Closed Loop Water Furnace

HVAC Gurus,

I did an inspection yesterday where the client had a gas fired split-system with a closed loop Geothermal Water Furnace. It was an odd set-up because the gas furnace was in the attic and the Geothermal unit was in the basement.

Anyways, the water lines (closed loop) from the ground were icing up. To be honest, I’m not that familiar with these units. The owner swore that the tech said that it was normal.

However, it seemed odd to me. Should there be ice on these lines?






Was it a “direct expansion” (DX) system or a glycol loop. Icing may occur on the return lines to the field due to (1) the coil system was designed to be too small for the heat load, (2) extreme heat needs above regular designed, or (3) near the end of the heating season, the ground in general has given up the available heat and the system is operating at the lower end of its optimum.

The return glycol may get as low as 29-31 deg F after the heat is extracted.


I don’t believe it was a glycol loop, but to be honest I’m not sure. Obviously, I need to read up and get familiar with these systems. I get the feeling I will be seeing more of them in the field.

I did measure the lines to be around 32.5 Deg F. I would agree with your first theory as this home was extremely leaky and they are running their system constantly, trying to keep the house warm.

Here are a few of the exterior images. However, I took about fifty thermal images from the inside. They have an addition to the original structure, and the furnace seemed way too small.




always liked popular mechanics articles, saved this one for some reason :wink:

Thanks, Barry!

I will read that one tonight :slight_smile:


Installed a few of these in years past

Kevin[FONT=Tahoma][size=2], a check of your weather records shows in the last 48 hours it was well below freezing at night (14°) and at freezing during the day. Previously to this, temperature was moderate.[/size][/FONT]

[size=2][FONT=Tahoma]The colder it is, the longer the run time, the colder Loop field becomes. Just because the loop is below 32° does not mean there is not substantial heat left in the line. Compare this to the 14° area temperature that air source heat pumps must operate under during the same conditions. Extracting heat from 32° brine is a whole lot more efficient than 14° air.

[/size][/FONT]Just because there is ice present, does not mean there is something wrong[FONT=Tahoma][size=2]. Though there could be. What we’re looking for is a temperature differential of the actual fluid inside the lines. In your pictures I can see that one line is warmer than the other, indicating that things are working properly.[/size][/FONT]

The actual temperatures that you will receive depends on the design of the system and cannot be compared with other systems or a rule of thumb[FONT=Tahoma][size=2].[/size][/FONT]

There is no abnormal ice formation visible, just frosting of lines. The amount of frosting is also dependent on humidity levels within the mechanical room area.

One common problem with geothermal units is sufficient water flow/quantity[FONT=Tahoma][size=2]. Scale builds up and reduces flow and will cool these lines to a colder temperature than desired. So, were not necessarily out of the woods. In these cases I generally ask who servicing contractor is and give them a call.[/size][/FONT]

Other geothermal designs are more consistent than the ground loop because water is taken from greater depths or have a better heat exchange rate than the soil[FONT=Tahoma][size=2]. The ground loop will probably give you the greatest consistency problems.[/size][/FONT]

I know this isn’t much help, but it may expand your concept of what’s happening.

Thanks for the responses.