Secondary return

[Johnyc86]

Hi
Just wondering can anyone tell me why a secondary return can not be plumbed with plastic pipe like pex or pb

 

[Scott_D]

Pipe manufacturers say it’s not suitable

 

[exedon2]

I know quite a few people at polypipe apparently it's the constant temperature that's the problem.
The tube is designed to expand then cool not be constantly expanded.
Apparently strips the inner core out .

 

[Ben-gee]

Interesting exedon; so it sounds like it should be ok in a domestic situation with a timed or PIR operated pump- but not in a constant or commercial setting.

 

[oz-plumber]

Yet the same type of pipe can be used in heating installations at presumably slightly higher temperatures and slightly lower pressures.

I've looked into this situation fairly deeply, a couple of years ago, and can find no manufacturers ( the ones I was going to use - not all ) that say their pipe can't be used in this situation.

If their pipe can't be used for secondary return systems, you would think they would emblazon the the fact that their product is not suitable.

Contact the Manufacturers of the product you want to use and get confirmation either way.
If they respond ( some didn't in my situation ), use the one's who say their product is suitable.

 

[SimonG]

It's the level of oxygen in the water as well as the temperature.

 

[Chuck]

I've looked into this situation fairly deeply, a couple of years ago, and can find no manufacturers ( the ones I was going to use - not all ) that say their pipe can't be used in this situation.

Hep2O is one that says 'no':

Push-Fit Hep2O: Your plumbing FAQs answered

We've compiled the most frequently asked questions about our Wavin Hep2O push-fit plumbing system.

blog.wavin.co.uk

"No. Continuously operated re-circulating systems (Secondary Hot Water Circulation/Ring main installations) are very different from conventional hot water supply and central heating systems found in domestic properties, for which Hep2O products have been tested."

 

[snowhead]

Polypipe say no, see note 1 bottom of table


.......................................^^^^^^^^

 

[Ben-gee]

Yes, but hep says “continually operated re-circulating circuits”

I would suggest this is not what one has in a domestic secondary return situation.

 

[ShaunCorbs]

Mlcp or copper only plastic is a no unless it’s on a pir sensor and max run time of 5 mins which isn’t enough for a src

 

[Ben-gee]

Do you know the run time to get hot for 12m of 22mm , and what size is best for the return in this situation?

I know 12m is about 4 litres, but I’m guessing you have to flow much more than 4litres through to replace all the cold with hot?

 

[EvilDrPorkChop]

I would suspect on a domestic system that would run maybe 2 hours in a morning, 1 hour at lunch and maybe 2-4 hours at night it would be fine. In fact there's systems that I know of that are like this. I think the main issue is when it runs 24/7. But it's certainly an interesting topic as no manufacture can give you the main reasoning why.

 

[ShaunCorbs]

Do you know the run time to get hot for 12m of 22mm , and what size is best for the return in this situation?

I know 12m is about 4 litres, but I’m guessing you have to flow much more than 4litres through to replace all the cold with hot?

0.11 per m for 15mm so 12m is around 1.4l

it all depends on your length as this needs to be replaced with hot / warm

 

[Ben-gee]

Length is 12m, pipe is 22mm - so that’s about 4 litres.

So 15 mm return of 12 m too (another 1.4 litres)

So 5.5 litres of cold in pipes. Turn pump on, how much has to flow until pipes are hot . Presumably quite a lot more than 5.5 l, but how much?
Aldo what speed is secondary return pump run at? (Slow I guess?)

 

[ShaunCorbs]

Normally around 1lpm max

 

[Ben-gee]

Oh ok, so takes .... about 15 mins in above scenario?

 

[ShaunCorbs]

Around 10 mins yes

 

[Ben-gee]

So PIR not going to be any good for this situation.
Timer best?
I’m guessing these can “learn usage” now like the immersion heaters and so on?

 

[ShaunCorbs]

So PIR not going to be any good for this situation.
Timer best?
I’m guessing these can “learn usage” now like the immersion heaters and so on?

timer could be set to constant so no

pir is the only option hence why it’s a double edge sword with plastic and why most manufacturers say no

 

[Ben-gee]

So with copper return is it normal to have pump on constant during day?
Does one put some sort of temp sensor to turn off pump when return pipe gets up to say 50 deg c?
I’m guessing cylinder needs to be on constant too , to reheat as cool water is brought back - coikk LG d get expensive pretty quick (obviously as much insulation as possible would help)

 

[exedon2]

Do a fair bit of this sort of work use tectite metal its rated for continues use easy to use as plastic not that much more expensive.

 

[ShaunCorbs]

So with copper return is it normal to have pump on constant during day?
Does one put some sort of temp sensor to turn off pump when return pipe gets up to say 50 deg c?
I’m guessing cylinder needs to be on constant too , to reheat as cool water is brought back - coikk LG d get expensive pretty quick (obviously as much insulation as possible would help)

yep temp sensor on the return these days you can get pumps with temp sensors attached so once upto preset temp it turn off

 

[ShaunCorbs]

Do a fair bit of this sort of work use tectite metal its rated for continues use easy to use as plastic not that much more expensive.

yep mlcp

 

[Ben-gee]

What do you use to lag the 15mm return?

Also the 22mm supply?

 

[ShaunCorbs]

Armaflex normally but it’s gotten a bit hard to get in the last 12 months so had to use some grey

yep the whole hot pipework needs to be lagged