This system has already been discussed on this thread....
//www.diynot.com/forums/viewtopic.php?t=239878&postdays=0&postorder=asc&start=0[/QUOTE]
Thank you Norcon, never saw that thread, or I would have joined in.
This system has already been discussed on this thread....
//www.diynot.com/forums/viewtopic.php?t=239878&postdays=0&postorder=asc&start=0[/QUOTE]
Thank you Norcon, never saw that thread, or I would have joined in.
Looneyfitter has a nice picture on page 5 I think you'll like.
However in my opinion the system has been designed/sized to have a stand-by boiler, and only one should be running with a changeover every 4 weeks or so.
I would therefor shut one boiler down and try it with the gate valve nearly closed, until an ABV can be fitted as a replacement.
I would also get rid of the system pump, if the pipes are sized properly.
Good post GT, in my opinion slightly over the top perhaps, and I think blenders would be too restrictive.
There is the potential for problems, and perhaps why the HX keep going if one looks closely.
I mentioned several times about the gate valve and also the primary pump in the system flow being wrong, sorry I didn't say why.
Look at the pic, and think boiler pump speed modulates right down as the OP rightly said.
Next look at the short circuit between the return, through the gate valve and back into the pump which is still running at full speed.
One has to assume now because we don't know, but if the much stronger pump is full speed and pinching all the return water, (being the easiest road) perhaps it can also stall the boiler pumps running on low speed.
I can see where you are going with the drawing but, consider if you close the gate valve, the circulation pump will be pumping in series with the boiler pumps and assisting rather than stalling. Opening the gate valve won't magically stall the boiler pumps but simply reduce the assistance.As the drawing shows, the larger pump is stalling the boiler pumps when they modulate down
I can see where you are going with the drawing but, consider if you close the gate valve, the circulation pump will be pumping in series with the boiler pumps and assisting rather than stalling. Opening the gate valve won't magically stall the boiler pumps but simply reduce the assistance.As the drawing shows, the larger pump is stalling the boiler pumps when they modulate down
BTW, I think all three pumps might be similar or even the same.
Given that the boilers shut down if I close the gate valve and run with only max 8°C temperature rise if I open it wide, I am sure that both boiler pumps in parallel push much more water round the primary circuit than the circulation pump pushes through the rads. Even at part load, the gate valve stays at flow temperature so is not taking reverse return flow. Also, the temperature drop across the rads is greater than the temperature rise across the boilers. Finally, the flow temperature through the circulation pump to the rads is the same as the flow temperatures from the boilers so there cannot be reverse return flow through the gate valve.even if all 3 pumps are the same duty, the system pump will always be the stronger because the other two have the resistance through the boilers to overcome first
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