Converting gravity fed to fully pumped

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I want to convert a gravity hot water system to fully pumped to include a new cylinder. Complicated by the fact the old cylinder is a primatic. Current system layout is cold water storage tank in attic, primatic cylinder on 1st floor and boiler on ground floor. This is what I plan do.

From the boiler use the current HW flow and return pipes for the supply pipes and cap off the old CH pipes. As they are 28mm reduce them to 22mm. Add appropriate valves to feed HW and heating and room and cylinder thermostats.

The pump is currently on the CH return pipe so this will be moved to the flow pipe.

Put new f&e tank in attic. However, although it’s a primatic cylinder there are already two vent pipes which both currently discharge into the cold water storage tank. One vent pipe is teed off the HW outlet from the cylinder. The other is teed off what I assume is the HW flow pipe to the coil in the cylinder. Therefore, can I simply move this vent pipe over to the new f&e tank or does this pipe need to run all the way down to the boiler?

Other issue is where to connect the cold feed from the new f&e tank into the system? Could this be teed into the return pipe close to the cylinder coil or should it be run all the way down to the boiler? I understand that there is some issue about positive/negative pressure with regard to where the vent pipe, cold water feed pipe are connected in relation to the pump.

Not sure whether it's relevant but the CH is a single pipe system.

Thanks for any responses.
 
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Lsst thing - single pipe system - is a worry.
How old's the boiler?
CHanging it in the next few years likely?
New boilers don't like single pipe systems, so it's worth thinking about changing that to 2 pipe now. Probably sounds lke a nightmare, but often not so bad once you've worked out where all the pipes are and where they need to be. I've seen it done piecemeal, which sounds barmy but can "work" while you do a couple of rooms at a time.

Next thing would be to pressurise the system and do away with the header tank. It does make a cleaner quieter system, generally thought to be more efficient. Probably is, but no figures...
Many new boilers insist on it.

The normal gravity fed, fully pumped system is like this, splits are for CH and HW, but that's still about 30 years old advice....

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Lsst thing - single pipe system - is a worry.
How old's the boiler?
CHanging it in the next few years likely?
New boilers don't like single pipe systems, so it's worth thinking about changing that to 2 pipe now. Probably sounds lke a nightmare, but often not so bad once you've worked out where all the pipes are and where they need to be. I've seen it done piecemeal, which sounds barmy but can "work" while you do a couple of rooms at a time.

Thanks for the reply. The boiler (Ideal Mexico) is 'only' 15 years old and therefore assume it has some life left!

I do plan to change to a double pipe soon as the system is painfully slow to warm up. Almost all the pipes are above ground so it should be not too bad-just time consuming.

Thanks for the diagram, so basically its boiler-vent pipe- cold water feed pipe-pump. What size should the pipes be - 22mm for vent and 15mm for cold feed?
Thanks again.
 
Don't even think about it.

Rip the lot out and start again.

Moderns combi and unvented cylinder if you have a good pressure
and flow. Or a glow worm ultrapower a cross between combi and unvented
cylinder.

Don't even mess with your old system waste of time.
 
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Check exactly which model of Mexico you have, but I think at that age you should be able to go straight to a pressirised system. I would, it's easier. (It would have to have a high-limit, aka Overheat, thermostat). You would need to add a 15mm pipe poking out through a wall as an overpressure relief.
One benefit is the ease of filling and emptying, which in your case will matter!

But yes, 22mm vent and 15mm feed is the standard.

Mexico 15yr old... You could probably save around 15 -20%, on your bills with a fairly modern system. More if it has no fan, more again if it has a tall flue. The sums can be compelling.

Having had a primatic, it'll probably be fairly rusty/sludgy inside.


Edit - if it were mine I'd certainly do what DC suggests, but I suspect you may not be wanting to do that ;) So I started off with what you asked!
 
Check exactly which model of Mexico you have.

Edit - if it were mine I'd certainly do what DC suggests, but I suspect you may not be wanting to do that So I started off with what you asked!

The boiler is a Super Mexico RS80.


I appreciate the comments about replacing the whole system, but at the moment I want to keep it :). I'm keen to DIY it and obviously could not put in a new boiler!

I assume that how I plan to do it is as described in the first post is OK. I have two other questions:

1. I've noticed that when the HW is on without the heating the lower pipe into the cylinder coil warms up first. I would assume that the higher of the 2 pipes into the cylinder would be the flow and the lower would be the return, but if that was the case shouldn't the higher pipe warm up first?

2. with regard capping off the old flow and return pipes I can only cap them about a meter from the back of boiler and therefore will that be OK?

Thanks
 

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