Perhaps;

Like you I am stretching the delta T and using more Alphas. Long term, low temperature, low operating cost goals dictate my practice (while still staying in business, naturally.

From the educated homeowner / designer club, let me offer you some advice. Use the glycol. Pay the price and sleep well. I had a no glycol install in my garage and lost a pump one snowy night. By the time the contractor made it out to my house about 3 days later, I'd lost a tube. And my design temp is 0F. Murphy usually wins if you let him. It only takes once.

Or you could have a backup heater,rent a space heater, pull your car into the garage and vent the exhaust out while its running, buy an extra pump.It takes very little heat to keep a properly insulated slab from freezing a fan will do it in many cases, leaving the lights on in others.

If you are completely helpless, you should always go with the glycol and pay more to heat the place as long as you are there. And don't forget to have the glycol checked every year, as bad glycol is worse than no glycol at all.

Posted By BadgerBoilerMN on 04 May 2010 09:18 AM
Or you could have a backup heater,rent a space heater, pull your car into the garage and vent the exhaust out while its running, buy an extra pump.It takes very little heat to keep a properly insulated slab from freezing a fan will do it in many cases, leaving the lights on in others.

If you are completely helpless, you should always go with the glycol and pay more to heat the place as long as you are there. And don't forget to have the glycol checked every year, as bad glycol is worse than no glycol at all.

...or you could start a fire in the middle of the slab, etc, etc.  Point is, the system needs to operate somewhat independently.  And again, if the power is out for multiple days (not uncommon here) I am left with fire as a heat source.  Then I might as well put in a wood stove and forget radiant all together.  The cost of a backup heater/car exhaust ventilation/other methods + freeze alarm will outweigh the savings gained by using water vs glycol.  And how about the savings of being able to shut the system off for 3 weeks in the winter when I'm out of state...how much does that save when fuel oil is $2.71/gal (yes, that's was the price last month when I filled).  And then at $145/CY + $1.75/SF labor for the slab, you won't find me jackhammering up much concrete if I can help it.  

BUT, water is more efficient that glycol...point taken.  However, a higher efficiency doesn't always mean least expensive or best for every situation.  Nonetheless, I'm still not sure what way I will go...

Anyways, enough on that...

I've been looking at lots of different controller options.  I think the best option is going to be with some type of 3-way mixing valve, because if I don't have to worry about the return temp, I will only need one circulator...the fewer moving parts the better!  I don't really like the idea of the "dumb" 3-way, but it would help some.  Because of the constant source temp, I'm guessing that a thermostatic 3-way probably won't be doing much more than a "dumb" valve anyway.  Neither are really going to give me too much control I think.  

Therefore, to get better control, I'm thinking the motorized 3-way with outdoor reset might be the way to go.  I've been looking at the iSeries and some of the Tekmar stuff...does anyone have any other suggestions for a controller for a simple 1-zone 1-temp system like this?  If I go with this, I'll probably have a constant 60 deg or so return temp.  Does anyone see a problem with condensation when feeding the water heater 60 deg water at 1.5 gpm?  I figure that won't be a problem since my domestic water heater normally sees 38-40 degree groundwater at 3 gpm when filling the bathtub. (Yes, our groundwater is that cold; I've measured it.)

My other option is simply to feed 120 deg water right to the tubing.  My buddy has been hammering on me to just do it that way...he says I am making this way more complicated than it needs to be.  I know that isn't ideal, but I'm not clear on what the drawbacks are.  I don't mind if the indoor temp varies +/- 5 deg.  What other problems occur by feeding hotter water than necessary?

Smart guy.

Keep the water heater high enough and condensate will not be a problem. Of course this is a set it and forget it control. Turning it on for Saturday hobby will not work.

You're not the first to accuse me of being a "wise guy".    No offense meant, just wanted to point out my thoughts.

I was planning on leaving the water heater set at 120 deg and never changing that, do you think that'll be fine?

Any thoughts on my buddy's idea of just sending 120 deg water through the tubing?

I would look at going to 12 to 18" centers and 5/8 then you would be around 900 ft of tube. A 3 way mixing valve with a remote sensing non electric head used for space temp control, one grundfos 15-58 set on speed 2 add a strainer and a summer winter switch for pump and pipe it in 3/4. I would install a smaller nozzle in the burner to minimize short cycling and then size the flue to the new setting on the burner. So say you can install a .5 gpm nozzle you would then be at 70,000 btu input. Your tank is going to maintain 120 all the time and you dont have a lot of piping so outdoor reset is not going to do much...Insulate with 25psi eps under and perimeter...Eric