Hi. I posted a while back about a heat pump installation (http://www.refrigeration-engineer.co...ight=smartrads). Simply put I've asked them a number of questions and wanted a second opinion as to whether their answers make sense - I've pasted their answers below. They only do two pumps (a 12KW and an 8.5KW) and want to install the smaller one. I asked them about the unit multi cycling due to being oversized, and about whether the cost would be more than running a smaller pump, and whether the on demand nature of fan convectors would lead to multi-cycling. So far these guys have answered all my questions/been far more professional than the other one I've looked at, but I'm keen to get opinions of those more in the know!
> In regard to your questions, a heat pump will NOT multi cycle as long as a buffer tank is used AND provided enough water per KW output of heat pump is contained within the buffer. In the case of our ***** PWR12, at least 130 litres of water in the buffer tank is needed. The unit can never multi cycle if this MINIMUM requirement is met.
> In the specific case of this small heat load (1.4KW), the buffer tank is charged with heat and then the heat pump automatically switches off. It will only switch on again when a 4 degree temperature drop in the buffer tank occurs. In the case of* a small heat load, this will take some time, maybe 20-25 minutes. The heat pump will then switch on for approx 5-7 minutes and then switch off agin. This would be the approximate cycle in the case of a small heat load. The guidline for maximum cycling of heat pumps is 4 to 6 cycles per hour, as you can see in this case, we are comfortably below this. If there was a larger heat load, the heat pump would be on for longer in the cycle and off for less.
> Provided certain criteria as set out above is met, the heat pump will never multi cycle. This will then leave the heat pump to take care of the heat demand. The cost to run a heat pump of say 6kw output against a heat pump of say 12kw output will be exactly the same since the 12kw will be on for half of the time the 6kw would be on for.
> Regarding the question of the fan convectors, this makes no difference to the cycling of the heat pump since the heat pump is only connected to the buffer tank.
> I would recomend installing a programmable room stat. This will enable the user to programme temperatures for different times of the day. For example, at 7am the target should be 18C until 9am. From 9am to 4pm 15C, then from 4pm until 10pm 21C and then from 10pm until 7am the next morning 15C. Heating the home like this provides a stable enviroment whilst maximising the efficiency of the heat pump. The costs to heat the home will be very similar to the heat load/costs that we have provided.