High-mass installations use a sand bed or pit underneath the building and a network of Pex tubes to gather, store and deliver heat to a radiant floor. They are extremely economical to run and provide very comfortable heating for the whole home. Remember to complete your installation in spring, so that the system starts gathering heat as soon as possible!

The sand bed should always be located beneath the building if possible. It is feasible to locate the sand bed outside of the building's heating envelope – in other words, somewhere other than underneath the building itself – but this introduces additional heat transfer requirements to and from the bed and complicates matters. Consequently, a home which needs a lot of heat must have a deeper pit rather than a wider one, which can increase the time it takes to recoup the initial installation costs.

The bed itself is constructed as an insulated sand pit within the building's foundations. It is contained by two-inch extruded foam insulation on all four sides and the bottom, with a concrete floor on top (the radiant slab) bordered by treated wood. Near the bottom of the sand bed, a gridwork of Pex pipes is installed, running back and forth across the space. These pipes carry the hot solar fluid from the solar collectors to the sand, which is poured into the pit and compressed – as with all heat storage mediums, the more mass there is available, the more heat can be stored.

For your own calculations, sand weighs about 210lbs per square foot of floor space when it is compacted and laid two feet deep, which is the norm (105lbs per cubic foot). That much sand can store enough heat to last several months, although sand is very inefficient compared to water, the standard against which all heat storage and transfer mediums are measured. One cubic foot of sand stores as much heat as about three gallons of water. Thankfully, sand is really cheap, you can use a lot of it and you don't have to mount it on anything – it's going to be buried under the house – so its inefficiency and weight aren't so important.

Most installations use a bed that extends under the entire building. For a fairly modest home of around 1,200 square feet of floor space, a standard system using a 350-gallon water tank would weigh about 3,000lbs (1.5 tons), whereas the equivalent sand weighs 126 tons!

The layout for the solar collection part of a high-mass space heating installation is no more complicated than any pressurized solar thermal system and is shown in the diagram. As a general rule, dump systems need about twice the amount of collector space as for a similar liquid-based system, while pressurized installations need a similar array to those used in liquid-based storage systems – and will provide more heat because they can benefit from high-sun months when a liquid-based system is switched off.