hydronic floor heating system efficiency - using concrete as thermal energy store or not?

Question

I am renovating my house and would like some advice on floor heating for my living room. I have dug this room out to a depth of 97 cm to create a lower level. I have lined this box with 2.5 cm thick polystyrene and then a double layer steel reinforced concrete floor of 15 cm.

I am now looking to add a hydronic floor heating system to this lowered area. As I understand it I have two basic options.

1. add the pipes directly on the concrete with a layer of screed or
2. add a layer of insulation in which (or on top of which) the pipes are fastened and then add a layer of screed

I would like to know which system would be the most efficient in my case. Reading up on option 2, it seems like this heats up the room faster, but you are not utilizing the concrete floor as thermal storage, so you lose the heat very fast as soon as the boiler is switched off. With option 1 it takes longer to heat up, but then you are also using the concrete floor as thermal storage.

EDIT: I fear I may have under-insulated by only adding 1 inch of insulation (extruded polystyrene), thereby negating the effectiveness of the thermal store.

Could someone please give me some pointers?

Answer 1

Disclaimer: I'm not a scientist or an engineer, so it's entirely possible that all of this is wrong.

For the last part of your question regarding whether the insulation you used was too thin -- extruded polystyrene typically has an R-value of 5 per inch.

Here's the part where it would be great if someone who knew "the maths" could help me out. I've calculated out at 60 lb/ft^2, 6" thick, and specific heat of 0.21 BTU/ft^2*F, the heat capacity should be 11.55 BTU/ft^2*F, or ~65.6 W/m^2*K

Anyway, if my understanding and math is correct, which it frequently isn't, it should require about 65 watts per square meter to change the temperature of your concrete by 1C. Your insulation of 1 inch has an R-value usually of 5, which means that:

((Concrete Temperature C - Ground Temperature C) / 5) will be the amount of watts per square meter of heat lost. So -- 10C temperature diff = 2 W per square meter of heat loss, or 32.5 hours to lose 1 degree C per square meter. Without that 1 inch of insulation, you would be generating 10W per meter square of heat loss, and it would only take 6.5 hours to drop 1C -- so yes, that 1" extruded polystyrene can make a significant difference. (I assumed it was extruded and not expanded... I hope!)

This obviously doesn't account for other types of heat loss, and is theoretical, but is to really say -- that 1" is a whole lot better than nothing. However -- this also means that to heat your concrete evenly, it will require applying 65 W per square meter of heat per degree (plus 0.5W per degree for each hour it takes to apply that), which means it is going to take a long time to warm that puppy up unless you start it off in the summer.

Answer 2

A very important factor in deciding to use a thermal mass to store energy is the energy source. Solar energy is the classic example. The energy is free and the supply is periodic and unpredictable. Energy storage is a must.

You mentioned a boiler, so you likely are burning fuel, directly or indirectly, or at least paying for the supply. In this case, energy being of significant cost and in virtually ready supply, energy storage is not warranted. The inherent losses in the store/release cycle make it of little benefit. Especially with only 2.5cm of insulation.

Answer 3

The difference is in the use of your system and your personal preference.

Since you have insulated the ground beneath your slab, concrete thermal store can work in your favor.

Do you live in an area with consistent heating needs? Embedded in the concrete will take a long time to heat the slab, this will be continuous circulation time... and it will stay warmer longer.

The setback, though, is that if you want to cool down, or don't need heating for a period then you have spent the energy warming up your heat store... cooling the room "wastes" more heat.

Many people, for example, turn their heat down at night... your situation will dictate just how long the system must run to regain heat lost overnight if you practice this... and you'll have to schedule it to come on early enough to heat the slab to be warm and ready for you when you get up.