I agree with most statements of tejasrojas with one exception. If the building has large thermal mass such as floor heating embedded in concrete floor I would set the desuperheater temperature higher than the hot water heater thermostats to give it a preference. Loss of the heating capacity caused by heating hot water is usually only temporary (It will not cause noticeable discomfort) while it is significantly cheaper way to heat the hot water.
Fair enough, but this scenerio is correct only if the aux heat is disabled. Otherwise, when outdoor temps fall to 85-100% design temps, aux heat is going to kick in to maintain the hydronic loop. It's really more of a question of capacity and how often the system demand is near or at full capacity.
I mention this because, as you know, some of the desuperheater controls are quite primitive. The default mode for one paticular unit consists of nothing more than a high limit set at 140F attached to the desup inlet line. The desup runs anytime the return water is under 140F even if aux heat is on. Adjustment consists of moveing the high limit to the desup output to lower the temerature. One has to pull the fuse to disable it.
Other systems have adjustable desup controls that one can match to the WH control and have additional control logic to shut down desup when aux heat is called for.
Some systems extend this control set with multiple stages so desup can be shut down before aux heat to allow the system time to meet the demand while running at full heat pump capacity. If the system still cannot meet demand additional stages of aux heat are turned on.
The better control for hydronic systems monitor outdoor temperature and use this information as part of the control loop. I think this is clearly the best way as the floor loop temp can then be matched to demand. The desup can be shut down as system reaches max capacity.
The logic that turns the heat strip on in forced air system should have reasonable time delay so the direct heat will turn on only when it is certain that the heat pump can't supply sufficient heat. It shouldn't turn on instantly when the temperature drops too low below the set point. In example when door is or window is opened for short time. I personally would disable the heat strips and re-enable them only in emergency. The short loss of comfort is not worth the money it costs.
I have never disabled ours as I have to travel too much to see it as worth the risk. Because ours is an air source heat pump, we make an attempt to provide suplementle heat (wood stove fireplace insert) whenever the outside temps fall below ~35F for an extendend period. This is not because of aux heat, but we try to minimise system usage when defrost cycles are required.
