Benefits of radiant floor and a question about practice.

nibs

One of the big benefits of a radiant slab;
"Dear, why did you drop your coat and gloves on the floor"?
"So they would be nice and warm when I put em on to go out" One answers.

Serious question:
Asking for your experienced knowledge please,
Have a thin slab radiant floor which is performing well. The boiler starts once a day if it needs to.
Have an insert fireplace that has an equal BTU output to the boiler, and in the evenings we like to fire it up, for the ambiance.
So when we light the fireplace, the boiler shuts down and stays off until early in the morning (depending on outdoor temps). 8 hours later +/-.
While the fire is lit, the slab cools, and we do not mind that, and in the morning the slab is warm again and we do like that.
Am I saving gas by burning wood, or is the boiler having to burn more gas to bring the slab back up to temp.

GroundUp

Does it take more electricity to turn on a bulb for an hour or leave it on all day? Burners like to work, not cycle. I have no factual data to back this up, but the way I see it is while you-re burning wood you're heating the space so the boiler doesn't have to. If this goes on for 8 hours, that's 8 hours the boiler doesn't have to work. That cold slab might take an extra 10 minutes of run time to heat back up in the morning, so would the boiler have run more or less than that 10 minutes total during the 8 hours you were heating with wood?

JUGHNE

That is somewhat like setback, the fireplace affects the tstat.
Fireplaces are often heat losers....negative efficiencies I have read. But introduces air changes in the house....not all bad.

But are you comfortable? Heating, especially radiant is all about comfort. If this does not cut into your food budget I would not worry about it.

CMadatMe

Slab is a mass - Let it cool - Takes longer to bring that mass to setpoint.. Can't get around physics.

hot_rod

The fireplace needs a source of combustion air, if it is pulling that from the room via an opening somewhere really the fireplace warms only what is in line of sight (radiant) and may be cooling the room? So possibly adding additional load for the boiler when it fires?

Or it may have combustion air from outside directly piped to it?

Either way be sure to have a working CO detector in any solid fuel appliance room.

Most radiant surfaces, even low mass, struggle with supplemental heat being brought in and out.

I guess if you are comfortable, that's the bottom line.

I much prefer the warmth of a constant radiant floor to scorched air blowing around, if the insert has a fan assembly?

nibs

Comfortable, yes the radiant floor is fantastic thanks in large part to those here who talked me through the install.
My question is do I save gas ($$$) by burning wood?. Wood is free and gas is expensive here, but we have to rewarm the slab after burning wood....... am I over thinking it?.
Kinda like setbacks, which we don't do, we use the aforesaid Honeywell mercury switch.

JUGHNE

I doubt if you save any gas money, but don't really loose much.
Your tstat senses air temp that the fire place raises and shuts down the floor.
Fire goes out and room temp drops and floor starts again.
Again a form of set back.
If you enjoy the fire, I would continue with it.
But having burned wood myself, the romance goes out of it eventually as I learned it wasn't always "free".

hot_rod

My opinion would be any heat put into the space by another source, like the wood burner, lessens the amount that has to be put in with gas fired appliances. How could it not be? The space needs X amount of heat input at any given time. It really doesn't know or care where it comes from.

Also the cooler you keep the inside temperature, the lower the heat load, it that ∆T thing again Less heat energy and $$ to maintain a space at 65F compared to 72F. so setback does lower the heatload and as a result the $$.

The house doesn't magically need twice the input to bring it up from setback, it needs what the ∆T between outside and inside dictates, wind factors in as does solar gain of course. The heat load on a space changes second by second, it's a dynamic number.

The setback function is more about comfort, you don't want to wait hours for the space to become comfortable after a setback, 3- 5 degrees maybe is reasonable.

My concern with any wood burner is how much heat it sucks from the space. With the gases going up the flue, and combustion air, O2, needed for the fire to burn that has to come from outside the space at some point.

kenjohnson

I have installed radiant heat in a retrofit and also plan on having a wood stove there as well (I have a wood stove in my current house).

Yeah, if you are putting wood heat into the house you won't be using gas to put heat into the house. As the house cools with the wood stove burning down, the thermometer for your radiant system should sense that and start circulating water to warm up the floors again. As long as it is not a high-mass slab to heat, I would think it would work pretty seamlessly and you wouldn't notice it much.

I won't get into the details of whether your wood stove creates a more drafty house or not - I'm assuming you know what you are dealing with there.