Hot Water Boiler - Return Temp

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Old 01-19-14, 04:28 PM
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Hot Water Boiler - Return Temp

I have a hot water boiler, gas fired, Weil-Mclain CG-4-SPDN, made in 1994. This is an 80% efficiency unit. It has 2 zones, one for the house and one for the garage. I also replaced a hydronic garage heater last year and installed a 2 stage temperature controller. See the attached link where I was seeking help with the garage heater to get some idea of the setup if you more info/photos.

Link:
http://www.doityourself.com/forum/bo...-controls.html

I only use the garage heater when I need it (work to do in the garage, etc.) One stage of the two stage temperature controller is set up to run the fan on the heater when the temperature of the water is hot enough to blow hot air through the unit until the thermostat is satisfied when heating the garage. The second stage is set up as a low temperature protection, so if the temp hit 40 degrees, it would fire up the boiler and call for hot water, heating up the water in the pipe but not turning on the fan and avoid any risk of freezing.

Under normal circumstances, the water temp in the garage rarely hit 40 degrees, so it circulated infrequently. Now being in the midwest and going through the major cold snap we had, the unit was calling for hot water more frequently. I started to think that maybe this was not a good idea in how this was set up. I'm not a tech, but using common sense, I began to wonder if having 40 degree water flush in to the boiler to be heated, mixing with approximately 140 degree water may not be a good idea. I mean I wouldn't want to jump out of a hot shower and start making naked snow angels. With that being said, even with the influx of cold water, the boiler would fire and the temperature on the boiler guage would not fall much off the 140 degree mark

I tried researching what return water temperature should be on a boiler. There's a lot of conflicting or vague information out there. It seemed a target of -20 degrees was ideal. I'm not even sure if that is right. The boiler operates in a range between 140-180 degrees. I started to measure temperature on the return line of the house zone. The water comes back on the house zone at 110 degrees (or lower if the boiler has not fired for a while).

So that was a long set up for:

1. Is the 40 degree water coming in to the boiler from the garage an issue and/or does it depend on the frequency of this occurring?
2. What is the ideal return temp for water coming in to the boiler?
3. Instead of having the garage circulate water at 40 degrees, should I match the house return temp of 110 degrees and wrap the garage pipes with insulation (if so what type of insulation)?

As always, thanks for the help! It seems the people here seriously know their stuff!
 
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Old 01-20-14, 07:00 PM
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Chacho
1. Is the 40 degree water coming in to the boiler from the garage an issue and/or does it depend on the frequency of this occurring?
2. What is the ideal return temp for water coming in to the boiler?
3. Instead of having the garage circulate water at 40 degrees, should I match the house return temp of 110 degrees and wrap the garage pipes with insulation (if so what type of insulation)?

A1: If the boiler is cold (and not firing): not a problem as long as the return water temperature rises quickly enough to avoid excessive condensation of combustion gasses. Rapid chilling can cause section cracking. To protect the boiler from cold water a boiler by pass loop, three way valve arrangement, or primary/secondary boiler loop can be used.

A2: Minimum 130 F return water temperature to the boiler.

A3: Not quite on point; 40 F is only 8 F above freezing, not much margin. Would glycol or drain-back be a better solution?
 
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Old 01-21-14, 06:38 PM
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Thanks for the feedback. I think it was one thing to set up the temperature controller as a true, emergency freeze protection, but the way I was running it having it routinely move extremely cold water was bad execution on my part.

In the short term, I'm going to change the temperature controller in the garage to fire the boiler and move hot water when the water temp in the garage lowers to +110 degrees to match the house water. I'm sure I'll be cycling water pretty often, but it is better than sending 40 degree water to the boiler. Would it be helpful to insulate the pipes in the garage to slow the cooling, if so, what type of insulation? Would this suffice?

6RXL048118 - K-Flex 6RXL048118 - 1-1/8" Pipe (O.D.) x 1/2" Wall Insul-Lock Pipe Insulation, 6'

The other option would be to simply use the garage heater to keep the temperature up a little when it is extremely cold.

I like the idea of a mixing valve somewhere in the setup, but that will be an off season project.

Can someone explain the pros and cons of glycol? Seems some people say to use it and other are hell bent on DON'T USE IT.

Sorry if this stuff is rudimentary.
 
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Old 01-22-14, 03:02 PM
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That insulation is ok, check your pipe outside diameter.

The heat exchanger is the expensive part. I don't think insulating the piping and or maintaining 110 F in the loop protects the system in any useful way.

Maintaining temperature in the space and delaying the start of the fan are good strategies as is decreasing the heat loss of that space.


propylene glycol -- Anti-Freeze in Hydronic Systems

Watts Radiant: Sludge in Radiant Systems
 
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Old 01-22-14, 04:44 PM
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Watts Radiant: Sludge in Radiant Systems

I wonder if Watts, if contacted, would stand behind that? With a closed hydronic system and an appropriate air removal device, and without unnecessary routine draining and refilling, I don't think corrosion will occur, at least it hasn't in my 60-year-old system. The circulating water will turn black, but that is not important - ignore it, and don't try to flush it!

And sludge can only come from excessive water makeup, possibly due to a leak.
 
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