Hydronic Unit Heater - water temp control

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Old 04-14-12, 09:28 AM
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Hydronic Unit Heater - water temp control

I just recently replaced an old hydronic unit heater in my garage. I live in northern IL and the first year I spent in this house, the old unit heater cracked in a really severe cold snap. Iím just getting around to replacing the unit heater with a Modine HSB47 with the top/bottom inlets. So I have hung the new unit heater and now I have to plumb everything. I have a Weil-Mclain GC series 12 boiler that heats my house as well as feeds the unit heater in the garage. It is a gas fired unit. I do not need the garage heated all of the time but I do not want the new heater to have issues with freezing water. I would not call myself a tech by any means, but more someone that is somewhat capable and willing to learn with the preference of not being ignorant of how things work in my house. Iíve been doing research on aquastats, aquastat relays, thermostats, wiring, etc. I canít seem to find the silver bullet to safe guard myself from having the unit heater or pipes freeze. Ideally I would use a thermostat to control heat when I want to have the garage warm enough to work in during the winter. When I don't need the garage heated, I would turn the thermostat way down and use some mechanism to control water temperature going out to the garage to keep the pipes from freezing. Is this possible? What components would I need? Any help in how this would be wired would be appreciated also.

I have 2 zone valves, one for the main house and one for the garage heater. From what I can tell there is only a Honewell L4080D on the boiler to control water temperature, and it looks like only a high temp limiter. There is a thermostat in the main house and one in the garage to control room temperature. From what I can tell, the thermostat triggers the boiler to fire and calls for water. Iíve attached photos and documents on the equipment used. I donít want to waste anyoneís time so if I start to think Iím in over my head, Iíll call it what it is. At the least it would be nice if I could get the components and plan the plumbing.

Boiler Manual:
http://www.weil-mclain.com/en/multim...cgx2manual.pdf

Unit Heater Manual:

http://www.shophmac.com/product-cata...stallation.pdf




 
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Old 04-14-12, 10:05 AM
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Turning the thermostat way down should be adequate. Keeping it at 55 degrees will keep the garage warm enough to prevent the pipes form freezing. If you are concerned about the cost, spend the money on insulating the garage. Insulation is fuel you only buy once.
 
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Old 04-14-12, 04:26 PM
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That's pretty much what I'm gonna say too... but I'll add that you do always have the option of running anitfreeze in the system... BOILER ANTIFREEZE! do NOT use auto stuff!

The only way yer gonna keep if from freezing is to either keep the garage above freezing by running minimal heat out there, or use antifreeze.

In ANY case, if the garage isn't insulated, insulate it!
 
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Old 04-14-12, 07:04 PM
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I appreciate the feedback. I guess there is always something to be said for keeping it simple. Not having much experience at this, I did not want to overlook something that I should be considering. The last unit heater install was not thought out well, no ball valves, drain, nothing...so I didn't want to miss anything prior to buying fittings, valves, etc. Would anyone be able to help me figure out how to wire the thermostat to kick on the fan? I'm not too concerned with blowing cold air until the water is up to temp, or would it just be better to use an aquastat? Sorry for these basic questions...gotta learn somehow.
 
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Old 04-14-12, 08:56 PM
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Page 6 of the Heater Installation Manual says:

Intermittent Fan Operation ó Hot Coil

A room thermostat starts and stops the fan motor. An aquastat
is sometimes strapped to the return piping to prevent fan
operation when heat is not being supplied to the unit heater.
So the manufacturer suggests using a separate a'stat to control the fan motor. In a nutshelll, the a-stat would act as the on/off switch for the fan motor, depending on the temperature of the water flowing out of the heater coil.

There are several makes of temperature controls that you could adapt to this application. One Example.

Question - that's not a piece of PVC pipe being used as a vent stack for the water heater, is it....? :NO NO NO:
 

Last edited by Rockledge; 04-14-12 at 09:05 PM. Reason: Correct Link
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Old 04-14-12, 09:33 PM
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Good eye on the water heater vent pipe...but it is not PVC. The previous owner was a little paint happy, as you can also see by the copper painted white in the background also. Thanks for the literature on the electronic temp control. One other question in regard to using an aquastat in a cold garage (in the winter), is there a performance difference with an aquastat that straps to a pipe and the type that uses a bulb well?
 
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Old 04-14-12, 10:08 PM
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...is there a performance difference with an aquastat that straps to a pipe and the type that uses a bulb well?
I would guess that the "immersion well" type of controls might be a little more accurate, but that's not really an issue, since you can set the temp. control to whatever keeps you comfortable. Once you find the setting that works, then you can just leave it there.

When using a control like the Ranco unit, after the temp. sensor is strapped against the pipe, it should be wrapped with with insulation so that it won't be affected by cold ambient temps.




 
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