I'm doing something very similar, built and installed a heat recovery pipe on our shower drain to warm the cold supply to the thermostatic shower mixer valve.
I did this as an experiment to see if it is worth doing as we are extending the house later this year and all the pipework will need altering again anyway.
My recovery system involves an aluminium tube 100mm x 1800mm (4" x 71") with 4 coils of 10mm (3/8") aluminium pipe wrapped around it.
To improve the heat transfer i coated it in glassfibre resin mixed with copper powder, i,ve no idea how much this helps but it seemed like a good idea at the time.
I have some sensors attached as part of my heating datalogging system.
As far as i can tell, it recovers heat quite well.
However the flow of cold water is so low that hardly any recovered heat makes it to the shower.
I'm considering altering the pipework to pre-heat the cold feed to our hot water tank so the flow through the recovery lines would be far greater and hopefully recover a usefull amount of heat.
The only way i can imagine this working properly in the cold feed to the shower is if the hot feed is at a much higher temperature and requires large quantities of cold to dilute it.
I need to check the logging software but i think our cold flow during a shower is only around 0.2LPM out of the 7LPM total shower usage.
Hope this helps.
Steve
