thermal battery

While a concrete floor and wall can make up part of a thermal battery, they can also be a path for heat to escape. Is it better to insulate the wall/floor or use it as part of the battery?

Why think of it as an either or situation? Are you talking about an existing structure, or one to be built? If you put insulation under the floor, then you have it as part of your thermal battery. Same with walls, insulate the exterior and the wall is part of your battery.

That depends on what is on the other side of the floor or wall. If you want heat from all surfaces of the concrete, treat it as part of the mass; if they are exposed to outdoors or the ground or an area you don't want to heat, insulate them.

Thanks for the response! It's an already existing structure. It's a basement -earth backed wall and floor. So I will insulate. I have lots of sand on the property ( no clay ) that I can use to insulate the floor ( 2 to 4 inches ) but I have a feeling that man made insulation such as 2 inch foam ( then 2 inches of sand would be more efficient.

a lot of unnecessary silicon.

Everything I've read says be wary of CO leaks .Every seam I could reach is sealed both inside and out .

Sand is fine but use a thin layer of foam board as a thermal break.

I would be concerned about foam board anywhere in that sort of totally enclosed system. With the heat confined, it will slowly build up and just might get hot enough to melt or burn the foam. A much safer method would be to put thin brick pieces spaced every 6-8" or so with a layer of cement board on top. This will break the conductive heat loss and allow airflow to carry away excess heat.

Thanks Glenn and Roy for your replies. Makes sense to have that thermal break.
So I'll create a thermal break, pile sand on until it just kisses the pipes,then build the thermal battery.
I don't have a enough of brick / rock to do all the thermal battery. What do you think about mixing pea gravel with clay slip to the consistency of concrete and pouring / packing it in. I think that this would be relatively quick and inexpensive.
Any ideas for a quick and inexpensive TB?

Tim Straw wrote:
So I'll create a thermal break, pile sand on until it just kisses the pipes,then build the thermal battery.

I don't think I'd carry the sand that high. It is an insulator, not a thermal mass; instead, I would have an inch or two of sand, as insulation, but I would want thermal mass to be in contact with the duct work all the way around, and thick enough to get good heat gain. For example, say you only had 1/2-inch of thermal mass on the bottom of the ducts, and then the sand below that, but had several inches of thermal mass on the sides and top of the ducting. What happens?

The 1/2-inch obviously heats up to the saturation point, and then begins radiating heat, trying to warm the sand.

The question is how this compares with the sides and top that have several inches of thermal mass, instead of just 1/2 inch. Which "pulls" heat faster? Thermal mass you are going to use, sucking the heat into the mass, or the hot thermal mass touching the sand? I'm not positive of the answer, maybe someone else will be. It just seems to me that since higher temperature differences speed heat transfer, it would make sense to reduce the temperature of the thermal mass touching the sand.

Tim Straw wrote:
I don't have a enough of brick / rock to do all the thermal battery. What do you think about mixing pea gravel with clay slip to the consistency of concrete and pouring / packing it in. I think that this would be relatively quick and inexpensive.

Good general idea, but way too much water.

It is good to use the clay to pack in around the pea gravel, that will drive out any air gaps (do some packing and tamping as you fill, to better discourage air gaps from forming). But if it is so wet you can actually pour it like concrete, you are using a great deal more water than needed, and that is only going to increase the drying time. I would suggest just using enough water to let the clay and gravel pack nicely.