Downsizing from one rural location to another, I was keen to avoid certain necessities of life in the countryside, such as septic tanks and LPG fuel tanks. If you’re going to avoid LPG or Fuel Oil then if mains gas isn’t an option then it’s likely that you’re going to be dependent on electric for heating.
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Back up heater for ASHP failures
©Harveywetdog |
That has certainly been our experience. In our case the main heating and domestic water heating is provided by a 8kW ASHP system with two types of additional heating in bathrooms and a log burner in the lounge. The ASHP drives under floor heating downstairs and wall radiators upstairs. All downstairs rooms have individual smart thermostat control. There is a single smart thermostat upstairs although all radiators have individual temperature control valves.
So what are my initial impressions? I think most people will be aware that ASHP heating and conventional boiler type heating are completely different animals. Whereas a conventional boiler will be typically called to deliver heat at specific periods each day, the ASHP will be left to run continuously and keep the house constantly warm. And that is my initial conclusion; the ASHP keeps the house warm whereas a conventional boiler heats the house.
I appreciate this is a philosophical difference, but I think there are times you want a source of heat for comfort purposes, rather than keeping warm. A blazing fire has a better psychological effect than looking at a thermostat and saying yes it's definitely 22 deg C. You're better coming in from a cold dog walk and burning your bum on a Rayburn than acknowledging the under floor heating is doing a good job. I suppose that it is why the architects provided the log burner. It is a point of focus, it heats the room up far more than it really needs to but at the end of the day it makes you feel good.
But back to our basic ASHP system which comprises a single Stiebel Eltron HPA-O CS Plus (GB) Air Source Heat Pump coupled to a HSBC 200 S (GB) Direct Hot Water Cylinder. The HSBC unit is actually a Hot Water Cylinder and a Buffer Cylinder located one above the other to save space. It appears to be the brains of the unit deciding whether the ASHP provides heat for hot water or room heating. We have the ISG fitted which allows us to monitor the system remotely either by phone app or through a PC. The phone app is pretty superficial but the PC gateway provides a lot of data from the unit together with the ability to adjust settings remotely. Both phone app and PC gateway don't provide a lot of trending data, but Stiebel Eltron can do this remotely if required.
I should have said the HSBC unit contains the WPM heat pump manager which is the main user interface if you don't have an ISG. It's ok, and I suppose you get used to it, but using the PC is much easier.
As we moved in early July I didn't have a lot to do with the ASHP to start with. I think I may have adjusted the DHW temperature using the WPM following a complaint from the boss. I then got the link to the PC and the phone App working. I recall I had to contact Stiebel Eltron to get the App working. The builder had installed an ethernet cable from the plant room to the cupboard where the router was so it was just a case of linking the heating ethernet to the router and then searching for the gateway on our home network and instructions must have been good because it worked straight away.
Ultimately your system is only going to be as good as the architect who designed it and the plumber who installed it. Although it is early days I'm a bit nonplussed by the open ended nature of heating system. The Stiebel Eltron system produces hot water for the underfloor heating and upstairs radiators but the temperature is controlled by the separate Heatmiser controllers. I can see why they've done it, and being able to set a different underfloor heating temperature for all the rooms is very nice, but I'd like to have a single integrated window on the system which showed what the ASHP was doing as well as the temperature in the rooms.
Anyway back to my story; in the summer I realised some of the radiators were coming on overnight so I set about creating a sense of order with the controllers. Although people tell you you should leave the system running all the time I have radiators off from 11:00pm until 4:30am, underfloor heating off from 11:00pm until 5:30am and DHW off from 11:30pm until 4:00am. At least that shuts the ASHP up overnight!
We have the Heatmisers set to ~22 deg C room temperature and the hot water set to 49 deg C. On the App I have the underfloor heating set to 23 deg C and the radiators set to 24.5 deg C but at the moment I don't know if that makes any difference so that'll proabably have to wait until my next update. Internally the underfloor heating circuit temperature is set to about 32 deg C and the radiator circuit temperature is set to 45 deg C.
After this early dabbling and setting our comfort profile via the Heatmiser controllers, all went well until late November when we experienced our first really cold night, well at least below -2 deg C.
What I hadn’t appreciated is that the ASHP design struggles with below zero temperatures and relies on two immersion heaters to progressively cut in and take over from the ASHP. Now I was aware that we had two immersion heater switches adjacent to the HSBC unit, simply no one had explained to me that were essential to the operation of the system. In fact there appeared to be an apocryphal belief on the site that one of the heaters should be turned off.
The bottom line was that when the cold weather hit and the unit called for the DHW immersion heater to cut in then the immersion heater didn’t come on and the unit decided to stop calling for heat from the ASHP. One of the disadvantages of a low energy home is that they hold onto their warmth and if something goes wrong you aren’t immediately aware. This was certainly the case for me as although I suspect the ASHP tripped out in the morning it wasn't until the evening that we realised that all wasn't as it should be.
When things go wrong with something you certainly start to learn quickly and begin to understand what you've got and what can go wrong. I phoned the system installer, well his name and number were proudly displayed on the HSBC unit, but as it was now about 10:00pm on a Friday evening he didn't feel the situation warranted his attention. I tracked down a Stiebel Eltron system handover document, the sort of thing a responsible builder might have given you when you moved in, and this told me I should either force a reset of the system (not quite turn it off and then back on but the ASHP equivalent) or if that fails run the system in Emergency Mode. Running the system in emergency mode basically means letting the immersion heaters carry the full load and as that sounded a bit drastic I went for the reset option and I'm pleased to say the system burst back into life.
To be honest I think I was lucky that the outside temperature had warmed up a bit since the system had tripped out in the morning and so the system was no longer reliant on the immersion heaters. I found one heater was turned off at the wall and one was turned off at the main fuse board. I should also point out that neither the phone App or the PC gateway reported a fault. In fact the gateway proudly informed me "your system is operating correctly; no faults".
As my installer didn't appear interested I dropped Stiebel Eltron an email last thing Friday and I got a reply on Sunday telling me they'd made some adjustments to the system in order to avoid a repeat. I never got a reply as to why the system had not reported a fault, one of the people I spoke to suggested the situation is configured to favour the heating side over the DHW function, but that didn't really make sense and I remain convinced that the system is simply poorly designed - von Eseln entworfen as we say in Germany!
I felt very proud to have got the system and also felt confident enough to turn the immersion heaters on as the surest way of avoiding a repeat. Of course as I'd only found one heater turned off I didn't know for sure if that had been the reason for the system failure - this all required further investigation as I will explain in Part 2!
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