Alter ASHP hot water hysteresis?

[Tom]

Hi all - my Ecodan ASHP comes on once a day to heat water but is set up in such a way that if it is only 1deg below target it comes on. Is there a way to set it so that it would say need to drop 10deg before calling for heat? I cant find this in the normal or installer menus. 

Thanks all

[Tom]

One at a time please.

 

So, on further I research I see that the hot water cylinder should have two temperature probes: one half way up that calls for heat, and a lower one that shows when up to temp and shuts the heat off. Here's the thing: my cylinder only has one probe, the lower one. So does this mean that, assuming the lower probe remains below temp and not "shutting of" the heat, the absence of an upper probe means it will always call for heat?

[SimonD]

It's difficult to help without knowing exactly what model you have installed with what indoor unit and cylinder. But there should be a menu for DHW on the controller where you can access the settings - it's in the locked menu IIRC. 

[Tom]

Thanks @SimonD, it's an Ecodan ASHP (monobloc so no indoor unit) and the cylinder is from Tempest designed for ASHPs.  I have entered the installers menu on the Mitsubishi controller and see that you can set one parameter "max temp drop" which is set at 10deg, research today indicates this is the hysteresis. So, with this set as it is, it should mean the cylinder doesn't call for heat unless it has fallen by at least 10 degrees below the set temp. However, as above, mine does - so I'm wondering if it is the lack of a second temperature probe - the one further up the cylinder - that is causing the problem and making the controller always believe the tank is >10deg below target.

[JohnMo]

I did plenty of testing. On a light use day a big hysterisis may mean your whole cylinder drops well below a usable temp. A normal use day it may be fine, but suspect you will have difficulty conversations with the other half, when the shower is like warm at best. So be ready for your head on a plate

[ProDave]

I think mine is set to 2 degrees hysteresis.

 

It is more about where the cylinder thermostat probe is placed.  Initially mine was in the upper pocket (I only have one probe) and that meant the ASHP did not come on for a long time, as with normal HW usage the water does not mix and remains in layers and with the probe near the top it was a long time and a lot of water usage until the hot cold transition reached the probe and the ASHP started heating.  For certain members of the family that like long showers that could mean the shower ran out before time.

 

Moving the probe to the lower pocket meant the probe sensed the cold water much earlier on in the usage and started re heating sooner.