Posted by: @mike-camelot

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don't have a room (central) thermostat. That was actually my question. My system has been working quite well while the OAT has been cool. Now we are seeing higher temperatures I was considering fitting a central thermostat (NIBE BT 50) so that I could set it to a temperature that would more efficiently reflect the demands of the house. I was asking for feedback from anyone who may have this unit fitted

It may be a bit house dependent.  In my first year I used pure weather compensation without any room influence for almost all of the season, except at the end where I used the sensor in the heat pumps own controller as a temperature limiter to deal with excess solar gain in March/April.  This year I haven't done that, its just on pure WC even now (I switched it back on today).  In between last year and this I tweaked the high oat end of the WC curve so the min permissible ft is a few degrees lower.

I did, very early on, try using the temperature sensor as a controller layered on top of reasonably well adjusted wc, but it was a very poor way to control, resulting in much greater temperature swings and far inferior comfort than pure WC, nearly as bad as when I was running a boiler at the high flow temperature so favoured by plumbers!

Hope that helps

Many thanks for your post. I assume that the "house" thermostat (BT 50) setting is included in an algorithm that controls the heat pump. Without any house temperature input I have noticed that even with OATs of, say, 10 - 12C the solar gain from the south west glazed area and distribution of air with the MVHR still results in heat pump supply. I realise that I could adjust the WC, but it is just right for the cooler nights that we have been getting recently. In these conditions I was thinking that if the "system algorithm" had the internal house temperature input it would reduce the heat pump run cycle. I hope that makes sense?

Posted by: @mike-camelot

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In these conditions I was thinking that if the "system algorithm" had the internal house temperature input it would reduce the heat pump run cycle. I hope that makes sense?

 

Whether your heat source is a boiler or a heat pump the problem with any algorithm that uses house temperature directly is that it's rear view mirror stuff, houses take a long time to react, you really need to know in advance what's happening before it hits the house.   This is particularly the case where the house may otherwise overheat as we usually don't have active cooling so are relying on loss from the house not the heating system to deal with the situation. 

This is one of the advantages of weather compensation (again whether your heat source is a boiler or a heat pump) it does know in advance most of what is going to happen because it reacts to the principal outside stimulus not the house itself.  For many houses this results in the best control, aided by the fact that the emitters will anyway emit less heat if the room is over temperature thus reducing the run cycle, put another way low temperature heating is a bit self regulating.

Of course in principle an integrated controller could learn about your house response over a period of weeks then use this plus OAT plus forecast solar , wind etc from the internet to make a decision what to do.  The heat pump specific ones (homely, havenwise, adia and passive) do that to a greater or lesser extent.  Regular thermostats generally do not (and are generally bad for heat pumps) and many people prefer to have a heating system that is not dependent on the cloud for it's operation.

The bottom line is that there is no perfect control system for any heating whatever your heating source and so we have to live with something close.  What that is depends on the house, but properly adjusted weather compensation is almost always a good place to start and may well be the best place to end.

@jamespa 

Need to find a good thread on the effectiveness of WC control approaches.. Particularly for Vaillant, Mitsubishi and Grant 

Posted by: @batpred

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@jamespa 

Need to find a good thread on the effectiveness of WC control approaches.. Particularly for Vaillant, Mitsubishi and Grant 

Loads of discussion on the forum over many years.

To first order WC is WC independently of manufacturer and if you operate pure WC there is not much to choose between the controls from the manufacturers.

The differences are what they layer on top and how its presented.  Vaillant for example present theirs with a single parameter more easily adjusted by the user.  Furthermore the timer/programmer (where you can change 'set temperature with time) actually shift the WC curve rather than invoking a temperature sensor and resorting to on/off behaviour.  Ideal did something similar with their R32 models at least, and I think Nibe do also (it seems to be a European thing - I presume because the europeans had WC for boilers).  Im not sure about Grant, Mitsubishi. 

In addition Vaillant and Mitsubishi have some sort of auto adapt, again I cant remember if Grant does. 

All of this stuff layered on top matters only if you actually want to depart from pure WC or pure WC with a limit.  I doubt that there is a universal answer to that question because it will almost certainly depend on house.  As It happens I have one of the more functionally rich controllers and don't currently use much of its capability because I operate almost exclusively on pure WC.