Low limit aquastat help

CHRISHALL

I have read through many threads on freeze protection solutions for attic air handlers. I do not want to go the glycol route as I have three large zones that I flush every couple of years for maintenance purposes. I just replaced 5 circ motors and have my radiant dialed in and working perfectly with some help from others on this thread. I now am concerned for my back-up hydro air heat that is my primary AC over summer months. It is located in an unconditioned attic space and want to be sure the circ motor cycles when the attic temp reaches near freezing temps. I read about three solution; Delta T circs, Freeze Stats, and low limit aquastats. I believe the simplest solution (access and wiring) would be to go with the low limit strap-on aquastat but most don't go below 100 degrees for min temp. So my first question is; any recommendations on low limit circs? Second questionI have is most of the wiring diagrams show the aquastat interrupting circuits for protection and not engaging a circuit and I only want to send power to the circulator (not the fan) when it reaches a low water temp. If I wire it directly to the circ. pump wiring isn't this just like wiring a second thermostat in parallel? The wiring system induces a thermostat, a Unico system with cooling and hydronic heat and a Taco SR504 switching relay. Any help or discussion would be greatly appreciated!

EBEBRATT-Ed

you already have a circulator. You need a strap on aquastat that "makes" on a temperature fall. Put that in the attic. Run a 2 wire cable to whatever starts the circulator now. This could be low or line voltage depending how your wiring is hooked up. Then put a single pole switch (like a light switch) in series with one of the wires. The hot wire if it is 120 volt. Mark the switch "summer-winter" switch is closed or 'on" in the winter "open" in the summer

pecmsg

If the attic can get below freezing then you need glycol!

CHRISHALL

Wiring is low voltage from Thermostat to Unico air handler relays then onto a Taco switching relay and then sends line voltage to the circulator. I was hoping I could power the aquastat from the heat side of where the home thermostat is wired to the Unico relays and bypass the fan relay switch and wire the power out of the aquastat to the circulator relay switch. Hoping it would engage just the circulator when the supply water gets down to the set point and turns off when it reaches the differential temp. Not sure I need a summer/ winter switch especially if I can get an 40 degree low limit aquastat.

pecmsg

So, what happens when the powers out?

CHRISHALL

The whole house generator kicks on. My house is over 170 years old with portions that are dirt crawl spaces and granite stone foundations. If I loose power I have many areas of my entire home plumbing that will become compromised. This is not a new system and it includes two hydronic manifolds and 8 circulator pumps. If it were cost effective to isolate the attic system I would reconsider using glycol. I am just looking for a non-glycol solution to circulate water in a backup heat zone during extreme winter months.

SuperTech

Unfortunately the best thing you can do is add glycol. Aquastats, relays and circulators all fail. Are you prepared to deal with the aftermath? 

Why are you "flushing the system" every couple of years for maintenance? Seems completely unnecessary and counterproductive.  At least if had glycol to replace every few years the flushing would make some sense.  

EBEBRATT-Ed

If you glycol it you will have to glycol the whole system. Or if the attic AHU is on it's own zone you could install a small heat exchanger and just glycol that zone

HVACNUT

CHRISHALL said: If it were cost effective to isolate the attic system I would reconsider using glycol.

You can. A heat exchanger for that zone. Otherwise, it's been my experience over 35 years that if you don't use glycol, or are HERS rated, where the attic would be considered conditioned space, then you've got a big mess begging to happen. 

What's the primary heat source?

CHRISHALL

OK so thanks all for the discussion on glycol when I specifically requested a discussion on non-glycol solutions. My experience with this current system of 20+ years is that previous use of glycol in this system had contributed to completely corroding 4 of my 8 circulators in less that 5 years. The system consists of a hydro air zone manifold directly off my boiler. That consists of DHW zone, two hydro air zones and one radiant injection zone that feeds another radiant heat manifold in another basement. That zone has three radiant zones, one concrete floor zone in my garage and two staple-up zones for bedroom and family rooms. The only zone compromised is one of my hydro air zones and its a primary zone for summer air conditioning but a back-up heat zone in the winter during extremely cold nights where the radiant heat can't keep up with heat loss in large cathedral ceiling bedroom and living rooms. In the 20+ years, 15 of which did not have glycol I have only had one freeze-up and luckily no damage. I'm not just putting a glycol mix in 50 feet of copper, I have three short 20 feet zones, one air handler about 120 feet from the main manifold and a radiant manifold 60 feet away from the main manifold. That along with 2000+ feet of radiant tubing. It comes down to a $$$ versus risk game. I could spend another $700 or more on glycol for a system where only 10% needs it and risk chewing through more circs in another 4 years. OR I could spend $1500 repiping one of my zones to isolate for glycol use. OR I could add a $200 low limit aquastat to mitigate a freeze risk in an attic over my garage. I would love to hear from anyone with low-limit aquastat experience before I end up at Home Depot looking for heat tape.

SuperTech

I'm not a fan of glycol either, but it's the only solution that I can promise will work without having to worry about it failing to function.  If I were you I would consider the heat exchanger option. Everything else you have mentioned is something I have seen not work as intended. 

yellowdog

use a johnson a421 aquastat. it is digital and will allow you the lower setpoint temps that you want.

HVACNUT

CHRISHALL said: I would love to hear from anyone with low-limit aquastat experience before I end up at Home Depot looking for heat tape.

I have plenty of low limit aquastat experience, just not in that scenario.
Your situation isn't at all atypical. Many a ceiling has come down over the years, all across our great nation from frozen heat pipes or the coil itself, which is usually where it happens. You're not getting "outside the box" comments because its all been tried before, and the general consensus has deemed, glycol, or a hot roof are the only bulletproof solutions. 
There are aquastats that offer freeze protection. 
I feel like I'm arguing with my kid to put a coat on when it's 25° outside.