I’ve been running with this relay for most of the winter season, and have found it works pretty well. The technique I used was wait for the compressor to disengage at the end of a cycle, and drop the target flow temperature by 6C. This would convince the heat pump to stay off for a few minutes before starting a new cycle.
However, I did eventually realise a couple minor issues with this approach:
if the flow temp hadn’t yet reached the target temperature, then a 6 C dip below the target doesn’t quite have a big enough affect, and the next cycle starts up soon after. This could be seen as a feature, as it allows the heat pump to reach the target earlier, but this isn’t what I want for my oversized system.
the circulation pumps keep on running while the compressor is off, which seems a little wasteful. It’s only about 120 W, but it’s still going to be dragging my COP down by a tiny fraction.
So, I’ve switched to something more like John’s original suggestion: when my program detects the compressor stopping at the end of the cycle, it turns off the power to the heat pump for 10 or 15 minutes to let the whole system cool down a bit. Then, when the heat pump is turned back on again it will begin a new cycle from a lower flow temp.
I need to put in some smarts to decide when the pause should be 10 minutes or 15 minutes. The shorter pause is better in the morning when trying to heat the house up, while longer pauses are better in the afternoon when we just need to maintain the temperature. I might just stick it on a simple timer for now.
Indeed. Had it been correctly sized to begin with, and been installed with the right controls, I wouldn’t have had to do this. I may at some point fit better controls so that I can use the built-in “autoadapt” feature, which seems to work well on other systems I’ve seen it on. But for now, I can get better performance for “free” by using a software hack.
I think the EcoDan has it’s own “wait X minutes between cycles” feature, but I’ve not be able to find it on my system.
Auto Adapt is something I have wanted to study using graphs, but I have never done it methodically enough… and given up. Interesting the observations that speed goes lower in auto-adapt. Not sure why that should be. Is it because it knows the room is warm. But if 3rd party stats are used, it assumes some rooms could be quite cold, hence the mild panic and rev-up?.. I don’t know
Anyhow, looking at the way Auto Adapt seems to look at the rate of rise in room temperature, it should be possible to trick the room sensor and ‘tame’ the system. It might take a while to fathom it out though
Morning All,
I’ve been running my system under Home Assistant for a few years now.
Before I got rid of my Grant UK unit, i was playing around with the reduce capacity switch of the heat pump.
This would be controlled Via a Shelly relay, under HA automations like, CO2 grid data, Octopus Tariff Data, House grid consumption etc.
My planwas to be able to reduce the output by 50% via ( a flick of a switch ) unfortunately the Grant Unit would only go down to 50%, so if it was running at 80%, the switch would take it to 50%, if the unit was running at 40% it would stay at 40%.
This would work in the colder days where the unit was working hard, but in the shoulder months, not much use to you at all.
Instead of trying to control the heat pump, why not install a throttled zone bypass valve across the primary flow and return, like a 240v Actuator TRV, this would increase the return temperature and then in turn the unit would shut down due to a tighter DT.
I expect the pause should take into account the length of the last cycle, for example 30Minues-lastCycleLength, then the heatpump will automatically be allowed to run more when more heat is needed.
Degree minutes (for starting and stopping a heat source to maintain a set delivery temperature) PLUS a minimum time between restarts (to protect motor starter / compressor) PLUS a random time for electricity grid safety (wait a random length of time of up to 3 minutes before starting) in the event of blackout/grid cold start.
I’ll dig out the manual. Shouldn’t be difficult to replicate the maths.
If anything I’d say that the AC vendors can’t use such crude controls because nobody likes blasts of hot and cold air from fan coils with no thermal capacity.
Also in many countries fan convectors are more common than UK style radators. But I wish the UK regulations was updated to require that all new heatpumps offered Degree Minutes as an configuration option.
In the “winter of discontent” my dad was running the power cuts for large part of London, they had to reconnect small area at a time and wait until all the eletric powered DWH had reheated before reconnect next set of areas. If all roads fed by a substation was reconnected at same time, then the transformer would be unhappy.
Once most homes have heatpumps, unless a way to remotely force them not to start, it coud take well over a day to connect a town after a long power cut in cold weather. So people will not have lighting and communication for much longer unless population agree to “big brother” having control over when they can use electric loads.
One option would be to require that heatpumps shutdown if grid frequency is very low, and that they have a max flow of 15c (for protect against activating freeze protection values) when frequency is a little low. But grid frequency can only be set national wide.
Shame smart meters don’t have a function to temporarily rational electricity useage in an emergency, so a person who uses electric heating/cooker get disconnect by their meter for 20 minutes.
Frequency sensitivity is an eminently sensible feature. Most units lack it. Many shut down at about 208VAC for motor protection purposes; but that only considers local effects not national grid availability.
SMETS meters do have auxiliary contacts that are remotely manageable and could be hooked up to the smart grid inputs (must not run; can run; should run if possible) on heat pumps.
The incumbents currently managing the grid lack the imagination to regulate distributed assets sensibly though. (else they’d have mandated, for any permanently connected material grid assets, both frequency based load shedding/load adding and voltage based load shedding/load adding as a foolproof means of grid management)
I realise that I only have experience of degree-minutes and fixed speed compressors. This issue of achieving minimum compressor speed seems to be a tricky juggle. How do they do it? for non degree-minute,. if we operate on a heating curve and room thermostat control, then in normal conditions, the system should achieve the room temperature, then stop, and after some time , and the room (or rooms?) cool, and it re-starts. However, its not necessarily ever at minimum modulation, and if we were to drop the curve, it may keep running (no cycling) and achieve a good house temperature. This is the essence of auto-adapt etc.?? The way i often run mine is to fix the flow temperature for periods (say beween 30 and 36). The old 5kW unit modulated nicely down to minimum, and may eventually cycle off because the radiators have crept above the set point setting. i have never really thought this through properly, … its a big topic, but there must be various ways of getting the best revs (not too high, not too low) and to avoid cycling. Maybe the gains here are relatively small, so that is why some contols to date have been somewhat crude.
Basic option: run at best efficiency point; ramp up output if the supply temperature cannot achieve setpoint; ramp down output if supply temperature begins to exceed setpoint; cycle once the minimum output is too much.
Potentially with some minimum runtime or minimum runtime at a set power level (exceeding supply temperature target) for oil return type reasons.
This appears to be the behaviour of most units if you watch “a cycle” on the charts.
Hence why they can “go nuts” when coming off a setback or there are sharp changes in the commanded setpoint.
