Reasons for Ecodan overshooting target fixed flow temperature

I am having a few issues with my 11.2 Kw Ecodan. Basically it overshoots the target flow temperature and turns off. I had been running it in weather compensation mode 40 @ -1 30 @ 15, and the unit would overshoot the flow temperature indicated by the “curve” and then cycle off, then spring back to life a while later. To be fair I had a good conversation with my installer yesterday and he seems committed to rectify things. As an experiment, the installer has suggested running at a fixed flow temp of 45 degrees, which I am doing. What happens now is that once or twice an hour the temperature shoots to 60 degrees and then of course it shuts down for a while. If when the flow temp is high like this there happens to be a hot water demand, the immersion heater kicks in. What has struck me is the almost non existent dT when say 1 of the underfloor heating tracks is demanding heat. Even when both zones (rads and underfloor) the deltaT is never more that 2.5 or 3 degrees and there are maybe 2 cycles in an hour, which on the face of it isn’t that bad… but I am sure there is some issue. A bit about the set up… low loss header with a radiator circuit and an underfloor circuit coming off of it. Has anyone experienced similar?

Hi. I have different setup but problem is similar. I have a 8kW system with cylinder unit + PUD-SWM80 outdoor unit and a 2-zone kit without buffer tank. All UFH (water capacity around 75 litres). Should be sufficient since minimum for PUD80 is 29 litres.

Problem is: Zone 1 (bathroom) overshoots after a while. Zone 2 (other living areas) stays at the asked flow temp. For example at the moment it is -11C (I live in Finland) and I have hysteresis limits set at +5/-3 and running on weather compensation (actually I have tried with fixed flow temp and it acts the same). With -11 outside the curve is asking for 32C for UFH. Since hysteresis allows +5 the flow temp will rise until 37C and then compressor shuts down. After a few minutes it starts again and runs for 30 minutes. Downside here is also when Zone1 keeps getting hotter is that the valve for Zone2 is getting more and more closed. I think that will affect on the system and it shuts down.

Below an image stating this behaviour. I have set the cylinder unit main pump to 2 which means 11-12 LPM. The pumps on the 2-zone kit are set constant pressure (because of UFH). Zone1 flow based on calculation is 3,65 LPM and Zone 2 if 10,5 LPM. I know the sum is more than cylinder unit pump flow but this way it works best for now. My aim is to have the valve half way closed to then the sum would be less than the cylinder unit pump flow. Now the Zone1 and 2 are set to similar curve and no vertical adjustment. Still it does not provide the 32C water to Zone1 like it does for the Zone2. My assumption was that these would be same flow temps and when I adjust Zone1 vertically like +4 then it would give 32+4 hence 36 to Zone 1. But with this current situation the process is not predictable/adjustable.

I think I have some insight into my own issue. So I think whats happening is for some reason the radiator zone circulation pump is being switched off inappropriately, when the heat pump continues to produce heat and the primary pumps are running. This inevitably means that all that hot water is forced through the low loss header shortcut so the flow temperature rises sharply and excessively, the unit cycles and remains off for a while, during which time the radiators go cold. So, it seems the radiator circulation pump is controlled by the heat pump controller, not just the zone thermostat and I think this is probably incorrect in this situation. The zone thermostats should probably be the only thing controlling the circulation pumps. The heat pump will of course control the pumps on the primary loop only… there are 2, not sure the reason for that!

Good to know. My setup doesn’t have any thermostats. I built it simple because if wanted in the future there is a possibility to add wireless room thermostats (and control box with actuators above UFH manifold). So my original plan was to use only weather compensation. Not sure if I could gain anything by adding Mitsubishi wireless room controllers for both zones. Not even sure if 2 controllers could be used at the same time. But I wouldn’t want to invest if not certain it would work. My friend has the same ASHP as I do but he has only 1 zone (with thermostats). He’s running weather compensation without these kind of issues like I have. His ASHP keeps running constantly with only defrost cycles.

It is -10C outside so not much changes there but now the ASHP has gone mad like this: