Mats Bengtsson

When is a heat pump overdimensioned

Mats Bengtsson mib over the years

What defines an overdimensioned heat pump

Now and then there is a discussion about a heat pump being overdimensioned. Often, such discussions do not define what is meant by overdimensioned. This means that the word is used by different individuals to mean different things, or the same individual to mean different things at different times. This is a pity,since the question about an overdimensioned heat pump is quite an important question.

If the heat pump is underdimensioned, this should mean you could have made a better saving or gotten better results with a bigger heat pump. If it is overdimensioned, that should mean that the pump is so big, that a smaller pump should have served its purpose better.

What should define an ovedimensioned heat pump

To be able to in a structured way say that a heat pump is overdimensioned, there should be a possibility to realise what disadvantages you get through the too big size of the heat pump. Thus, we need to list the possible disadvantages there is with a bigger heat pump, and how to recognise them. We must also define how to compare them to a more favorable situation.

Overdimensioned might mean different things for idfferent individuals (since different invdividuals might consider different things more or less important). Also, overdimensioned might mean different things in different situations, since a heat pump is a part of a whole, and that hole might look different in different houses and at different times.

But even if ovedimensioned will be evaluated differently by different invidviduals, and be judged differently in different settings, it is important to define its factors. The factors that should define overdimension is those factors which if coming from a too big heat pump negatively affect your heating.

The factors for overdimensioned heat pumps

The information I have about what factors define an overdimensioned heat pump is given below:

  1. The size of the investment (if the investment increase of going up one step is soo big that there is an economical loss from doing it, then the lower size will server you better)
  2. The number of starts and stops needed (if the number of starts and stops increases too much with the heat pump, then the life span will be shortened due to those starts/stops)
  3. The minimum running time of the heat pump needed to heat the water (if this time is to short, not only will it contribute to more start/stops, but it will also lead to lower COP, since the COP is lower during until the heat pump has warmed up).
  4. The minimum volume needed in the heating system in order to achieve low enough values on number 2 above, and high enough values on number three above (if this volume is too high, then the heating system will take too long to varm up or cool down, causing too high variations in inside temeprature)
  5. The minimum flow needed to keep the heat pump operating at an optimal level (if this flow is too high, there might be noices and resistance in the heat pipes, or a need to have two circulation systems if one was desired from the beginning)
  6. The minimum size of the heat pump itself (if the heat pump is too big, it might have lower COP and thus cause higher costs than a smaller pump)

How to decide when the pump is overdimensioned

This is the part where it becomes personal and depending on a wholistic view of all parts. The heat pump is overdimensioned only when the sum of the above is worse off than the sum of the above for a smaller alternative. The sum of the above can not be expressed with a single value or in a single unit, it has to be weighed together by the person doing the judgement.

Some of the parts can be evaluated together. For example 1 and 7 are both concerning economical factors and can thus be weighed together through comparisons, for example by using the form for temperature based calculations on heat punmp costs and savings. Factors 2,3,4 can actually be calculated on the same page as 1 and 2 above, since they are depending on the same factors, and since there is a mutual dependency between those factors and the heat pump itself. However, evaluations of them has to be based on rules of thumb or other means. For example:

  • More than 24 starts and stops per 24 hour period is too much
  • Less than 20 minutes minimum run time of the heat pump is too little.
  • More than 25 liters of heat water volume per kw heat pump is too much

Factor 5 is worse, it depends on the piping and is hard to give a rule of thumb for.


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