Using oil instead of electricity is about the same cost, so the heat could not be lowered by any high degree by changing to using the oil furnace (we have an oil furnace as well as an electric furnace). The heat cost had to be lowered by changing the amount of energy needed to heat the house. Luckily enough, the hous had a built in water borne heating system, making our choices many.
There were two different routes to follow, and we plan to follow them both:
- Make heating more energy efficient (described below)
- Make the isolation of the house more efficient (still have not done very much here, although have started on isolating the attic which in our house is the most important isolation improvement)
It is actually not easy to know what alternative is the best route for saving energy at a given moment. A good normal bet would be that in an old house, improved isolation will save the most. In our case, it turned out that due to a bad design of the heating systems, the biggest saving was in the heating system. There will normally be a lot to save for most really old heating systems. Also in modernised houses where like in our case described below, the consequences of changed heating system has been missed..
The heat pump already installed in the house was described to be useful down to minus 10 degrees Celcius outside (it stops at temperatures lower than that). This means that at temperatures down to minus 10, it is still more efficent than using just an ordinary electrical heater.
But if I turned the electrical heater off, the heat would drop in the house well before that. In fact, the heat pump was efficient for heating the house only down to about plus 10 degrees, which really indicated a problem. I noticed that the inside temperature would drop quicker in some rooms than in others, and started to wonder about how the radiators were calculated for the different rooms.
This lead me to an interesting knowledge:
- Radiators are calculated to work during certain conditions.
- An air to water heat pump does not operate at the conditions radiators from an older house (oil based furnace) are calculated for
Putting it simply: An oil based furnace in an old house was expected to deliver 75-85 degrees Celcius warm water. An air to water heat pump normally deliver up to around 50-55 degrees Celcius. And the colder it gets outside, the lower the temperature it is capable of delivering.
The difference means that the heat pumped out of the radiators will be a lot lower than what was calculated with when the radiators were sized, meaning that the house using the same radiators will loose heat a lot earlier with a heat pump than with an old fashioned furnace.
In fact, you will loose roughly half of the radiator Power in Watt from reducing the temperature in them from the normal oil furnace temperature to the normal heat pump temperature.
The colder it gets outside, the less heat is produced by the air to water heat pump. That is the same as saying that the more heat is needed, the less you have to use for heating.
Once again, this differs from the heat coming from an oil furnace, it can deliver its heat to the pipes independent of the outside temperature. It does also mean that the difference when calculating radiators for an old house and when calculating them for a heat pump increases at the time of the year when the need for the heat pump is growing, and thus radiators need to be adjusted accordingly.
I will eventually exchange the heat pump for a more modern version. Maybe even two to have something that works well in the time of year when it is really cold, without having to use a too big heat pump during the time of year when a heat pump can handle the whole house without being too big. But according to my calculations, such an exchange is not motivated by economics until the old heat pump need to be replaced. The difference in efficiency is not big enough to recover the investment in a short time.
Increasing the size of radiators makes the heat pump more useful, so is also the case when extending number of radiators in the house.
One of the reasons the heat cost was high was that the two heating systems competed for control, and thus increase the cost. This was fixable, although not that easy.