Hello all. I’d be grateful for input on the repiping of my home, ahead of a heat pump installation by Octopus Energy. I’ve no prior experience of heating systems but now seem to be part-designing one!
My house is a 1980 end terrace, 115sqm. The current pipework is mainly 8mm copper microbore, with 15mm copper in a 10 year old extension.
The original Octopus heat loss assessment was ~7.5kW which tipped me to the 9kW Daikin. My own assessments (1. Heatpunk, 2. a Gas Usage v Degree Day comparison), plus a Heat Geek assessment suggest heat loss of 5-6kW.
After much to & fro, Octopus agreed that the 8kW EDLA would suit, and have advised that max output from this unit at their 50c design flow is ~7kW.
I think I understand how 7kW of heat would be distributed between the radiators, assuming they remain open / lockshields used to adjust flow. This leads to the following pipework design - a small amount of 22mm from the cylinder, then dropping to 15mm.
The aim being to ensure velocity is about 0.9mps in the main pipework.
Assuming actual required heat output at -3.5c is between 5kW and 7kW, and assuming a dt of 5c, my spreadsheet suggests velocity thru the main pipework is 0.75 - 1.05mps. nb. although, on a typical cold winter day i suspect velocity would probably not exceed 0.5mps
-Does this all seem reasonable?
-Is there anything about the Daikin and how it’s best controlled that changes the picture?
I think the answer depends on how things are plumbed.
You’ve labelled “cylinder”, but this would normally mean the (tap/shower aka DHW) hot water, which is a completely separate circuit; the heating water from the EDLA goes into that tank but transfers heat in a “heat exchanger”.
I would expect the standard Octopus plumbing would include a buffer tank and a secondary pump which propels the water around the emitters (radiators) and the primary pump (in the EDLA case) moves water around the DHW heat exchanger and buffer.
The pressure which the secondary pump exerts is a factor to consider, and whether it is speed-controlled according to demand (I doubt). I suspect that these are generally over-powered, and designed to work in houses with 10mm pipework. Sure, they can shift enough heat but at the cost of extra elec consumption at the pump and pipework noise, and you end up turning the lockshields down to ensure that there is enough temperature drop across the rads. My experience with mostly 22mm + 15mm branches to emitters is that there was excessive noise. I build my own speed controller to get over the issue (noise almost gone at 50% speed and power down from 30W to 7W).
I would expect (given provisos above, and from my own property experience only!) that what you propose will work. A more balanced approach might continue the 22mm to the next “tee”. Also, if it makes sense in terms of routing, include the guest bed in another branch, as that will probably make balancing easier.
Thanks Adam. Yes, ‘Cylinder’ is perhaps not the best label. The repipe will initially hook into the existing combi boiler. Then, when the heat pump is installed, a DHW cylinder will be located in the place currently occupied by the boiler. And there’ll be a volumiser in the loft above the cylinder location, but no secondary pump.
Octopus don’t install a buffer that I’ve ever seen. They use Cordivari buffer/volumiser which can be installed as either as it is two ports on each side as well as top and bottom, but they install it as a volumiser on the heating return to help with defrosts and to avoid short cycling.
