CH Pumped Return OK?

[TheScullster]

Hi all

My present S Plan CH system has a pumped return rather than flow - is there any reason why I should swap? I am getting quotes for a replacement condensing boiler and intend to do some re-piping at the same time. Does this have any impact on the answer to the above question? ie pumped return OK with new style boilers?

The system currently suffers from pump-over which I would like to resolve. As the pump is in the return to the boiler, the feed to the upstairs HW cylinder continues past the cylinder to become the vent pipe. The motorised valve connection to the cylinder is teed off this flow line, so does not obstruct the clear vent. But the water make-up into the primary circuit connects to the return from the cylinder coil. So, if the hot water motorised valve closes, it effectively separates the vent from the water make up (this I believe may be causing the pump over). Is there any reason why I shouldn't move the water make up connection to the other side of the HW valve? This would position it in the flow/vent line close to the tee for the cylinder coil supply. If I do this, it would be similar to a combined feed and vent arrangement, but with the feed entering at HW cylinder level rather than at the header tank IYSWIM.

Hope this lot is clear

TIA

Phil

[Martyn Pollard]

Is there any reason why you aren't having a sealed system? It would make life a lot easier for yourself and simplify the heating system. Also, low water content boilers tend to operate better (quieter) on a sealed system anyway.

Martyn

[TheScullster]

"Martyn Pollard" wrote

Thanks Martyn

I am hoping that someone with serious experience of vented systems will comment here! If I can stabilise the system and prevent the pump over, modifying the existing layout will be the most "comfortable" route.

Yes I have considered/am considering a sealed system, but I am concerned about the age and state of the piping and its ability to cope with increased pressure. The ground floor is solid concrete with buried pipes and some plastic has been introduced upstairs (not sure if it's barrier quality). The original install is 1970s and this is probably the worst period in UK history for build and service quality IMHO.

Phil

[Andy Hall]

On Fri, 30 Jun 2006 08:28:17 +0100, TheScullster wrote (in article ):

I've installed and worked on both and migrated from a vented to a sealed system.

- If you want to retain an open vented system, then the solution to pumping over is to make sure that the vent and feed/expansion pipes join the circuit close together (within 150mm); or more conveniently use an air separator like a Myson Aerjec to do the hookup. The important things to avoid are certainly having them connect on opposite sides of the pump and almost certainly on opposite sides of the boiler. Putting both on one side of the boiler avoids the pressure differential that can otherwise arise.

- Making a system sealed does not mean that you are taking it up to mains water pressure or anywhere close. You are going to take it from around 1bar to 1.5 to 2bar. If the pipes are going to have a problem with this (which I think is very unlikely), then it won't be long before they do with a vented system.

- If there is going to be an issue, it is more likely to be radiator valves leaking and seeping around the stems. As part of a system refurbishment, it would make sense to proactively swap these out anyway. The system is going to need a thorough cleaning, and the summer is the perfect time to take the radiators outside for a thorough pressure wash. The whole process of doing that and swapping valves can easily be done in a day.

- This cleaning should be done regardless of whether the system remains open vented or goes sealed. What I did last time was to take the old boiler out of service (switch off but leave installed), swapped in the sealed system components. Then I had mains pressure available to flush the pipes as the radiators were removed. Considering you have buried pipes downstairs, this would be a good way to get the crud out of them.

[TheScullster]

"Andy Hall" wrote

............snip

Many thanks once again for such an informative response Andy. Can I prevail on your time/kindness and ask you to look at the schematic

please? This shows the flow from the boiler to the vent and the relative position of the make up feed. It looks like I do indeed have the worst case when the hot water motorised valve is shut. With the valve shut, both the pump and the boiler are "between" the fill and vent connections. ie fill to pump suction, vent after pump and boiler. To get around this, can you see any problem with moving the fill to the point where the water tees from the flow/vent to the h/w cylinder? This looks like it would create an arrangement close to a combined fill/vent but not quite. (The schematic shows system items at approx the right relative elevations).

Thanks again

Phil

[Andy Hall]

On Fri, 30 Jun 2006 14:12:57 +0100, TheScullster wrote (in article ):

I had a similar arrangement to what you are describing when the system was open vented except that the pump was on the flow side after the vent tee and before the motorised valves.

If you're going to do this feed repositioning, I would use an air separator like the Myson Aerjec.

part no. 11334.

Mine was set up like the third diagram, although any will work. The fittings are copper tails which will go into any solder fitting.

The other advantage of these is that it makes initial filling/venting of the system much easier since the small bubbles of air that initially tend to circulate with the water can escape easily through the vent and tend to end up less in the radiators.

[TheScullster]

"Andy Hall" wrote

Thanks a lot Andy I will have a look and see how this would map-in to the existing piping

Phil