Heating Pipe Runs

[TerryE]

As I mentioned in other posts, I have replaced my SunAmps with a UVC.  All done, but one consequence is that there is now an extra 3m of 22mm copper pipe between the storage unit (the UVC) and the DHW manifold.  This has introduced a noticeable additional lag in the time to run hot from cold when I run a H/W tap.  The total amount of heat absorbed by the pipe is quite small (1.5 Kg Cu pipe  × 0.385 kJ/Kg°C specific heat Cu × 30°C) or 17.3 kJ or roughly 1/200 kWh per "run from cold".  Given that we only run from cold maybe a dozen times a day, this heat loss is quite small -- especially as for half the year we are doing heat top up through the slab and this can just be considered another form of radiator.  It's the increased delay in flow response that is noticeable.

 

So I thought to myself: would replacing this copper pipe by HEP2O make a noticeable improvement? So I ran the numbers for PeX.  PeX pipe is over 4 times lighter but the specific heat of PeX is actually over 5 times higher than that of copper so the heat absorbed works out still more. However the HEP2O barrier pipe also includes an Aluminium layer which drops the overall figure back again.  Even so I estimate the heat absorbed to raise the HEP2O by 30°C is still around 18.5 kJ so the bottom line is almost counter intuitive (to me at least) that I am better off (in terms of thermal responsiveness) using copper than HEP2O for these runs.

 

I thought that some of you boffins might be interested in this observation.

 

Edited April 4 by TerryE

[JohnMo]

3 minutes ago, TerryE said:

HEP2O barrier pipe also includes an Aluminium layer 

Are you sure, mine is devoid of aluminium layer.

 

5 minutes ago, TerryE said:

extra 3m of 22mm copper pipe between the storage unit (the UVC) and the DHW manifold.  This has introduced a noticeable additional lag in the time to run hot from cold

Could you run 15mm inside it? That would reduce the lag 

[TerryE]

@JohnMo we are talking classic trade-offs here.  Dropping pipe diameters significantly reduces both mass-in-pipe and unit-length pipe mass and therefore heating lag.  I did my manifold design 7 or 8 years ago and it work well so is not worth replacing. Heat lag on flow-from-cold just wasn't an issue that I considered. TBH even now that I am aware of it, I am not sure how I would change things.

 

I use closed cell insulation for HW pipe runs from the tank to manifold, and the HW manifold itself is in box that is packed with lagging. The system leaks are ⅓°C / min at 40°C as you can see from the plot below of the UVC top and bottom temps, plus the out from the UVC to the manifold, and inside the Manifold itself.  You can see when someone ran the hot until "hot enough" at around 1:30, plus a few other mixer runs where there was some hot draw but not enough to reach the tap (e.g. a quick hand-wash after a pee in the night).  Jan was doing stuff in the kitchen 10-12:00, enough to get the hot to mid 30s but not enough to need to run hot. No showers or baths in this period.

 

Total heat loss isn't really material.  It's just that we don't use H/W that much but when we do want hot, it takes 30+ sec to run to a decent temp.  This would have been better if we'd used 15mm main runs but then this would really limit peak flow times (e.g. running a bath or 2 showers on the go at the same time).  

 

The one thing that I would have done differently is to drop all low flow hot runs from the manifold from 15mm Hep2O to 10mm.