Mats Bengtsson

Heat cost was high due to competing heat systems

Mats Bengtsson mib over the years

Our heat cost was high due to competing heat control systems

The house does have two major heat control systems in use. One is the heat system for the electric heater. The other is the control system for the heat pump. These two systems do not cooperate, in fact, they compete in such a bad way that the heat cost goes up, and that the comfort in the house goes down.

The electric heater has a quite good control system. It measures the outside temperature. The lower the outside temperature, the higher the temperature it delivers to the radiators. The temperature to the radiators can be tuned quite well. Both deciding what amount of temperature that is needed in a certain situation, as well as deciding how much the radiator water temperature shall go up depending on how much the outside temperature drops can be defined through a couple of knobs. However, it is not looking at the inner temperature to make slight corrections, for example lowering the curve slightly if the temperature is above expected.

The heat pump has a simpler system, but a system that is well tuned for prolonging the life of the heat pump itself. It measures the inside temperature. If the temperature goes below a certain temperature, then the heat pump goes on, and if it goes beyond a certain temeperature, then it goes off again. There is a gap between the temperature where the heat pump goes on and the temperature where it goes off. This gap assures the heat pump does not go on and off too frequently, which would not be good for the heat pump.

In the beginning there were problems in the electric heater

When we moved in, the electric heater behaved a bit strange. Sometimes the temperature where showing huge deviations form the curve temperature it was supposed to follow, or sometimes when I checked the electric usage, there had been used huge amounts of effect.

After a while, it turned out that all the contactors, but at varying degrees, had a problem with releasing. They might hang after the heater had turned them off, in simple cases for minutes, and in worse cases for hours. Eventually the problem was located and all contactors replaced.

The heat pump system had a major drawback seen from a people comfort point of view

The gap between on and off was about 0.6 degrees. But it takes some time from that the heat pump is turned on until there is enough heat in the radiator water to increase the heat in the house. In the same way, there is a time from the heat pump goes off until the temperature to the radiators has dropped enough not to keep on increasing the temperature in the house.

Thus, in total, the temeperature in the house moved up and down more than the 0.6 degrees that the heat pump had as its interval. The gap caused by the continued reduction of temperature after the heat pump had gone on until the radiators started to make the house warmer, and due to the temperature increase after the heat pump had turned off until the water temperature in the radiators was low enough not to make them warm the house made a temperature swing of more than a full degree Celsius. That is enough to feel the difference in temperature as a cause for discomfort.

Also, in order to make the control system work, some of the radiators, the ones close to the heat pump temperature measurement, could not have active thermostates. If they had, they would cause an improved comfort by not allowing the temperature to increase too much, but that would in turn mean the heat pump control system would not turn off the heat pump.

The competing control systems had a very high drawback in cost

When for example the electrical heater increased the radiator temperature due to the outside becoming colder, it might (should) increase the temperature so much that the radiators could heat the house enough to keep the inside temperature high enough. This could in fact result in the temperature being high enough for the heat pump to turn off. Which then meant that our house suddenly was heated only through the electrical heater without any aid from the heat pump, increasing the electrical bills.

Buying control systems or exchanging one of the heating systems is a high cost

There are systems on the market that handles the above by controlling both of the heat systems through a central control which takes over from the existing control systems. There is also electrical heaters that are aware of heat pumps, and contain control systems to control them. Both of these routes would solve my problem, but both of them would mean a heavy investment in the heat systems and they had (hopefully have) a long life left before needing a replacement.

The electrical heater has a good control system, the only problem was that it did not cooperate with the heat pump. The heat pump had a bad system in all ways except one: It was well designed to keep the life time up for the heat pump.

Designing and building a temperature control system

The solution we selected is similar to the expensive systems that can be bought. I replaced the control system for the heat pump with a design of my own, which both controls the heat pump, and knows about the electrical heater, and forces it to cooperate with the heat pump.




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