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Old 07-20-21, 08:40 AM   #1
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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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Old 08-05-21, 06:46 PM   #2
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Just got a free minute to check the forums. Life's been hectic for the last few months and all the side project stuff got put on hold.

It looks as if you are overestimating your compressor mass flow. Looks like a little pumpkin to me. If it's drawing around 500 to 600 watts, I would guess that it might move 1500 watts+ during ramp up, then taper off to somewhere around 1000 or so at equilibrium.

If you're using all copper fittings and tubing, you can use silfos rod with no flux. Once you start using silver, you definitely want to use flux. With the small stuff under about half inch (12mm), it's easy peasy. Just use an oxyfuel torch and the bare copper phosphorus rod. On the window unit and dehumidifier size systems, I use the little scuba tanks. They're super common, so it's not a big deal to exchange them.
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Robl (08-12-21)
Old 09-10-21, 06:57 AM   #3
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I finished the heatpump build, pressure tested with CO2, vacuumed it & filled it with propane, then turned on the heatpump for the first time yesterday!
I charged it with the pump on, aiming for pressures I'd found with Coolselector2, which used maybe 200g of propane to give ~ pressures of 5bar(low)/13bar(high). Sadly I wrote the "before" gas can weight down on the can, which got all frosty and the number rubbed off.... and I can't remember it. The can originally had 360g in, I wasted a load last week hamfistedly putting the regulator on, and there's still some left hence ~200g. It was supplied in the "on" state, and like the CO2 one I have, I thought I had to tighten it on to stop leaks - but no, that just made it worse.. Uhh, amateur! I am doing all this outside with gloves and safety glasses on at least.

There's a picture attached - all the heatpump parts are on the outdoor grade ply, the compressor next to it is used as a vac pump. I've attached hoses to circulate water for now.
I ran it with a 3 thermocouple logger and an elec power meter manually read, with "cold" water from the tap at 8lpm being chilled down by a heatexchanger and dumped in the garden. The hot loop ran from a 27.4litre tank of water, pumped at ~8lpm through the "hot" heatexchanger, and back into the tank. I pushed the data through excel and I get some graphs, attached too. I got excel to do a linear fit on elec power in and delta(output temp), rather than use the actual data to calculate the COP, just so it's a noise free result, which I think is fine.
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Old 09-12-21, 11:21 AM   #4
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I've run the rig a few times now logging data, and analysed the data a bit better now, taking into account losses of the 27litre water tank to air - I did this by only running the circulation pump allowing the 27l tank to cool down, and then curve fitting to get the losses. I find 15W/degC relative to air, so when running hottest I get 300W extra loss, which is significant given the system is small.

I tried working out the superheat, which started at around +20C, then added some more R290, then I got +10C superheat. I added some more again - I think too much, not sure if I had any superheat at all at this point, and the efficiency dropped. I've added a picture of the calculated COP versus DT, and all the runs are similar except for the one with the highest charge, which gave to worst result.
Today I planned letting out some gas, to get back to the better performance - only to discover some must have leaked out - my pressures were a bit lower again, and the efficiency was good again. Hmm, clearly something isn't quite gastight. I have 4 heatexchanger connections that rely on nitrile rubber washers, and I left the manifold guage set on, I think it will be one of those things.

There's some vibration on the thin capillary tube, at the end, after the section soldered to the liquid line. It makes a white noise sound which I can deaden by touching it - I presume there's bubbles of gas in here? Is this a sign of something I can improve somehow? Should more of the cap tube be soldered to the liquid line, or the length be more/less? All the 4 charge levels I used made this happen, I think in a similar way. There's a pic of where this is coming from attached, I circled the pipe in red.

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