Yes indeed, and Nibe have been using degree mins for very many years. I guess many of the the air con derived systems have controls that are somewhat crude. Its a complex topic to grapple with.
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The FTC5 manual says it does, running the water pump after the heatpump shuts down to discourage scale build-up.
I still suspect there is a large but relatively simple formula for T_flow = f(T_int_target, T_int, T_ext, T_ext_forecast, T_ext_forecast+1, E_n, E_n+1, E_n+2, cooling_coeff, heating_coeff) linking interior and exterior temperatures and forecasts with energy prices and some things reflecting the thermal performance of the house, finding the current flow rate for minimum cost to hit the target temperature. I’ve not figured it out yet, though!
Or maybe there isn’t and that’s why manufacturer heat pump control programs seem to perform worse than enthusiast ones… 
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For me it’s the n+k things that become interesting because that’s where humans think they are adding value by controlling things. Unfortunately I’m missing the bit that predicts when a teenager will take a shower to know how long I can defer the DHW.
BTW, just to give you hope, we created suite of machine learning algorithms at my old job and in the end found that you could get similar results with linear regression which would only need 3-5 terms in the polynomial to give reasonable accuracy.
It might amuse you to see there’s even a library to explain ML models like you are 5:
Here’s another snapshot showing the effect of a forced pause (gridlines every 10 minutes) (Dec 4th am)
The first couple pauses didn’t really work, as HP had already decided that it wanted to start another cycle before it got the memo. CoP was around 3.7. The third pause (dip in the middle of the chart) did take hold and resulted in an hour long cycle with better CoP (4.0) even thoug the temps were 2 degrees higher.
Nice.
However, I have to ask why your flow temp is exceeding the desired flow temp? I’m mostly asking because my “walking up” of the desired temp ends up with the heat pump hugging the blue line pretty tightly. However, my deltaT is nothing like as good as your 5 °C
I’m impressed the deltaT stays so healthy as your flow temp rises so much.
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My EcoDan sees the target flow temp to be more of a guideline and will happily exceed it. No idea what is going on in it’s little brain, but performance is okay so I’ll leave it to do what it does.
I’ve since tweaked my “walking up” algorithm, and I’m seeing tighter correlation of flow to target temps.
(90 minute run here)
Yeah, I’m pretty happy with the dT. Shortly after installation, I was getting barely more than 3C, and it was very noisy. The engineers then reduced the pump speed to reduce the noise, and the dT also improved.
I also keep the TRVs as open as possible, 5 in main rooms, 4 in bedrooms to limit overheating.
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I’m learning that my (oversized) heat pump has a minimum output between 4 and 6 kW, depending on the outside temperature. If the radiators can’t emit that all that energy then the flow temperature will keep on rising. Meanwhile, the heat loss of the property is only 4 kW at 4C, which is less than the minimum output of the (11 kW) heat pump. (I’ve see it go as high as 14 kW!)
(MCS calculated peak load of 9.5 kW at -3C. I’m seeing actual load of 6 kW)
- HeatingEnergy (W) is the measured heat output from the heat pump
- EmitterOutput (W) is computed the wattage of all radiators for the flow temperature
- HeatLoss (W) is computed based on inside - outside temperature x heat loss coefficient
When HeatingEnergy is below EmitterOutput (assuming all TRVs are open), the flow temperature will stabilise, and the HP will run for a long cycle. Otherwise, the flow temp creeps up and up until the HP stops for a breather.
Also, as HeatingEnergy is often above the HeatLoss of the building, the average temperature of the whole house is either increasing (when it’s on) or decreasing (when it’s off). Like a good ole gas boiler. /sarcasm
So, during milder days (i.e. above freezing) my HP is doomed to cycle, either bouncing the flow temp or the house temp. I don’t know if fiddling with flow speed will help, or I need to add more radiators. Probably needs a smaller heat pump.
On the other hand, the house is warm, and it’s still cheaper to run than gas, so I should be content with that.
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That’s pretty much the point I’m at.
My heat pump is clearly capable of generating enough heat to keep us warm from the evidence of the last three years.
I still remember ten years ago when it was -15 °C for a number of days and I need something to keep us warm when that next happens in a few years time. Talking about how efficient my heat pump is when it’s warm won’t help.
So I’m willing to tolerate a fair amount of inefficiency to ensure we will always be warm.
In my case we have a lot of large multi-layer radiators which are good at releasing heat, but most of the time my flow temp wanders up like yours and the house would overheat if we just left it running.
When we get better insulation it will clearly be much too large and will produce too much heat at its lowest setting so will have to work in bursts. The bursts with the oil boiler were bad because the radiators got to 60 °C and it was “too hot” and then later “too cold”. With the low flow temps of an ASHP the radiator is just warm and then later just cool so the variations are more bearable when the heat pump is shut down.
I actually quite like having a “transit van” of a heat pump because I wanted something that would just chug along and do it’s job without needing the device to have lots of specialist intervention. My tinkering with the algorithm is just a hobby and the Ecodan would be OK without me doing that.
I believe that if I slow the flow rate the heat pump will turn itself off because it can’t get rid of the heat it has collected.
Of course we’re all looking forward to heat pumps that can modulate better. It’s easy to forget that they didn’t modulate at all in the past so it was much harder to get things working smoothly back then.
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This lack of accurate estimates for the design and a fear of under specifying is a serious issue IMHO. It is seen in so many different ways.
It is the question of whether the main heating source should be sized for all eventualities? My thoughts are that as long as it can deal with the average cold conditions, then in the long term accepting that you may need a couple of fan heaters to cover the extremes, may be the most efficient overall.
So a smaller HP, that can do what you need efficiently for ~90% of the time, and then supported by other direct heating options for the remaining extremes, might be more efficient than an HP that can cover all eventualities.
For afar, it seems a smaller HP, that can modulate right down, is likely to be the better option.
As an aside (but related) I’m looking at this Direct Water Heater - https://www.stiebel-eltron.co.uk/en/products-solutions/dhw/instantaneous_waterheater/compact_instantaneouswaterheater/dce-s-plus/dce-s-10-12-plus.html. It seems it will take warm water up to 55°C and I’m hoping it can be fitted inline to the DHW. If I can, for me this would mean I can run the gas boiler more efficiently and not heat the tank so high (with the associated losses) and for a HP, it might mean not needing the DHW heating period, so again possibly increasing overall efficiency.
Because of the location and planning of my house, I have quite long runs to the furthest shower, so the water in the tank need to be hot enough to cope with that.
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Yes, the oversizing problem seems to be one of the consequences of the various grant schemes to replace fossil burners which say the old heaters can’t remain installed in a way that could be used as primary heating. So some useful things like clean infrared and storage heaters have to go, while we can keep our dirty wood stove for the rare times it’s below -7.
It makes sense in one way, in that they don’t want less efficient things like IR heaters kicking in at peak times in deep winter, right when everyone has heat pumps running at max and they’re less efficient and renewable electricity production is at its lowest.
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True, but from a whole system view, a smaller HP that needs some ‘inefficient’ boost, is probably going to be more efficient overall, plus as a bonus, should require lower capital investment thus whole life cost is lower.
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A New Thing which would mostly have the effect which you desired, without sneezing the complicated and unknown internal shufflings of heat pumps (or fridge-freezers) would be for the manufacturer to provide a way to “bump the Thermostat”.
Such could be retrofitted to any device which you are ok to take the lid off which contains an “NTC” type thermister (there is more explanation about a 3.9kOhm@25C thermister somewhere near to this forum) and in which the ace dabbler can safely find the (?two wires?) going to that thermister. The mod is to cut one of those wires and solder in a new proposed board containing a small series resister (for example 200Ohms to increase R at 45C from 2kOhm to for example 2.2kOhm unless a single transister bypasses that extra R). Then drill a hole in the case to feed in GND and a +3.3V DC “bump” signal which goes into the transister base (or gate) through quite a large resistor such as 33kOhm. While bump=3.3Volts, the temperature sensor and appliance behaves (roughly) as it used to. While bump=0.0Volts, the appliance aims for a higher setpoint T, for example heating the room preferentially while bump=0.0 but sufficiently while bump=3.3. The appliance thinks that its T sensor is seeing (for example) 2.2 kOhm instead of 2.0, which can be used to encourage it to try harder to do heating for as long as the bump wire remains 0.0 or until its internal (and unknown) workings decide that it is time to stop; for example if its NTC warms up to 1.8kOhm then it sees 2.0kOhm and its usual thermostat decision says “warm enough, stop”. This is a two wire interface which should be compatible with raspberry pi GPIO pins (two pins amongst the eight rows to the right of the RF shield if you have one) as long as your raspberry pi ground is the same as the NTC sensor ground.
The choice of for example 200 Ohms to go with a 4kOhm@25C NTC near 45C defines how far apart the two setpoint temperatures will be so you do need to know the R(T) curve of your thermister and where on it you expect to be aiming for.
The best place to “bump the thermostat” could be at room target temperature, which does get recalibrated for two setpoints (T while solar power is plentiful OR T while solar power is scarce)
Has anyone looked under the lid inside their heat pump and found out what sort of temperature sensors are in theirs and how they are connected? Vaillant gas combi boilers use the water pipes as ground, which is annoying, because I’d want to insert the “bump” device between ground and sensor.
It might be more efficient for you and lower cost, but it might not be when you take system cost into account. If you need more networks upgrades and more peaking/low renewables plant, perhaps hydrogen fired at 35% round-trip efficiency, and operating at very low capacity factor, then that could quickly cancel out your benefit, certinaly from a whole-system cost perspective.
True. But I’m not going to be thanked or rewarded for taking that into account, so what is best for me will always win. That’s why HPs tend to only be done by those with sufficient capital and the social conscience to spend it that way. The rest buy new cars and take foreign holidays. This is also true for home insulation, electric cars, solar panels etc. etc. Until the financials stack up, it will only ever be the minority that take it up and they usually have money to spare to do so. And as ever, those with the capital benefit (look at solar panels and early FITs and the money made off them) and those without are penalised.
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Well i guess thats for people like me, working in BEIS clean heat policy, to sort out!
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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.
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Quite phenomenal though, that a bit of kit worth multiple £K needs to be forcibly switched off to be most efficient! 
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.
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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.