Heat exchanger for a garage?


I have a heater in my garage that runs off it's own zone on the house hot water system. This will be the first winter. My concern is that it might freeze if I lose power. I don't want to put Glycol in the entire house. A co-worker told me about a heat exchange system that essentially isolates the one zone and would only result in minimal water being lost if a freeze did burst the pipe. Can anybody give me more insight into this?
-Jim
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Zoning would minimize impact on the house system but frozen pipes in the garage would expand and crack the concrete floor. Best plan is to try to find a system that allows glycol in the garage zone. What is wrong with glycol in the entire house system? Saves a lot of hassle. That is second best plan. There are options to glycol such as percentage alcohol mixtures that wouldn't cause as much damage as gylcol if there was a leak.
Alternatively, if your garage pipes are not already installed into the concrete you could use an installation for the pipes that prevents pipe expansion from cracking the concrete - place a thin layer of something around the pipes so they can expand a bit without breaking the concrete. Maybe a 1/16-inch thick layer of foam? Then use polyethylene pipes which can stretch just enough that they don't crack when frozen water expands. The water would freeze and block the flow until it thawed. You would have to heat the garage with temporary heaters to thaw the pipes which might be a hassle.
Another option - put an auxiliary heating system in the garage to prevent it freezing if power fails. Kerosene heater? Small generator that you put outside and run a cord into garage to run a heater? Require you to know power is out and operate manually. A ceiling fan also run by the generator would make sure there is not a layer of frozen air along the floor if you running just a small heater. A larger generator would allow a heater in the garage and one or two in the house to get you through a long power failure.
If you super-insulate the garage and don't open the doors when power is out then it should last against freezing almost as long as the house itself. Good insulation would allow a small heater to prevent freezing. If it is an attached garage and the house has a basement against at least one wall of the garage then there will be a lot of heat stored in the ground under the garage which should add a few hours to any freeze-up time.
I was looking at large emergency generators that automatically turn on in case of power failure. I got estimates from $8,000 to $5,000 for our house (5,000 sq ft with a 5-ton, a 3-ton geothermal heating system and 2 separate 200 amp boxes). If your house is smaller and less complicated you could get it cheaper but I don't know by how much. These use inter- connection to the house power supply and expensive bits so they come on when required, turn off when power is restored and only serve the necessary electrical loads. Would protect the whole house for a long power failure. We pay a lot for insurance which would cover any damages so it is hard to say if these systems are worth it.
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Wow, thank you for taking the time to type all of that. Let me explain in more detail what the setup is. The garage is attached to the house. The heater/blower (about 2' square) is hung from the rafters. There are 2 plastic (poly?) pipes that come out of the shared wall behind it (~12-18 inches) that feed the water to it. That's it. The garage is on a slab. So the only pipes that are vulnerable to the cold are the ones coming out of the wall and whatever runs inside the heater/blower unit itself, which are probably metal.
The exchanger that was described to me by a co-worker would go in the basement somewhere. The napkin drawing presented to me showed the heat pipes coming from the boiler looped through a unit of some type that passed heat to a similar unit facing it via some kind of radiant method. Then the water passing through the receiving unit would flow to the garage. The benifit here being that this water could be treated with (a small amount) Glycol. This way the main system would not have to be treated and any leakes elsewhere would not have Glycol in it. Make sense?
-Jim
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Jim;
Sorry, I just assumed you were referring to an in-floor radiant system. I am in the process of designing my mechanical system for a swim-in-place pool in the basement. You are referring to a water to water heat exchanger.
In my travels through the internet I found the heat exchangers at the site below which do what you describe. I am going to get two of the pool versions to get the desuperheaters from my geothermal system to heat my pool water. Same concept as you describe. I think the internet cost was around $500.
http://www.heatexchangersonline.com/poolheaters.htm
You could google heat exchangers and perhaps find a cheaper one.
To do the circulating of the glycol to the garage heater you could use the Grudfoss circulating pump at the site below. I used one for my house's hot water circulating loop and it has worked fine for 2 years now and is dead quiet, if that matters. I ordered mine from a plumbing supply in town. They have a large selection of circulating pumps and you should check the literature, or phone them, to ensure that glycol is an OK liquid for that pump. Find one without the thermostat or timer option because those features don't apply to your application. You will have to wire the pump to start when the heater does.
Get a circulating pump that has about 1 gpm near the middle of the pump performance chart and a head of about 5 feet. In the size you need, these pumps draw about 25 watts and are high quality construction. If the power spec is over 25 or 30 watts it may be too large. Stick to the UP series - they are small and should be cheapest ones. I forget the naming convention but the letters at the end of the pump name sometimes refer to the type of plumbing connections; threaded, brazed, flanged, etc. Careful how you specify them - I got one that was way too large the first time around and was flanged.
http://www.grundfos.com/web/homeus.nsf/Webopslag/D6600B14F5C7C73386256AE900 5326AE
If the circ pump is too small you won't get enough heat transfer. A rule of thumb is 1.5 to 2.0 gpm per ton of heating capacity. A ton is 12,000 BTU and 1.0 Kilowatt is 3413 BTU so you could get the garage heater capacity from it's plaque and calculate the required flow. You should allow for the fact that the heat exchanger will have less than 100% efficiency and try to find the efficiency in their web site, if not use 70% and the 2.0 gpm/ton value - should be close. Insulate all the lines or you will take a hit on efficiency. Over-sizing the pump is bad if high flow velocities damage the garage's water to air heat exchanger. The pool exchanger seems pretty solid from the descriptions. Over-sizing also wastes electricity.
The circulating tubing size is tough to design without knowing more about the system but I suspect that 3/4-inch will be more than large enough. You could likely use 1/2-inch if the runs are short. Use only ball valves or plug valves. If you do use a different valve then go to 3/4-inch for sure.
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