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Old 07-20-21, 08:40 AM   #1
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Join Date: Oct 2020
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Default DIY W2W heatpump

We now have a relatively well insulated house in the UK, needing a peak of 1.5kW over the last year to keep it warm in the coldest of weather. That would be 1.5kW of heat continuously on for 24hours on the coldest day, in addition to the 500W of people and dog and standard electrical use. Our house warms up by 10degC per kW of continuous heating, and the existing water radiator circuit couples to internal air with 5degC per kW of heating, as measured at the boiler output. That simple statement took me a while to get the data for it - we have a "smart" gas meter, which logs daily measurements, which makes it a lot easier, and I've got a diy logger which gives nice graphs. From this, if we heat with 2kW continuously, the water circuit would operate 10C above the target room temperature of 21C, and the house could warm up even in the coldest of weather we get, to above 21C. The radiators are oversized - they're mostly original from 1963, but they could could keep up back before the house had external wall insulation fitted (big recommend!), and now the heatload is a lot lower.

I would like to replace our existing old gas boiler with a small capillary tube metered heatpump, and I'm musing with building my own gshp unit using an R290 compressor. AC Hacker and others on this forum that have persuaded me to consider it - so many impressive projects! If I did it, the unit would likely live outdoors next to our brick garage resting on a concrete block, the exposed sides having rendered rockwool insulation and a little airflow. The insulation means the air temperature in the box will be closely related to the garage next to the house, say 10-20C temperature all year round unless selfheated, and the render will keep it all watertight. It would be a lot simpler to put it indoors, but I think it will be ok outside, and it removes the propane worry.

To get the low grade heat in the first place, we'd need at least 100m of hdpe pipe buried in the back garden. This is another tricky part, but my hope is that a trencher would make it relatively painless to make a series of 1.2m deep 200mm wide cuts that pipe would be pushed into, giving 3 off 40m parallel loops. For the moment though I'm focussing on the heatpump itself, probably as it's most fun:-)

I picked up a 2nd hand compressor, NLE12.6MN, by Secom, 530W in and 2.3kW heat out from 5C up to 35C. I've hopefully attached a picture of the setup I'm thinking of. It's a bit complicated, as there's a direct "free" cooling path that I intend as well as the heatpump circuit itself.
I've plenty of diy plumbing experience, lots of electronics soldering, and I've watched lots of youtube vids on heatpumps with R290, some pro, some amateur.

Below are some details I've discovered randomly on the internet - please say if you think I've got stuff wrong!

Get the heatexchangers the right way round. The top should be the hot gas end, the bottom the higher density liquid. This doesn't matter so much when there's no phase change as the density doesn't alter much, but any refrigerant gas will have a density change. The left side ports (when "upright") should be the R290 side, so it's encompassed by water both sides.
An accumulator on the suction line reduces the likelyhood of liquid getting to the compressor, which is bad for it
Silver solder together the suction line and the capillary tube - Secop Capsel shows this, as do other places, but plenty of capillary systems don't seem to do it. I think it maybe helps nudge the operating conditions a little towards optimum keeping things more efficient when not quite right. Say there's too much subcooling, not enough superheat so the compressor might flood with liquid, this detail might prevent that.... I'm way into bluffing it territory now mind, I'd love somebody to point at something authorative on this:-)
Twist the remaining capillary tube up by bending it round something neatly, so it doesn't mind the compressor vibration.
Braze (15% silver) pipes to the to compressor, less critical joints elsewehere could be lower percentage silver soldered. The compressor joints must be brazed so that they are stronger and take longer to fatigue fail than silver soldered joints when subject to the continuous vibration stresses.
Bleed nitrogen or CO2 through the system while brazing, otherwise copper oxide flakes form on the inside and might damage the compressor.
Ideally all R290 joints to be brazed, possibly silver solder away from compressor - there seem to be strong opinions both ways about this - as I understand it brazing is so hot that the copper is weakened, but then silver soldered joints are not as strong. Least favourite are compression joints - all pipes and joints leak a little, but compression leaks most.
Secop Capsel is a great free tool that will calculate the optimum capillary tube size.
Danfoss Coolselector2 knows all the specs of Secop compressors (Secop=Danfoss), and is very powerful. I haven't mastered it - I don't know if it can simulate capillary systems or only more complex ones; I'm just using it to suggest appropriate pipe diameters and pick a compressor!

A few questions -
I'm assuming for a diy system, I need schrader ports on the discharge and suction lines so that I can connect up a guage manifold set and choose how much refrigerant to put in by matching running pressures to what Coolselector2 says. Do these have to be near the compressor, or actually better at the heat exchanger end of the pipes, so they're less affected by vibration? I'm just intending copper tee pieces for these connections.
I don't think I'll need the service valve on the compressor - I think that's just for when a cheap system is being comissioned, and then there's no need for 2 valves as no pressure measurements are made. I think I need the two service ports, as I won't know how much R290 to put in without measuring the running pressures. Is there a reason to braze on a schrader to the service port anyway?

Love the site, any advice taken with thanks!

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