Will it work?

I have a 16' X 16' single floor addition off my kitchen space. Currently we have a wood stove for heat but it really over heats the space. Just off the end of the kitchen is a 10 X 20 room that had roughly 25 ft of standard baseboard style radiators. I have removed them from that zone entirely. I should also mention that the 25 ft of radiator ahd a return from the end of the radiator to the same spot...so 50 ft of tubing...half was radiator half was simply to get back to the rest of the house. My feeling is that zone now has plenty of extra energy for me to add some baseboard radiators to the addition. Heck I took off 25 feet I should be able to add 15 without any trouble.
The addition is on a slab though. It does have R-19 fiberglass in the joist spaces under the floor. My thoughts are to run approximately 7 feet in the first joist bay and surface into the room to run 8 feet to the corner and then another 8 to 10 feet down the far wall before going back under the floor and running 16 ft back to the loop in the cellar space. This would eliminate a section of pipe in the cellar equal to the 8 to 10 foot run on the far wall.
My son (the engineer) and I have reasoned through the facts that if the original circulating pump was fine before the 50 feet was removed adding this 32 feet should be ok. After all 32 IS less than 50. The issue we are both worried about is the under floor tubing is going to be in "unheated" space unless you consider the fact it is surrounded by the R-19 fiberglass. His firm says NEVER put water pipes in unheated spaces. I agree but what if:
The rafter bays are 14.5" wide with 11.25" depths ( 2 X 12's were used for the joists) I would run the pipes as close as I can to center or the width and perhaps 1" below the subfloor. I would then line the edges of the space with 1.5" thick foam insulation. Then I would force the fiberglass R-19 back in and seal the bottom with another piece of 1.5" foam. All joints taped and sealed. What this should accomplish is a sealed "room" with R-19 Fiberglass PLUS R-10 to 12 Foam on 3 sides all around the pipe. The "exposed side is actually the underside of the room's floor. My feeling is that the zone will call for heat often enough so that the supply pipes carrying the 160 to 180 degree water would heat the spaces well above freezing. The insulation should keep them from dropping much at all between cycles.
Obviously a power outage in severe cold is the flaw.
Are there any other solutions? As the room is laid out there really is no way to enter above the floor in the heated space.
Thanks in advance to all.
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Your hydraulics are fine. A shorter pipe will have less rpessure drop and so the pump flow rate will go up a bit. No harm done.
The arrangement you describe should not freeze for a few hours, maybe days, in a power failure. No insulation between the pipe and the room will let the room heat keep the pipe warm until the room runs out of heat. The only thing I would add is insulation along the edges of the slab to at least 1 foot below grade but 2 to 3 feet is better. That would create a warm volume of soil under the slab that would hold enough heat to last for several days. You could heat trace the pipe and plug it into a generator in an emergency but unless you get long power failures it would be overkill.
You don't say where you are located so no info on how cold it gets. In central Canada wuth temps of 30 to 40 degrees below there could be problems if you go on a long vacation at the height of the cold season and turn down the heat in that room too much.
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I forgot to mention a few points:
The slab is a "frost protected slab" meaning that it is poured on 2" thick "blueboard" insulation that extends about 3 feet beyond the edge of the slab. I live in what was a sandpit so the soil holds little or no water and frost heaves are really not likely but this technology is used in very cold parts of the world so I built accordingly. The slab is actually 12 inches below grade so that it is below the pre existing cellar windows. I then poured walls to bring the joist level up to match the house. Actually miscalculated by a half inch but for a customer service rep I think I did pretty ok. So the cold space above the slab is about 20 inches and there are 2 window style vents set diagonally to allow for air circulation.
My main concern is that the supply line would run about 7 feet in an outer bay. That bay is protected somewhat by a deck above so no realy wind ever hits it. I have no way of insulating the outside of the upright wall because of this. That is why I am hoping that lining the inside with 2" of foam insulation will form enough of a barrier. Though the pipe can be in the middle for most all of the way it must turn towards the outer wall and go through the floor to meet the inlet of the radiator. I believe I would still have enough room for the 2" foam between the uplink and rim joist though. The return line will be approx 4 to 5 feet in from an outside wall so the bay it is in will not have as much loss I wouldn't think.
I do live in Vermont and long power outages are not common. 3 to 6 hours has occured but really rare. You do bring up a good point on the location of the thermostat. Right now it is in a different room because the woodstove would never let the zone call for heat. With the stove gone perhaps relocating it to the kitchen or the addition would be a wise idea. As to vacations etc...we are never that lucky and would have to leave it up anyhow for the others left back at home. We do try to keep it set around 66 and find that rather comfortable. Somehow short sleeve plaid flannel shirts look funny to me...gotta have the full length sleeve. And when would I wear the 100 sweaters I have been given over the years if not around the house in cold weather.
Again thanks for the ideas and it sounds like you feel it should be "OK". The idea of the electric tape is a good one but, as you said, likely overkill. Nonethe less if the power has been out for a real long time at 30 or less why not kill the zone and run the tape for an hour or 2 before turning the thermostat back up? The other zone and the Monitor heater will warm the rooms around the space just fine.
What are your thoughts on some sort of heating system anti freeze? Another responder mentioned it and I will have to drain that zone if I cut into it to add the radiators anyhow. I could easily refill the loop with a gallon before firing the system back up.

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Definitely use anti-freeze if that is at all possible. It would make your system freze proof. Geothermal heating designs have buried closed systems and use anti-freeze because they actually cause the ground to freeze.
Your slab having the ventilation space between concrete and underside of floor means that you don't get any benefit of the ground being a heat sink. You should close all the ventilating openings for the winter. Even consider insulating the walls of that space.
Make sure that all of the pipe has as much insulation around it as possible. Don't allow any gaps in the insulation. If the pipe goes through any holes make the holes large enough for the 2-inch insulation.
If you use anti-freeze then you don't need any insulation at all from the point of view of preventing freeze damage to the pipes. If the fluid doesn't freeze then there will be no expansion. Use the insulation though to maximise efficiency. It is like anti-freeze in a car - the right mixture strength has better freeze protection than pure anti-freeze.
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Sounds like you make sense. There could be enough heat to keep the pipes from freezing. I don't know if this is legal, but you could put some RV non-toxic antifreeze in with the heating water. Change the freezing temp down a couple degrees.
You could cut a hole or two in the wall, put in air moving fans. Use "cooling only" stats near the ceiling. When the wood stove is going super nova, the fans would blow heat into the addition. Easier than fiddling around with hydrionic heat. Were you going to heat the addition off your existing boiler, or off the wood stove that you mentioned?
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Some things got a bit confused I guess:
The idea is to do away with teh wood stove entirely. The Mrs thinks I am too old and it causes too much dirt and dust in the house. More likely the second reason than the first though my new neighbors from Alabama really do not get much of a view when I toddle to the woodshed in my sweatpants and t-shirt at 5:30 AM. And it does get chilly in Vermont in the winter.
So yes the idea is to heat the addition off my existing boiler by adding the baseboard radiators.

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