Heat pumps are great technology, until they aren't, when something goes
wrong.
Then it can turn into a deep dive into technical diagnosis and frustrating
guesswork, often chasing subtle and hard-to-find problems.
The Daikin was pretty solid for six years after installation, other than the
waterlogged thermistor problem.
Then, just heading into the next heating season, one October day I noticed
that it seemed to be running for a really long time and not managing to make
much useful heat.
The superficial "diagnostics" I could get from the controller told me that
the overall output from the indoor coil was only 80 degrees or so, not up
to the usual 105 or 110 where it usually runs.
That might have eventually gotten the house somewhat warmer, but at the
expense of how much time?
Clearly, something wasn't right.
The "service mode" on the control panel also showed the outdoor evaporation temperature wandering around at a fairly ridiculous low level, like -20F in maybe a 40 degree day. Normally the system runs the outdoor coil about 10 - 15 degrees under ambient, which is enough to pick up heat from the outside air and bring it inside. This is a little nonintuitively puzzling -- if the outdoor coil is so much colder than the air around it, why wouldn't it be picking up *plenty* of heat to transfer in? Well, the properties of refrigerants aren't that simple, and the numbers I was reading were actually quite misleading. To transfer heat efficiently, refrigerant has to change state evenly across the entire area of an exchange device, aka a coil with fins on it. If liquid refrigerant flow is low and it tries to expand into a given volume of piping, it can all flash to gas immediately and then spend the rest of the journey through the exchanger fairly uselessly at ambient temperature, picking up almost nothing. So it might get really cold early in the process, but there just isn't enough mass of it flowing to maintain that state-change process through the entire length of piping. |
The first time this problem began, though, I had *no* idea what was going on. Was the compressor shot? The reversing valve internally leaking? An expansion valve not opening to the correct point? All kinds of guesswork flooded through my head because I didn't yet realize what the real problem was. Parts of the system were still a deep mystery, despite having all the service documentation from Daikin on hand. Whatever the cause, I figured that it would ultimately need professional eyeballs and tools. This was all pre-pandemic, so the first go-round or two of trying to call a contractor weren't quite as terrifying and frustrating as later on. |
Anyway, with nothing obvious *I* could find wrong, it was clearly time to
call the professionals.
And here's where we start condensing [ha!] about three years' worth
of story into one general rundown on what the diagnostic and recharge
process entails.
And after getting into some serious deep technical weeds, my eventual
elegant, cost-saving solution.
So we start with Technician #1, who seemed fairly impressed and amused by the things I already knew about my own system. During his visit he took some phone pictures of my hacks and the whiteboard diagrams on my kitchen wall, "for the guys". |
I pointed out that I'd installed the plenum access hatch for a reason, so he did due-diligence probing all around the underside as far as he could reach. Still nothing. |
Technician #2 was a different guy from the same company as before, and for
one of the most bizarre reasons I'd ever encountered.
Apparently the first fellow they sent out, who seemed to appreciate the
geek factor of my system and my understanding, had suddenly died of a brain
aneurism about a month after that visit.
Perfectly healthy otherwise, competent and contented in his job ... and then
out of nowhere, blammo, just never woke up one day.
You just never know when something like that can happen.
That was unfortunate in many ways, but one of those was that I had to basically start over and train up another guy on what my system consisted of and what had gone before with it. He came up to speed soon enough, and along with more fruitless leak-sniffing, we agreed to try a couple more things to proactively seal the system better. With it once again emptied of about the same remaining amount of refrigerant as before, I suggested opening the flare fittings for a good look and possible improved sealing with a bit of lubricant on reassembly. Now, these were the same flares that had been sketchily installed in the first place, the larger of which was visibly leaking the day after and all the installer did on the callback was crank the nuts down tighter. These had never been disassembled since, on the assumption that if they weren't leaking in any obvious way, they were best left alone. But Technician #2 seemed confident that because they were always suspect, they could be opened, inspected, and reassembled in a more leakproof way by applying the Nylog that the original guy should have used in the first place. The surfaces actally looked fairly good, so he spun a little Nylog around the threads and the back of the flares and buttoned things up again. He resisted the concept of putting anything on the *mating* surfaces, even though Nylog is supposed to be safe for systems if a little bit of it gets into where the refrigerant circulates. It's basically a super-thick polyolester oil. Still, color me skeptical of the sealing ability of a raw metal-to-metal junction. What's ironic is that now they actually make crush seals for flare connections now, for exactly this issue, but the guy didn't offer to install those at the time or possibly even didn't know about them yet. Next time these get opened, if ever, maybe that's something to do as well. |
Now, a vacuum-rise check has an inherent flaw, that when piping is evacuated,
hairline cracks at joints can actually get pulled together and self-seal
as opposed to being forced open wider under high internal pressure.
On the heating cycle, the entire lineset and indoor coil is part of the
*high* pressure side, increasing the likelihood that pressure-variant leak
points might actually leak more under those conditions but not others.
I had never seen any issues with cooling; the issues seemed to only show
up once the system kicked back to heating mode and put high pressure in the
whole vapor side and indoor coil.
This made me continue to suspect a problem on the indoor side.
To facilitate longer-term diagnostics, the guy suggested putting some fluorescent dye in the system which might help locate a tiny leak over time. He assured me that it wouldn't harm the system at all and wasn't any kind of leak *sealer*, which is still the subject of raging debate across the industry. A simple tracer visible under UV light might give a better chance of letting me eventually find the leak, so I was on board with that. He understood that I would be perfectly comfortable continuing with my own diagnostics, and that an appropriate light source would be inexpensive. |
Here's how the dye kit works. This is forcible injection over system pressure with a little plunger pump, and some of the later kits simply use high-side pressure to push a quantity of dye into the low side. Whichever it is, it is generally done with the system running, to immediately mix the dye through the refrigerant and distribute it. The plunger method can get a little messy, as not all the dye leaves the injector and when the fitting is unscrewed from the access port, some if it can spew back onto whatever's nearby. Which, of course, it did here... |
A few days later I had an appropriate 360 nm UV flashlight in hand, and was
now equipped to do as much leak-chasing as I cared to.
But the Nylog job must have helped quite a bit -- for the rest of that
heating season and about a year and a half further on, the system performed
as intended.
That took us well into the pandemic, when the last thing anyone wanted to
worry about was HVAC problems, and everyone was in lockdown and terrified
over the idea of a contractor having to work in someone else's house.
I made it through the token cooling/dehumidification period over the summer
of 2020, and then as I swung back into heating that Fall and was starting to
feel a little more confident that the leak might actually be gone,
guess what.
*Argh*. Still with the Covid-driven apparent impossibility of finding someone to come out and do the $800 "refill dance", in despair I simply shut off the outdoor unit and started heating on the 3 kW "backup" resistance element. Good thing I had done the secondary thermostat integration to let that work independently, because now I really needed it as "emergency heat". While that is *just* enough to heat the house through most of the winter, it is decidedly the expensive way to do it, as there's no benefit from the COP of the heat pump *and* it has to run longer to produce the same overall BTUs inside. With the refrigerant side of the picture out of commission again, I was almost resigned to having to go through the entire winter on the resistance coil and who knew when I'd be able to ever get the heat pump fixed. And the kicker was that I *still* couldn't find any evidence of an identifiable leak point, even scanning all over everything with the UV light and trying to sniff key points with an ancient leak-finder I'd had for years and filter out all the false alarms. That was only really a gas density detector, not specific for refrigerants, and would start indicating on just about anything including dust in the air. At one point I slit into the lineset insulation at a couple of points to see if I could detect anything along the piping, and the wheezy old sniffer started alarming on the *outgas* from the cut foam cells. By now I knew that all the system really needed short-term was a simple top-up of R410A, about three pounds worth. But that wasn't something I could just hop down to the hardware store and buy. Sale of refrigerants is quite restricted, to certified industry professionals, and a bit of specialized gear is also needed to fill a system. One of my nearish neighbors works in the HVAC industry and has his "ticket" to legally work on this stuff, and at some point we got to talking about my system. He offered to wander over and take a quick look, and after another couple of superficial and negative sniff and bubble tests, he came up with a couple of useful ideas on how to proceed. One was that he could bring his gear over and add the three or so pounds of refrigerant needed, which could hold me over for a while if the leak rate remained steadily low. This is called a "gas-n-go", and while it isn't the most environmentally friendly approach, is done all the time in the industry. For small systems, escape of a pound or two per year of HFC refrigerants isn't considered so big a deal -- especially compared to some of the allowable leakage from large industrial systems. The right answer, of course, is to fix the leak, but if it's proving just about impossible to *find*, we run into the limits of practicality and labor cost. We geeked a bit about all of this, and he seemed somewhat impressed that I knew what I did about the system and what to add and get the system charge back to nominal. |
At some point in the midst of all this I decided to construct a little box to make the "voltmeter pressure-read hack" more convenient. Two little tiny digital meters, in red and blue, a 9 volt battery, and my original connection cable. This and my P/T voltage chart makes high and low pressure reads fairly easy, and those are always measured at the compressor, independent of the reversing valve. | |
It was after putting this together and thinking about it a bit, that I finally
realized that the "thermistor" figures that the wall controller shows me about
the outdoor unit are actually doing the same thing: equating the voltages from
the same sensors to what the temperature *would* be for R410A in
saturation at the given pressure.
This is invariant, and one of the most magic properties of refrigerants --
whether you have two ounces or twenty pounds of liquid in an R410A jug, its
pressure will be exactly the same at a given temperature.
Watching the meters during the system's "struggling" phase showed the low-side
voltage dipping fairly drastically down, to like the equivalent of 50 PSI or
thereabouts.
Suddenly the "ridiculous superheat" scenario became clear, as the "temp"
reading of -20F simply equated to very low suction pressure as insufficient
refrigerant was coming through the expansion valve.
Oddly, even in that pathological under-fed state the suction pressure never reached the cutoff point and the system never threw an error, it would just keep trying even if it was just wasting energy for very little heat by then. |
Another thing my neighborhood buddy had mentioned was the possibility of
becoming certified to work with refrigerants, so that supply houses would
be willing to sell them to me and maybe treat me with a little more respect
than is afforded "homeowners".
He seemed impressed by what I knew already, and said that the tests weren't
that hard after a bit of study-up, and that's all that was required rather
than having to be a full-blown "licensed contractor" on paper.
This would allow me to do my own top-ups, and given the observed leak rate,
I figured one 25-pound jug of R410A could last me the next 5 or 6 years if
things stayed as they had been over the last three.
This idea took root and grew, and along with possibly getting myself EPA 608 certified, as it's known, I could already buy HVAC tools on Amazon without even having that status. After observing my various service visits and a few Youtubes I knew I could easily and safely handle this stuff, so why not? If a bit more money and passing a test involving some science were all that stood in the way, it was a project well worth tackling. I started making a list of what I'd need as permanently owned tools to work on the system, and easily found a lot of good EPA study guides all over the internet for free. While the neighborhood guy had recommended the two or three evening training course that he'd taken over at S.G.Torrice, it became clear that I could just learn this stuff on my own and only bring in an external agency to proctor the test when I was ready for it. And to hasten this process along, this latest top-up only lasted about a month before the system was wheezing again and giving me "the finger" of premature evaporation. Either the leak had gotten worse, or my neighborhood buddy had done something wrong when he unhooked and closed up the fittings again. I didn't feel right about pestering him to keep coming by with his pink bottles of salvation, and at this point, *fuck* if I was going to keep paying for $800 "professional" truck rolls. I was determined now: I'd go ahead and get myself able to pursue this whole mess on my own, and began working on several fronts to make that happen. |
While too much liquid inside a gauge set can be dangerous in the same way
an overly hot tank can, this wasn't enough to be a concern.
Next time I used it, I would try to dump most of the pressure back into the
running system and leave only vapor inside, and leave a gentle holding
pressure to make sure no air got in during storage.
So the next thing was to finish studying up and schedule my 608 test. This process was rather fascinating, because there was a lot I didn't know about other types of refrigeration systems and correct handling of their contents. Industrial low-pressure chillers using R-123 or old R-11, for example, run at both above and below atmospheric pressure, and invariably suck in a little bit of ambient air -- and have special "purge units" to collect and eject it automatically! I learned what those little dead-end copper tubing stubs sticking out of small refrigerator compressors and such are for. I delved further into what recovery machines can and can't do. There are all kinds of arbitrary facts on what year certain restrictions kicked in, and how much negative pressure a given size system needs to be recovered to, and even how much a responsible technician or company can be fined per incident of illegal venting to the atmosphere. Much of it is interesting science; some was just arbitrary cramming that I've already forgotten but could easily look up in the material I collected. And all of my study material came for free with a little google-fu. Taking the test was a bit of a saga, leading to one total fail for completely unrelated reasons at the company my buddy had recommended, and then a successful test session at their competition. And I *aced* the test, only two wrong out of 100, handily earning my "Universal" certification to work on all four types of equipment. The same shop where I took the test accepted the temporary PDF printout of what my card would look like when I went back the next week, and I drove triumphantly home with a new pink jug of R410A and gave my system the three additional pounds of juice that it had been craving. Now I could at least cater to its drinking problem without outside help, while continuing to chase the leak conundrum. As more time went by it was clear that the leak was still present, albeit a little slower than after my buddy had taken his stab at it, so there was some maddeningly variable factor still in play. Even armed with a better sniffer by now, I was still basically getting nothing on my leak-hunting efforts. I got a few one-off sporadic readings around parts of the indoor coil, and couldn't quite determine if the UV light spotted a tiny hint of bright green?? buried deep in one section of the finning. Probing the indoor coil is easier since the air around it can be made absolutely still, and refrigerant tends to drift downward in air. But those hints weren't consistent, just like for the first tech who visited this thing when the problem first began. The primary suspects were still elsewhere, however, and one of the purchase cycles from Amazon included the gear necessary to change out the access-port Schrader cores. The guys responding to my thread at HVAC-Talk agreed that in a system eight or nine years old, the original valve cores could easily be suspect. |