Mitsubishi Ecodan "Smart Grid" - what does it do?

[joth]

I've been playing around with the Ecodan smart grid inputs, mostly to try and effect a "boost" and "set back" UVC temperature for times of cheap (free PV) / expensive electricity.

 

Curious if anyone else has looked at this?

So far it seems the "Switch-on recommendation" and "Switch-on command" are pretty similar in effect, not quite sure why I need both.

But more problematic, the Switch-off command. (IN11 open, IN12 shorted) has the strange behaviour of triggering all 3 pumps (primary + 2 zones) to run constantly, even though the heat pump itself is off. This seems completely counter to expectations: if the electricity is currently very expensive, why would anyone want all three pumps running?? I've already disabled the "Freeze stat" mode and we don't have any secondary heat source (booster / boiler) so it's not circulating "just in case" for any of those reasons.

 

TBH the thought even crossed my mind to put a relay on the entire FTC6 power supply and hard shut it off at times I don't want it wasting power..   Just don't want the risk that it might not like being power cycled frequently (especially if powered off in the middle of operation).  (And, the MELcloud nonsense does bleat a lot of it looses connectivity).

 

Anyone got a direct support line to Mitsubishi that might shine some light on the intended behaviour and thinking of these modes?

 

The MIs on it are fairly thin on details.

https://library.mitsubishielectric.co.uk/pdf/book/Ecodan_FTC6_PAC-IF071-3B-E_Installation_Manual_BH79D843H02

 

Quote

In DHW or heating operation, the commands in the table below can be used.
IN11 IN12 Meaning
OFF (open) OFF (open) Normal operation
ON (short) OFF (open) Switch-on recommendation
OFF (open) ON (short) Switch-off command
ON (short) ON (short) Switch-on command

 

[joth]

I tried using IN4 demand control instead to signal OFF. This looks promising as it can be configured either to mean "boiler mode" or "off" however it has exactly the same issue, whenever it is asserted, all the circulation pumps run.

So, I have complete control over heating, and can boost DHW,  but just lacking a way for telling DHW not to charge during peak price electricity. 

[joth]

Got a bit of tech support conversion going with Mitsubishi, still a bit vague but it sounds like Smart Grid features are all intended for use with a (large) buffer/mixing tank with embedded thermistor, so when the power is cheap it can charge up the mixing tank, and when expensive it runs the circulation pumps to run the heating circuits off the buffer tank only.

While I have a buffer tank, it's pretty damn small, and has no thermistor. The relevant thermistor input is left floating open and maybe confusing the controller into thinking "damn that mixing tank is way too hot, while the electricity is expensive I should do something with all that heat". This is kinda bobbins as I am intentionally not calling for heat on either zone while prices are high, and even if I did it should be very frugal in doing so during high energy prices as those pumps aren't free to operate. And moreover, it's running the primary pump too, which is shipping all the (imaginary) stored heat energy out to the outdoor unit and being lost to the atmosphere -- surely it should just run the two internal circulation pumps.

 

The mixing tank thermistor is PAC-TH012HT(L)-E   I'll see if I can get my hands on one, or at least the detailed spec sheet for it (e.g. what resistance is it when 0 and 60ºC) so I can simulate scenarios and figure out what state it needs to be in to stop the pumps (if it ever does).  

 

Taking a step back, as my goal is just to stop DHW heating when the electricity is expensive, I could mess around with the UVC thermistor e.g. to increase it's resistance (i.e. lie and pretend it's hotter than it is) to effect a "set back" mode during those times.

Edited April 29, 2021 by joth

[joth]

3 hours ago, joth said:

Taking a step back, as my goal is just to stop DHW heating when the electricity is expensive, I could mess around with the UVC thermistor e.g. to increase it's resistance (i.e. lie and pretend it's hotter than it is) to effect a "set back" mode during those times.

 

LOL. It's all a bit heath robinson, but this actually works. I used a relay to switch a 5K6Ω resistor* in parallel with the UVC tank thermistor, and magically the controller believes the internal temp has jumped up by 10ºC meaning it will need to get that much colder before the heatpump will fire to charge it up when in this "set back" mode.

That'll do.

 

(* - experimental data: at 49ºC the factory thermistor reads 2kΩ and popping 500Ω in series drops the readout to 43ºC; putting about 6K in parallel reduces the effective resistance to about 1500Ω which increases the the readout to 59ºC. It's almost certainly non-linear so the effect of the resistor in parallel will be more pronounced at lower temps, somewhat unfortunately. I guess if I really wanted I could pull the thermistor out of the tank and put in different temp cups of water and find out.)

 

 

Edited April 29, 2021 by joth

[ProDave]

4 hours ago, joth said:

Taking a step back, as my goal is just to stop DHW heating when the electricity is expensive, I could mess around with the UVC thermistor e.g. to increase it's resistance (i.e. lie and pretend it's hotter than it is) to effect a "set back" mode during those times.

Not so silly as it sounds.

 

On my LG heat pump, there is no "input" to control the DHW functions, just it's inbuilt timer.  I wanted to control it from an external timer.  So I used a small changeover relay to switch the thermistor out and in it's place put a fixed resistor, making it think the tank was at 85 degrees and so turning off the DHW heating.