ᐅ Insufficient depth of the pipes leading to the ground probes?

Hello,

briefly: From the house, both pipes run from the basement at about 1.5m (5 feet) depth to the probe manifold, and then another 5m (16 feet) at only about 50cm (20 inches) depth to the 99m (325 feet) deep boreholes.

in detail: We have now successfully passed the first two winters in our new single-family house. The annual performance factor (seasonal coefficient of performance) of the ground source heat pump is just over 4. Not record-breaking (there was little planning in advance regarding underfloor heating or the sizing of the heating system), but I think it’s acceptable. For two years now, I have repeatedly wondered whether I might be losing some efficiency because the pipes to the two 99m (325 feet) probes lie very shallow in the ground. I didn’t measure it down to the centimeter, but they are no deeper than 60cm (24 inches). Since we also want to remove some soil above one of the pipes, that might reduce the depth to only about 50cm (20 inches).

So this is not actually below the frost line. I understand the pipes are regularly “flushed” in winter, and they don’t freeze at 0°C (32°F). But I just have the feeling that it can’t be good for efficiency that probe water at around 9°C (48°F) is led out of the house, probably cooled down again over the first 5 meters (16 feet), then warmed up over two 99m (325 feet) boreholes, and finally cooled off again over the last 5 meters (16 feet) before it reaches the heating system.

Question: Is this shallow installation likely to have an impact on the annual performance factor or even on the system’s lifespan?

If yes: Are there ways or recommendations to insulate the pipes over the first 5 meters (16 feet), or possibly to insulate the soil above, for example with aerated concrete blocks? Or do I need to try to uncover the pipes again and lay them deeper?

I'll take the floor.

Normally, the manufacturer of your heat pump should be able to provide information about the required depth for laying the pipes. Have you asked them politely? Can it be ruled out that the pipes were not already insulated when they were installed? At a depth of only 50cm (20 inches), that should logically be the case. Are there any photos from the time the pipes were installed?

It’s not ideal—if the pipes are deeper (>1 m (3.3 ft)), uninsulated pipes could actually be beneficial, so you might get some advantage there. However, the impact on the annual performance factor is minimal.

Insulating the pipes afterward is possible but can be a bit tricky. You can relatively easily insulate the pipes themselves with Armaflex or similar materials (make sure they are approved for underground use). There are slitted versions that can be slipped over the pipes, depending on how closely your pipes are laid together. However, you must not place any weight directly on the insulation since compression reduces its effectiveness. That’s why insulated pipes are usually installed inside KG pipes (corrugated sewer pipes) to isolate them. This might be difficult in your case, but there is a fairly simple solution—you can build a kind of U-shaped protective casing or shaft around the pipes using XPS panels, which then allows you to backfill soil on top safely.

I would only do this if you need to carry out other earthworks there anyway.

Allthewayup schrieb:

Let me take the floor.

Normally, the manufacturer of your heat pump should be able to provide information about the required depth for laying the pipes. Have you asked them politely? Is it possible that the pipes were already insulated when they were installed? At a depth of only 50cm (20 inches), that would be the logical conclusion. Are there any photos from the time the pipes were installed?

What does the heat pump manufacturer have to do with how deep the pipes run to the probe manifold? There should be standards for that, right?! The manufacturer only specifies or recommends the temperature difference between inlet and outlet – how or whether this is achieved is the heating engineer’s responsibility. The heating engineer told my excavation contractor how to bury the pipes to the probe drillings. Most likely, he just told him to bury them, which is why they aren’t very deep. I’ve attached a picture.

Harakiri schrieb:

It’s not ideal – deeper (>1 m (3.3 ft)) uninsulated pipes can actually be beneficial; you can gain a bit that way. But the impact on the seasonal performance factor is minimal.

Why is it beneficial from 1 m (3.3 ft) depth? Even then, it’s still not 10°C (50°F) in winter, right? From a purely technical standpoint, it would make sense to insulate the pipes from the depth where the ground temperature is warmer than the fluid temperature. Anyway... it never reached 1 m (3.3 ft). I hope/think it’s more than 50cm (20 inches), but definitely not more than 70cm (28 inches).

Harakiri schrieb:

Retrofitting insulation on the pipes is possible but a bit tricky – you can fairly easily insulate the pipes themselves afterward with Armaflex or similar (make sure it’s approved for underground use). There are split versions that can be pushed over the pipe (depending on how closely your pipes are laid together). However, you must not place any weight directly on the insulation, as compression partially reduces its effectiveness. That’s why insulated pipes are normally laid inside KG pipes (rigid sewer pipes) to decouple them. That might be difficult in your case – but there is a relatively simple solution: you can build a kind of protective cover or shaft in a U-shape around the pipes using XPS boards, then you can cover it with soil.

But I would only do this if you are doing other earthworks there anyway.

These are great tips! I could imagine that placing a lengthwise halved KG pipe over the pipes could also effectively distribute the weight above?! We’ll see. I did have an excavator on site recently for a rainwater tank, but that didn’t coincide with this. I think I’ll hold out for a while longer until something structural is changed there.

I wouldn’t worry about freezing. On the one hand, because of the constant circulation, especially in winter, and on the other hand, the probes are filled with brine, meaning the freezing point is not 0°C (32°F). I would be more concerned about the way the pipes are covered and how to protect them. We have a terrace above ours ... everything works fine.

At 1.5m (5 feet) depth, I also wouldn’t worry about heat loss. Yes, you’re basically right, but I think it’s negligible with about 200m (660 feet) of probe length.

Respect for the probe depth—two times 99m (325 feet) for a single-family house is quite impressive. Is the heat extraction capacity that low where you are and/or is the house that large? You can also save on drilling meters (= money) by drilling deeper than 99m (325 feet) and installing just one probe, but well, it is what it is now.

guckuck2 schrieb:

I wouldn’t worry about freezing. On the one hand, there is constant circulation, especially during winter, and on the other, the probes are filled with antifreeze, meaning the freezing point is not at 0°C (32°F). I would be more concerned about the structure above the pipes and how to protect them. We have a terrace above… no problem at all.

At 1.5m (5 feet), I wouldn’t worry about heat loss either. You’re basically right, but I think that’s negligible for roughly 200m (660 feet) of probe length.

Respect for that probe depth—2 x 99m (325 feet) for a single-family house is impressive. Is your heat extraction efficiency that low and/or the house that large?
You can also save drilling meters (=money) by going deeper than 99m (325 feet) and using just one probe, but well, it is what it is now.

1.5m (5 feet)? The pipes to the probes run about 6m (20 feet) each and only about 0.6m (2 feet) deep here. I don’t worry about freezing at all. Even if the system only starts once a day or is off for two weeks, it’s unlikely the antifreeze temperature would drop to -10°C (14°F). I think that’s fine. My main concern is the system’s annual performance factor (seasonal coefficient of performance). If I knew the heating system would work even 2% better by insulating the pipes or burying them 30cm (1 foot) deeper, I’d probably spend a little extra money on that. The pipes are located where nothing is built above them yet. Eventually, there will be paving there.

The new build happened so quickly that we barely had time to study many things properly. The heating installer just chose this setup. We have 220 sqm (2,370 sqft) of heated floor space. That includes the unoccupied basement, which is heated and suitable for use. Five people live here on roughly 150 sqm (1,615 sqft). Originally, he wanted to install an 11kW heat pump and drill 2 x 99m (325 feet). Fortunately, because of the official change from KfW 70 to 55 standards, we managed to convince him to go for an 8kW unit instead. Honestly, I always prefer to have two deep drillings now rather than finding out later that the seasonal performance factor is worse. Whether the additional cost was worth it, I cannot and do not want to calculate now.