Making a concrete slab for a heat pump: 7 step method

Have you purchased an air-to-water heat pump? Or are you thinking about it? And you've found the perfect location for the outdoor unit? Now all you have to do is build a good concrete slab to accommodate the outdoor unit of the heat pump.

Follow this article to get the 8-step method to build a solid and durable concrete foundation or heat pump concrete slab.

Step 1: excavation of the heat pump slab

That's it, you've found the perfect location for the outdoor unit of your Air to Water heat pump!

If not, you can read all our tips for finding the best location here

Let's get down to business: the first step is to remove the soil. Decking is simply the removal of soil from the defined area. This prepares the ground.

1. Mark the desired dimensions of your heat pump slab on the ground with stakes or a spray paint can. (1m x 1.3m for example). Check that the angles are straight with a square.
2. Dig the ground with a shovel to a depth of 20cm. And don't forget to remove stones and other roots, to ensure a good future stability of the concrete slab.
3. Soil compaction: you have dug 20cm. Now you have to tamp the soil so that it is solid, compact. Use a vibrating plate or a rammer. This will stabilize the soil and avoid any subsidence or deformation in the long term.
4. Check that it is level with a spirit level in various places. Adjust.

Step 2: the gravel layer

To improve stability and durability, you will now put a layer of gravel:

If your soil is very wet, or not very stable, you may consider placing a geotextile surface between the soil and the gravel.

1. Crushed gravel is a good choice. It is more stable. The ideal gravel size would be between 10 and 20mm. For example this gravel goes well.
2. Spreading the gravel: spread the gravel evenly over the entire excavated area (with a rake) to a thickness of about 5-10cm. The slab will then be well wedged.
3. Compact: Take your compaction tool and compact the gravel to eliminate air pockets.
4. Check the level with a spirit level and rake it out a little if necessary.

Step 3: Reinforcement of the concrete slab for the heat pump

The idea is to install a wire mesh or reinforcement (steel bar)

1. Choose a welded mesh, which is a set of steel bars welded together. These bars have a diameter between 6 and 12mm

2. If you use steel bars, weld them to make a welded mesh to the size of the slab.

3. Installation of the mat: Install the mat so that it does not touch the edges of the defined area.

4. If you use steel bars, secure them together so that they do not move during the subsequent pouring of the concrete

Step 4: Drainage

If you have a water drain, you can connect to it, if not, you will have to create a slope to ensure the water flow.

If you choose to connect to the existing manhole, a sloped pipe that connects to the drain is required, as well as a siphon to avoid bad odors.

If there is no manhole: you will need to create a slight slope of 2% when pouring, using a spirit level. The idea is to direct the water as far away as possible from the exterior heat pump unit.

Step 5 : Formwork for PAC concrete slab

A temporary wooden structure will be put in place to hold the concrete in place until it is hardened.

1. You can make a formwork with wooden boards or plywood. It must be strong and rigid enough to support the pressure of the concrete.
2. Measuring and cutting the boards according to the dimensions of the heat pump slab. Assembling the boards to make a rectangle.
3. Formwork installation: Lay out your beautiful rectangle around the slab area on the gravel + welded mesh. It needs to be aligned and stable. Use whatever you have on hand to stabilize it. You don't want it to move while the concrete is being poured.
4. Check the level: the formwork must be level. The spirit level will help you to check the correct horizontality on the sides.

Step 6: Pouring the concrete

After all these preparations, we finally come to the formation of the slab itself. Follow the steps below.

1. Preparing the concrete: either you buy ready-to-use concrete or you make your own mix like a pro in the ideal proportions. If you do it yourself, you generally count one part cement for 2 parts sand, and 3 parts gravel, then about 0.5 part water.
   • Sieve the sand, remove the gravel. In the concrete mixer, mix the cement, sand and gravel until you have a uniform color.
   • Add water gradually while stirring. Adjust the water according to the consistency. You must aim for a concrete wet enough to be malleable but not too liquid to avoid cracks...quite an art.
   • Let the mixer run for about 5 minutes after adding the water
   • Use quickly as it hardens within 2 or 3 hours
2. Pouring the concrete: Pour the concrete mixture into the form from one corner to the other. Use the trowel to spread and push into the corners. Watch for air pockets.
3. Leveling: with a ruler or a cleat, level by going back and forth to have a uniform filling of the formwork. (or with a trowel)
4. You can tap the sides of the formwork a bit with a hammer or something to avoid air pockets. This can give a better quality of slab.

Step n°7 : Drying, stripping

1. To ensure perfect drying, protect the slab from rain and sun with a plastic sheet that covers the entire surface. The tarp must not touch the concrete surface.
2. Drying time: let dry as indicated on your ready-mix concrete. If you made your own concrete, if you let the concrete dry for 7 days, it will have a strength of 70% compared to 28 days which will give it the maximum strength. Plan a little in advance
3. Stripping: As soon as the concrete is hard enough, remove the formwork. Remove the boards without damaging the slab. Use a prying tool to loosen the boards if necessary.

If you have followed all these steps, your concrete heat pump slab should be up and running!