ᐅ The Practicality of Ground Heat Exchangers in Mechanical Ventilation Systems with Heat Recovery

Hello everyone!

I have a question about the usefulness of ground heat exchangers in a mechanical ventilation system with heat recovery:

I understand that you shouldn’t let cold air directly into the house, and that the heat energy from warm air shouldn’t be exhausted outside. That’s why heat recovery seems practical, even though it involves a significant initial investment. It is said that up to 90% of the heat can be recovered.

So why does a ground heat exchanger make sense? Is it mainly to keep the heat indoors even more efficiently? The idea is that the ground warms the -10°C (14°F) cold air for free, while the heat recovery unit requires electricity to operate. Is the goal then to reduce the electricity consumption of the heat recovery system? Does that pay off?

Or is it primarily about providing the ability to bring cooled air into the house during summer? Usually, you read that the main benefit is in winter use, and the summer benefit is just a small positive side effect.

Best regards

jx7

This is not just a claim but a fact—I can observe it every day by measuring the temperatures in the air ducts.

The geothermal heat exchanger prevents the system from freezing during winter without the need for an additional heating coil in the intake duct. Without a heating coil and geothermal heat exchanger, the systems usually switch to frost protection mode, opening the bypass so that cold air is blown directly into the building without heat recovery. This causes the indoor temperature to drop.

The geothermal heat exchanger ensures stable temperatures, keeping the bypass closed.

In summer, the opposite effect occurs.

Without the geothermal heat exchanger, the bypass opens again, allowing warm air into the house which gradually increases the indoor temperature, even if only slightly. Combined with the heat from windows and other sources, this can make the entire house quite warm.

The geothermal heat exchanger counteracts this by providing pre-cooled air, helping to maintain a more or less constant indoor temperature.

Thanks for your reply, Mycraft!

So, I see three options:

(1) Without the heating coil, without the ground heat exchanger => higher heating costs when the frost protection mode is active and an uncomfortable indoor climate due to cold drafts. I’m going to rule out this option.

(2) With the heating coil, without the ground heat exchanger

(3) Without the heating coil, but with the ground heat exchanger

Option (2) probably involves lower initial investment costs compared to (3), but higher operational costs when temperatures fall below a certain threshold.

At what outdoor temperatures can we expect the heating coil to be used? Is there a specific temperature limit?

Most likely, the ground heat exchanger makes more sense in colder regions where this temperature limit is frequently crossed, and less so in warmer regions (e.g., near the Rhine), where it is less often exceeded.

Well, you can figure out the temperature limit yourself... I’ll just give you a little hint... at what temperature does water freeze?

With an enthalpy exchanger, you can intake air as cold as -10 to -15 degrees Celsius (14 to 5°F) – which is why I skip the geothermal heat exchanger and install the system in the attic. For me, this has only offered advantages in planning...

You always have to consider the local climate where you live – in the areas around Berlin, winter can often be very cold for prolonged periods (well, except for the last few winters), but here in the Münsterland region, you can count the number of deep frost days or nights on one hand per season. If it does get really cold for a couple of days, you just turn the system off. For that reason, I gladly avoid the geothermal heat exchanger along with the potential hygiene issues that can arise from improper planning of such a system.

Alternatively, instead of using a ground heat exchanger ("earth register"), a brine ground heat exchanger can be used, which avoids any hygiene issues.