Hi folks,
We’ve had our heat pump in for a month and I’m wanting to make sure that my schedules are correct. Current DHW setup is: Setpoint 45C, Schedule 1am - 3.30am but water isn’t getting up to temp and COP is low at 1.7 average over the week. My EmonHP is showing that initial heat delivery is good up to around 40C, then heat drops off but power usage remains high (topping out at 42C by end of cycle). I’ve tried a Setpoint of 50C which gets me up to 46C - but COP remains low. You view my emoncms here.
As you can’t see the DHW tank temp on the heap pump dashboard so I’m attaching a the graph from the system for the last hot water cycle. Any initial thoughts on what changes I could make? Hysteresis is set to 0C and recirculation is off. I was wondering about trialling with two shorter cycles.
Do you have any axis for the graph with the tank temperature?
The flow and return temp are almost the same during DHW cycle. Is it possible there is a bypass valve open causing the generated heat to bypass the tank somewhat?
Apologies Chris, I hadn’t clocked that the axis had cropped. I’ve updated the original post.The flow is controlled by a single ESBE actuator with DHW priority, I agree that it looks like the heat isn’t getting to the cylinder, but that seems to be a change that happens after the first hour?
how are the flow & return temperatures being measured? What type of sensor?
how is the tank temperature being measured? Where in the tank is the sensor?
how is the cylinder being heated? Coil or heat exchange?
how hot is the water coming out of the tap?
is there a way to vent air out of the DHW circuit?
I would expect the tank temperature to reach 50° with those flow temps. I’d also expect to see more heat delivered from the heat pump, so have doubts about the accuracy of those temperature measurements.
Hi Tim,
I’m not onsite right now, so my answers maybe a little vague,
the emoncms monitoring is via a Level 3 heat pump bundle (Axioma DN20) configured by my installer (who’s fitted a fair few of these). The flow return temps on the other graph attached are taken from the heat pumps own control (econet300) so I guess will be the heat pumps internal sensors.
From memory at the centre of tank.
I think the OSO is coil.
I’ve not measured independently, we’ve no thermostatic limiters on taps, hot but not scalding.
I’d have to check, but would assume so, there’s a spirovent just below the DHW diverter valve.
Weird. That looks like a nice cylinder and I’d have expected much better performance from it. It has multiple tappings so might be worth double-checking it has been piped correctly.
Here’s a snapshot from your MyHeatpump app for this morning’s DHW cycle. I presume that’s a defrost at 01:07. Like you say, it looks OK for a bit then goes off the rails about 01:30
Tim’s questions and your answers are helpful: if the water’s hot from the taps (i.e. 50+ rather than 40-ish) might the tank sensor be under-reading? If the tank was actually 50C at 01:45 that could explain why the return temperature is so high. Is the tank sensor well-seated, using thermal paste?
Some people set up a second instance of the MyHeatpump app and connect the DHW tank feed into the ‘Room temperature’ input, so the tank temp shows on the same graph (or you could temporarily swap over the feed for the Room temperature on your existing app).
My reading of this graph: the heatpump is targeting a flow temperature of 50°, which is the temperature the DHW is supposed to get up to. When the flow reaches that temperature, it modulates down to a lower power - my Mitsubishi does the same, and a reasonably sensible approach.
From 1:50 the flow temperature is barely above return temperature, and the measured heat drops way below the carnot formula (hp factor = 0.4), indicating either a) the flow temp is under reporting, or b) the heat pump isn’t generating as much heat as it should. Given that temperature is still rising at the same rate, I suspect the former. Could be useful to see a plot of the heatpumps own sensors against the emonHP’s
This doesn’t relate to the specifics of your question, but in general if your heat pump is Air Source, you will probably get better CoP from your DHW cycles if you schedule them for the middle of the day when the outside air temperature is highest. This minimises dT between outside temp and flow temp.
Clearly you’ll need to make a balancing judgement between imported electricity cost and energy efficiency when setting your schedule, but for installations with solar and battery, midday DHW makes good sense.
Thank you so much all for your replies, I’m sorry I’m so late in getting back to them, we had various issues (due to retrofit) around connection so were missing chunks of data.
Noting what @dMb said about the early part of the cycle being good, I went back to the Grant guidance and their suggested scheduling short 1 hour hot water cycles (at least one hour apart), up to 4 a day - I think to minimise the impact of hot water priority. I’ve tried that approach with two one hour cycles and moved the time to afternoon as suggested by @dmajwool. This gives much better results from a COP point of view. As requested by @Timbones here’s a emoncms and system data for the same period.
I’m going to test a similar cycle during the night as power is a third the cost, but know this will impact COP. I’ll also setup a HPM app for hot water on emoncms once I’ve got the hot water temp in there.
Many thanks for all your help, it’s a learning curve and still a LOT to learn.
Matt
Thanks for the update and the extra graphs. That’s looking better but I’m surprised the DHW tank temperature (the green line on the second graph, I presume) is so much lower than the flow & return temperatures. Based on the spec of your tank I’d expect much better heat transfer into the tank, with the tank contents only a degree or two lower than the return.
It’s probably worth checking your tank temperature sensor is properly installed, with a liberal coating of thermal paste.
