Insulated twin pipe, 32mm or 40mm

[HughF]

A simple question:

 

15m run of underground twin pipe, 9kW unit on the end... Should I step up to 40mm or would it be ok to stick with 32mm?

 

I'd prefer to stick with 32 as I have press jaws for that size and can go straight onto 28mm adapters.

[Beelbeebub]

Here's the heatgeek cheat sheet for pipe sizes. 

 

32mm should be good for 15kw, so plenty of margin as long as your pipe run doesn't look like a kids balloon animal.... 

 

Edited August 29, 2023 by Beelbeebub

[HughF]

Spot on, cheers....

[Dreadnaught]

To help any others reading his table above … "Here you have the external pipe diameter on the left, and how many kW/s of power the pipe can carry in the centre" … from the Heat Geek website.

[Beelbeebub]

37 minutes ago, Dreadnaught said:

To help any others reading his table above … "Here you have the external pipe diameter on the left, and how many kW/s of power the pipe can carry in the centre" … from the Heat Geek website.

External copper pipe diameter. 

 

Plastic pipe has a smaller bore and more pressure drop for fittings. I think you nearly halve the capacity for plastic pipes. 

[sharpener]

5 hours ago, Beelbeebub said:

Plastic pipe has a smaller bore and more pressure drop for fittings. I think you nearly halve the capacity for plastic pipes. 

 

Pressure drop is indeed an important consideration, see this link.

[HughF]

5 hours ago, Beelbeebub said:

External copper pipe diameter. 

 

Plastic pipe has a smaller bore and more pressure drop for fittings. I think you nearly halve the capacity for plastic pipes. 

32mm HiLine E-Flex PEX-a (the stuff you can get from Ingoodnic as a kit) has a 26.2mm bore, the same as 28mm copper.

[HughF]

9 minutes ago, sharpener said:

 

Pressure drop is indeed an important consideration, see this link.

This seems to suggest that at 1.8m head (from my HP to my airing cupboard), and given 35m round trip length (for flow and return), I can get close to 2000ltrs/hr through 32mm.

[sharpener]

4 hours ago, HughF said:

1.8m head (from my HP to my airing cupboard)

 

If you mean the static head between the one and the other that isn't the right figure, you need to take it from the excess head graph for the circulating pump in your HP.

[HughF]

2 hours ago, sharpener said:

 

If you mean the static head between the one and the other that isn't the right figure, you need to take it from the excess head graph for the circulating pump in your HP.

Sorry, I’m not the smartest tool in the box, could you explain a bit?

[Beelbeebub]

13 minutes ago, HughF said:

Sorry, I’m not the smartest tool in the box, could you explain a bit?

In a closed loop (like a sealed heating system, the static head due to the height difference between the bottom and top of the system, isn't relevant for the flow head. 

 

For example, if you had a 30m loop of pipe from the output of the pump back to the input. 

 

The flow rate for a given pump pressure will be the same if the loop was horizontal or if it went up 15m and back down 15m.

[HughF]

Gotcha, so it’s just the length of the pipe that matters, irrespective of the height (because what goes up, must come down)…

[Beelbeebub]

11 minutes ago, HughF said:

Gotcha, so it’s just the length of the pipe that matters, irrespective of the height (because what goes up, must come down)…

Pretty much. Length of pipe and number and type of bends are the most important.  A long straight pipe may have less pressure drop than a short length with multiple 90 degree elbows. Valves that aren't full flow will also have a big effect. 

Edited August 30, 2023 by Beelbeebub

[HughF]

28mm long radius bends then, gotcha. That was my plan once I’d come off the 32mm duo where I come out the ground.

[Gus Potter]

1 hour ago, Beelbeebub said:

In a closed loop (like a sealed heating system, the static head due to the height difference between the bottom and top of the system, isn't relevant for the flow head. 

I wonder.. if you go back to how an old back boiler in coal fire works. Hot water rises, cold water falls.. the old back boilers could set up a good rate of convection flow.

 

What happens if you apply these basic principles to you modern flow and return.. thus that means you put the pipes in with a fall so you are not fighting against the natural convection.. mind you think I might be splitting hairs here!

 

Main thing for me would be to look say 20 -25 years ahead and think.. what if the pipes silts up a bit or I get a daft plumber in that lets unwanted bodies into the pipe. How easy would it be to flush it all out?

 

Given the cost of excavating and preparation I would go for the larger pipe diameter 40mm as once in it's "there for life" ?

 

 

[markocosic]

Flushing pipe is easy. 

 

You'll f**k the heat exchanger in the heat pump long before you have to worry about muck settling in the pipe.