In April 2020, and in the interest of legacy, I penned a Wikipedia entry recording the thoughts and notable achievements of myself as Harveywetdog. I admit I was ignorant of the rules concerning self promotion on Wikipedia at the time; consequently my entry was unfortunately, although arguably correctly, deleted and my account expunged from the system. As a result my original words and links were sadly lost but nevertheless here is an attempt at a rewrite. Perhaps when I'm gone someone will be able to enter it onto Wikipedia on my behalf, to serve as a fitting epitaph for the exciting times I spent on the Harveywetdog Project.
Air source Heat Pumps - Are They Any Good - Early Experience Part 2
In part 1 I described my early experience with my ASHP and how it failed on a cold morning in November 2025. I described how I reset the system to restart it although this left me doubting if the back up immersion heaters were working. Now let’s look at how I went about investigating what was happening with the immersion heaters.
I said in part 1 how something going wrong with the ASHP had put me on a steep learning curve. As well as appreciating the function of the immersion heaters, I also realised that the pasteurisation cycle was not set.
For those who don’t know the pasteurisation cycle is required with ASHP systems as the DHW temperature is generally not hot enough to avoid the build up of legionella in the tank and pipework. To avoid this the recommendation by the manufacturer is to raise the temperature once a week to kill off any nasties. The recommended temperature is 60 deg C and, in order to achieve this, requires use of (you guessed it) the immersion heaters.
So putting two and two together and possibly getting five my initial thinking was that the reason the heaters hadn't turned on or the pasteurisation cycle was set was because the heaters weren’t connected. Now, I know that’s a bit of a leap, but my experience snagging the property was that there had been a general lack of attention to detail; for example I had found one of the underfloor heaters had not been connected so why not the immersion heaters.
I decided that the best way to prove the immersion heaters were connected was by running the pasteurisation cycle. It took me a couple of attempts to configure the system correctly to kick the cycle off, but eventually it started, warmed the water up to about 53 deg C with the ASHP, switched the immersion heater on and then just sat there as the DHW temperature drifted slowly down.
About this time I became better able to interpret the data on the PC gateway and what this told me was that so far the back up heaters had already provided something like 10 MWh of power over the lifetime of the system. You can work out the cost of that. While I hadn’t paid for this it did make me realise that the heaters must have been connected and working at some point. Having consulted the circuit diagram I flirted briefly with the idea that the heater thermal cut out had operated.
In order to investigate further I carefully lifted the cover off the HSBC unit to visually inspect the state of play. This told me that there wasn’t any loose looking cabling within the unit and that while I could find the thermal cut out I couldn’t convince myself whether or not it was in the tripped state.
I consulted Stiebel Eltron and they were not able to help based on the photograph I sent them. Putting the cover back on is not any easy job as it is not only heavy but also very awkward to grip to lift. Another part of the von Eseln entworfen design I referred to in part 1 is the fact that if you miss the cover you’re likely to bring it down on top of the electrical connections to the immersion heaters that are crammed in at the top of unit. You’re also hampered by the height of the cabinet which means you can’t see what you’re doing as you lift the cover; luckily my wife was able to stand on the steps and guide me. I’ve think I’ve seen the new design features a split front cover which is hinged to lift up like a car bonnet - much better.
I decided to run the pasteurisation cycle again and noted to the Stiebel Eltron technical team that once again the system had stopped heating as soon as it switched over to the electric heater. They sent me a graph of the key parameters as the system heated up and as I discussed it with them it dawned on them was wrong. The DHW maximum flow temperature (basically the primary circuit temperature) was only set to 60 deg C which wasn’t high enough to get the secondary circuit DHW temperature up to 60 deg C. The primary set point was raised to 75 deg C, I tried the pasteurisation cycle again and this time the immersion heater cut in and the system finally reached 60 deg C. 👏
I was impressed by the data in the graph that Stiebel had been able to present; clearly the system is collecting a lot of history about its performance (or lack of).
And that’s where we are at present. The outside temperature hasn’t fallen below freezing again so we’ll have to wait and see if the back up heaters can save the day the next time it does.
David spent approaching 50 years in Her Majesty's Electricity Supply Industry before retiring
He was part of the highly successful design team on the Sizewell B Nuclear Power Station Project before spending 25 years producing safety cases to keep our aging AGR fleet generating for the good of the nation
He is responsible for the Harveywetdog YouTube Channel which he maintains as an outlet for his creative talents
David has experienced blood cancer treatment twice but refuses to be a victim
All views are of course his own but might be influenced by the medication he's had to take
Comments
Post a Comment