Little Clanger

Thanks for all your help, John. The Stanley had a huge boiler, so that could explain the difference. A higher vent would be difficult, but not impossible, so I'll give it a try. Thanks again John

In that photo, the flow is from right to left, so hits the 22mm vent first (although of course the pump is on the return, so drawing through the flow rather than pumping). I gleaned the power data from the labels on the pump. Not directly comparable as the Trident is derived from 220v, the Grundfos 240v. The Trident draws more current (0.25 amps compared with 0.17), which at 240v gives 60W compared with 40.

Should have been more specific. The old Grundfos is a UPS 15-50. The Trident is a WRS 25/60-130, listed as a replacement for the 15-50.

OK, John, so the height from the pump to the water level on the F&E tank is 3.7m. By 'the pump head', I assume you mean the suction pressure of the pump. The blurb for the pump (a Trident) only says that the maximum head is 6m (presumably maximum pressure when pumping on the flow). The pump is set on the lowest speed, 1, although actual rpm aren't given, and I've a feeling it runs a bit faster than the old Grundfos. It uses a bit more power. With the glass of water test, the level stayed the same, but when the pump first started up - before I'd got the glass under the vent - water was pumped out of the vent a couple of times. Previously, we had a big Stanley solid fuel range, burning wood. Thanks John

Brilliant idea, thanks. Presumably a falling level in the glass means it's drawing air in? I did try stuffing a cork in the vent - temporarily - but that made no difference. Do I even need a vent now I don't have a boiler? Presumably any expansion in volume as the water heats up would be accommodated by the F&E pipe, since even if a thermostat goes, they have a safety cut-out?

Well I did the swap on Friday and bizarrely the noise has lessened a bit. Can't understand why. Almost tolerable now, but still not right. Considering moving the pump to the flow, although the only long enough straight run of pipe for it is just above the outlet on the cylinder, where vibration-proofing support will be tricky.

Thanks, Nick, will swap and see.

...... although I still don't understand how restriction on the F&E pipe would cause the noise. It's not as if there is a continual flow once the system has filled, is it?

Pretty sure it's right way round (arrow is on other side of body). F&E tank is on left of picture. And I checked it's fully open. But as you said it offers more resistance than a gate valve, so could have been a mistake to fit it. I'll change it for a gate valve and see if that sorts things out.

Both the 15mm F&E pipe and the 22mm vent pipe are teed into the 22mm flow on the first floor. They have been like this for the last 40-odd years without the noise. The only difference is that where the 15mm pipe exits the tank, there used to be a gate valve. This had seized solid and so I decided to replace it while the system was drained down, and did so with a stopcock that I already had of the type in the photo. I assumed that any expansion in volume as the water heated up would be accommodated by the vent pipe since it teed off the same flow pipe. Since the noise has only occurred after making this change, I now suspect my assumption was wrong, but can't see how that would cause the noise. But then I'm not a plumber! I suppose I could swap the stopcock for a gate valve and see if that makes any difference, but I would like to know how. Again, thanks, Nick

For now, emphasis is on sorting out the noisy return!

OK, Nick, thanks for your input. We have a small, rural cottage with pretty good insulation and, being a retired joiner, have pretty efficient doors and windows. And we are used to low temperatures. Even when we had the Stanley range, we only ever turned on the upstairs radiators on the coldest of winter days, so the demands on the CH is extremely low. We have an Everhot in the kitchen and there is an open staircase from the kitchen to the first floor. It's quite surprising how much it helps the general temps. So a far from standard situation. I'll see how we get on this winter, as you suggested, and review things in the Spring. Thanks again for your input

Thank you both. I did consider an online boiler, but the thermal store gives us some of our heating on the overnight Economy 7 rate. An air source heat pump would be ideal (although our neighbours are worried about the noise), but I was just attempting a shoestring system that used up bits and pieces that I already had hanging around. In In fact I remember that I've got some Taco air separators in my shed. Perhaps one of those would help. Thanks again

PS. We have a woodburner that provides most of our space heating (the heat seems to spread through our little house surprisingly well). The radiators are for top-up/background

Thanks, Nick. That looks like a good idea. Is DIY installation allowed?