see does the level start falling.

Looking at the R290 properties, 5.0Bar = 9C & 8.3bar = 24C. And 3.0bar = -6C & 12.54bar = 39.5C. The pressures in the attachment look like absolute, so add 1.0 to your pressures when (if) reading them. Refrigerant R290 Properties.pdf

Its not my boiler I have a oil fired boiler. It was running (CH only for first 2 tests) at a modulation level of 4, I think it goes to 5 max?, 16 rads, all fully open so even with flow/return of 64C/44C, dT 20C will still give a rad(s) output of 60% so no way is this boiler output down to minimum, a dT of 20C with a target temp of 64C gives a return of 44C and a nice bit of condensing. if the boiler can achieve this dT of 20C then far easier than spending hours balancing rads to achieve it?, I would say its a excellent feature if it works.