Hot water secondary return help

Hi All,

I'm a DIYer attempting plumbing of my house reno. I've got a new unvented direct 300l cylinder on the ground floor - heat pump is the heat source.

Kitchen is opposite side of the house to the cylinder - approx 16m of pipework away (goes to first floor and back down)
Furthest proposed bathroom is in the loft, with a rise of approx 6.5m. Total pipework approx 16m.

I'm looking at putting in secondary return but the layout of hot water outlets isn't as simple as diagrams in examples I've seen. I've attempted a sketch:

Red would be 26mm MLCP flow (22m equiv to copper) and yellow dashed is return (20mm MLCP - approx equiv to 15mm copper)

Is this correct?
Will I have dead legs?
Whats the max distance I can I have an outlet from the flow pipe?
Is it acceptable to have various Tees into the flow and returns?

I've read I will need balancing valves, as I don't have a simple single loop. What products are best for my situation and where should they be placed in the system?

The various videos and things I've read seem to have a simple arrangement with a single return pipe, with no Tees in but I cannot see how this would work in practice as house layouts aren't always simple with outlets close to the flow pipe.

First floor:



Second floor (loft)



Any help is greatly appreciated.

In practise on long runs 20mm MLCP has considerably less resistance than 15mm copper. 16mm would be enough, one of my clients has a similar setup to yours except theirs is a 5 bathroom 9 bedroom house and that's running great on 16mm.

Make sure you consider controls. Running a heatpump you need to take the least energy out of the cylinder as you can which is another reason to use smaller pipe - obviously use the preinsulated stuff and consider the use of PIR sensors by the hot draw offs rather than a timeclock

diytarded said:

Kitchen is opposite side of the house to the cylinder - approx 16m of pipework away (goes to first floor and back down)
Furthest proposed bathroom is in the loft, with a rise of approx 6.5m. Total pipework approx 16m.

Click to expand...

Have you tried to calculate how long it would take hot water to reach those outlets without the circulation? What about with smaller-diameter pipes? How much delay can you put up with? Can you wait longer for hot water at the bath tap than at the basin tap?

You certainly don't need 26mm for most of that pipework, e.g. the parts that are serving one or two bathrooms.

diytarded said:

I've read I will need balancing valves

Click to expand...

That seems likely.


Edit: pressed send too soon, ooops.

Most secondary returns are just done in 15mm as it only needs to be able to create a small flow to keep the hot water circulating a very small amount.

Obviously quite a lot of lagging is required on the hot water pipes which is something which is not always done in this country.

Just as a matter of interest how satisfied are you with your heat pump installation?

They do not always gain a very high satisfaction review but this is probably because of a lack of good instructions on how to best use them.

Agile said:

Most secondary returns are just done in 15mm as it only needs to be able to create a small flow to keep the hot water circulating a very small amount.

Obviously quite a lot of lagging is required on the hot water pipes which is something which is not always done in this country.

Just as a matter of interest how satisfied are you with your heat pump installation?

They do not always gain a very high satisfaction review but this is probably because of a lack of good instructions on how to best use them.

Click to expand...

Could I use 16mm MLCP which is about 12mm internal (15mm copper is about 13.6mm)?

Heat pump only just getting installed, no experience using it yet but I have a very knowledgeable installer and I'll be using it with underfloor heating and upgraded rads. House will be fully internally insulated (currently solid brick) so I am expecting good results.

I will lag all the pipes for sure.

Is there anything wrong with my layout? The fact I have numerous Tee's creating different loops? Any advice on where to place balancing valves and also how long can I have a tap/shower/whatever away from the main flow pipe?

Razor900 said:

In practise on long runs 20mm MLCP has considerably less resistance than 15mm copper. 16mm would be enough, one of my clients has a similar setup to yours except theirs is a 5 bathroom 9 bedroom house and that's running great on 16mm.

Make sure you consider controls. Running a heatpump you need to take the least energy out of the cylinder as you can which is another reason to use smaller pipe - obviously use the preinsulated stuff and consider the use of PIR sensors by the hot draw offs rather than a timeclock

Click to expand...

Is that 16mm MLCP, which is effectively smaller than 15mm copper? I assumed larger pipe = better flow but sounds like flow isn't an issue for hot water, unlike for heating circuit. Did they have various flow and return loops and how did they balance the flows?

endecotp said:

Have you tried to calculate how long it would take hot water to reach those outlets without the circulation? What about with smaller-diameter pipes? How much delay can you put up with? Can you wait longer for hot water at the bath tap than at the basin tap?

You certainly don't need 26mm for most of that pipework, e.g. the parts that are serving one or two bathrooms.



That seems likely.


Edit: pressed send too soon, ooops.

Click to expand...

Not sure how to calculate that, I'm a DIYer! Based on the distances and experience in similar properties before it would be 40-60 seconds to get hot water at the furthest points from cylinder.

Are you a plumber and can you advise on my layout, any issues with it other than pipe sizes? Sounding like I should go a size down for flow and return which will be slightly smaller than 15mm copper on return.

diytarded said:

Not sure how to calculate that

Click to expand...

You can work out the volume of water in the pipe and divide by the flow rate.

I presume you're planning on recirculation because you're previously been unhappy with slow hot water, right?

You mention 40-60 seconds. The volume of water in 16m of 15mm pipe is 16 x π x (0.0136/2)² = 2.3 litres. 2.3 litres per minute is quite a slow flow rate from a tap. 16m of 22mm pipe is 16 x π x (0.0202/2)² = 5.1 litres; that's still not much water to come out of a tap in a minute. Your numbers sound more like a combi system to me, i.e. the time is both the time taken for the boiler to start up plus the pipe.

Note there is some additional time because the first bit of hot water has used its heat to warm the pipe and is cold when it reaches the tap. My recollection is that this effect is not very significant, but I could be wrong.

diytarded said:

Are you a plumber

Click to expand...

No.

For myself I would make it very simple with just one connection at the effective furthest HW point from the cylinder.

I would also time it and use a thermostat to turn off the pump when the return from it became just warm.

My cylinder is on 1st floor and the farthest fixture from cylinder was 60ft and took 2 min 20 seconds to get hot water. I have gravity fed system for hot water. I put a pex 16mm (for secondary circulation pipe sizes are not relevant). I put a grundfos 15-14 BA PM circulator and now i get hot water in 20 seconds.

The only issue is that I keep getting air in circulation line as it is a retrofit so the path for recirculation line is 10ft up from the farthest fixture with multiple u bends then the entire length of the house 40ft horizontal run (into the wall with insulation) and then the last 10 ft dropping into the cylinder with pump. The cylinder has recirculation inlet. My only issue is the backflow and pump noise which I am suspecting is due to reverse flow from recirculation port.