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Issues with installer's use of pipework for Vaillant Arotherm ASHP

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(@jwilliams89)
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Hi folks, looking for some support with a discussion I'm going to have to have with our installer on Monday. Currently having a Vaillant arotherm + 5kw fitted in our three bed. I've been on here quite a lot over the past few months gathering advice, and the installers came in today to fit first fix pipe work. The house has been stripped right back and I'm renovating, so its all new pipework and rads throughout. Up to this point I've been happy with the installer, and comfortable with their knowledge and design specs. The pipework is all well fitted, so all in all was feeling very chuffed, until I went in today after they'd left and noticed they'd used plastic pipe right down to the copper tails. The main feed from the loft (tank) is in 26mm which then moves into 22, and then into 15 for each run to the emitters. The system used is multilayer. The system is not big, with our overall heat loss being around 4300kw at design, but I'm very concerned that the 15mm plastic in particular is not big enough. The bore is 12mm instead of 13.6 in copper. I'm trying to find internal sizes of the unions, but by the looks of it the unions on the 26mm have a much smaller bore than the pipe. As I've said up till this point I've been comfortable with the installer's ability but this is raising big alarms bells and I don't want to go ahead with the system as it is if it is going to be compromised from the start. Any input on exactly what to say to the installer would be appreciated - am I right in thinking that the main issue in terms of restricting flow is the 15mm runs. There isn't going to be a LLH or buffer in the system, but it looks as if he intends to boost the flow through the pipes with a pump which is not what I want.

This topic was modified 1 month ago by Mars

   
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(@bontwoody)
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Hi. There isn’t anything wrong with using plastic pipes as long as they meet the needs of the emitters. See this by John Cantor. I’ve used them in my own house 15mm plastic to the rads for ease of fitting and have no problems.

https://heatpumps.co.uk/technical/pressure-drops-flow-rates/

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(@jwilliams89)
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Topic starter  

I've been looking at the info on DT over on Heatgeek again and think I understand it better now. From what I can see the fact the system is being fed by 26mm (22mm ID) this fits within the 22mm copper pipework category, which would supply 6kw at DT5 with 22mm copper being around 2.4 ID. I've then calculated the use of each section of 22mm plastic (18mm ID) and they never supply emitters totalling more than 2.75kw, and finally none of the emitters which are fed by 15mm plastic are higher than the corresponding DT5 requirement for 12mm. So from what I can work out the pipework is not ideal but it does come up to that spec. If I'm wrong in these calculations please do let me know.

 

The issue I have is whether the unions take the system under spec due to the smaller internal diameter.


   
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(@jwilliams89)
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Joined: 6 months ago
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Topic starter  

@bontwoody Thanks for that, I read that article and got a lot from it. My main worry is/was that the plastic pipe hasn't been increased in size enough across the system because of the unions etc. There must be a reason but it seems mad to me that unions for pipework where good flow is a requirement aren't external like with copper.


   
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(@bontwoody)
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@jwilliams89 Your calculations look good at a glance. With respect to the internal pipe strengtheners, I have tended to use the hep2o ones which are stainless and thinner. However I do remember reading another John Cantor article in which he states that small lengths of narrowing don’t have a very large effect on the overall resistance to flow unlike say a crowd crush.

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
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Mars
 Mars
(@editor)
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Here’s some feedback from an installer with their thoughts on your case: 

The flow rate is low, even taking the 5kW at max it’s only 860L/hr, that’s a mass flow of 0.238kg/s
Via 22mm mlcp or plastics velocity is just about ok and Pa/m is 1200, given the lengths are likely to be short and each rad/emitter will be relatively low load again the pd is very low.
for an accurate assessment the author would have to include the pipe lengths in detail, the fittings used and orientation of flow to determine the correct zeta values. The cumulative pd is most likely within the scope of the hp residual head anyway.

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(@jwilliams89)
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Joined: 6 months ago
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Topic starter  

@editor Thanks for that, appreciated. I looked and the the longest run I can find is around 5-6m for the 16mm. I found the exact specs for this mlcp and its actually 26mm(20ID), 20mm (16) and 16mm (12), which still fits within the specs for DT5 but it does seem tight. To reiterate thats 26mm from pump into loft and back down into ground floor ceiling cavity, 20mm from there towards each area of emitters and then 16mm to each tail.

 

The system is Emmeti MLCP. Interestingly I managed to measure the internal diameter of all the unions and for the 20mm pipe the diameter is 10mm which is quite a drop.


   
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