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Saving 2.0GJ/yr heating by upgrading heat exchanger

(Updated: )

Now that I’ve been collecting data on my house for a while, I’ve used this to diagnose and fix a heat leak we had in our Eneco district heating (‘stadsverwarming’ in Dutch) heat exchanger (‘warmteafleverset’ in Dutch).

TL;DR: Our 20-year old heat exchanger leaked 2.7 GJ/year, after upgrading to a better insulated one this was reduced to only 0.65 GJ/year, saving 2.0GJ or 50 euros per year! The upgrade was for ‘free’, meaning it’s included in the yearly fee of 90 euro/year. It’s unclear why Eneco does not do this pro-actively, but you can also request an upgrade if you say the old one does not suffice anymore.

Update: replies disabled because of spam

Old heat exchanger idle use

I’ve always found the room the exchanger was located in quite warm, and to diagnose this I looked at the usage during our holiday, which turned out to be 80 MJ in 11 days, or 7.3 MJ/d or 2.7 GJ/yr!

It turns out there is a small heat reservoir in the heat exchanger that buffers heat in needed to bridge the gap until fresh hot water from the district heating is supplied, e.g. during the first ~30s during showering. This heat buffer insulation is not very good, so it needs constant heating to keep its temperature. In this case, it uses a constant 80 Watt heat load.

Heat exchange upgrade

I called Eneco and asked for an upgrade, which took about 30 minutes:




You can see I tried in vain to insulate the (hot) pipes of the old exchanger myself. The new unit is much better insulated.

Heat exchanger CW-classes

Note that there are different types of heat exchangers, indicated by ‘CW’ value (‘Comfort Warm Water’), which basically indicate the power of these units in water flow heating capacity in l/minute, e.g. CW3 can heat water to 60 degrees centigrade at 6 liter/minute. These different units also have different prices, so watch out what you get when you upgrade:

CW-class Flow Cost (Eneco)
CW3 6 liter/minute 88.49 euro/year
CW4 7.5 liter/minute 126.19 euro/year
CW5 9 liter/minute 140.71 euro/year
CW6 16.5 liter/minute 140.71 euro/year

New heat exchanger idle use

The new heat exchanger is much-better insulated, even in spite of my own efforts. New measurements indicate that the idle power is reduced by a factor 4! The new one leaks 17 MJ in 9.5d = 1.8MJ/d = 0.65 GJ/yr. This is 2.0GJ/year lower, and at current (steadily increasing) prices, this is equivalent to a 50 euro saving per year.

Bonus: setting heat reservoir to lower temperature

Although not user-accessible, there is a thermostat to set the temperature of the heat reservoir, thereby allowing to tune the amount of heat available to buffer the time until hot district heating water flows through the exchanger. This thermostat can be set during installation if you ask the service technician.

Initially I optimistically had it set to the lowest setting (1 out of 5), the default being 3. This gives an idle use of only 8.0 MJ in 8d = 1.0MJ/d = 0.37GJ/yr. However, this capacity was not sufficient to bridge the gap of district heating kicking in, and resulted in a somewhat unpleasant first warm, then cold, then hot shower-experience. Therefore I asked to reset this at the cost of a slightly higher idle heat use.

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