Garage Heating Design at 50F vs 70F Modcon Vs 80% Unit Heater - 2 Stage Design?
I have a 1000 sqft garage, full insulated walls (R17) and ceiling (~R50). Concrete pad is believed to be uninsulated. 2 west facing 10x8 garage doors R13.
Currently I am attempting to decided between using currently existing modcon or add gas fired unit heater. Modcon has lowest modulation of 21k btu which should be close to keeping garage at 50 at 10 degrees outside (60 degree delta).
My question is best method to design around the idea of 50 steady state, 70 when using for best efficiency. Seems like there should be a controls option available, perhaps pumping? If efficiency isn't easily achieved might as well might throw up standalone 80% 38k btu hot dawg and live with some short cycling. Currently running my boiler primary loop at 140-150 degrees with no plans to change. This points me to Modine lodronic or hot dawg hydronic (or similar low pressure drop models). My thought is I could get tricky with hot dawg hydronic and its multiple fan settings but its form factor isn't the best for my situation. Could easily wire this to effectively be a 2 stage with relay. Seems like a thermostat/pump controls system should already exist I just havent found it. Price is a factor becuase at some point, the system cost wont be covered but limiting boiler cycles has a value I suppose.
Comments
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Obviously a mod/con on low fire with 140* SWT is going to have a higher combustion efficiency than an 80% direct vent UH. What exactly are you asking?
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Best way to design around 2 temperature design considerations. Im very inexperienced in this field but the asnwer seems clear to me at 2 stage heating. How to plumb this in a cost effective manner whether it be 2 pumps, multi stage pump, or multi stage fan, plus appropriate controls. Is it as simple as running 2nd stage thermostat wire to another "zone" on a taco controller to fire another pump? Or I could effectively control the modine fan in a similar way with a relay. I don't love fan control as that impacts heat throw. I could solve this with another fan somewhere else in the garage though.
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What type of emitters are being used. The existing mod con is/was doing what?
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if you just want to use the unit heater at 2 temps, size it for the higher temp and use a thermostat or programmable thermostat to keep it at the temp you want it at a given time. If you want it to have less output at certain times, use a 2 stage t-stat, connect 2 relays to 2 blower speeds and have the t-stat select a lower fan speed for the first stage. Use the first stage to control the zone valve or circulator.
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Existing modcon is controlling house radiant heating.
I think mattmia is suggesting controlling fan for 2 stage control, not 2 pumps? That certainly seems like easiest option but does limit me to unit with fan controls. I have an extra TacoSR502 laying around if plumbing second stage is an option. Seems like a 2 stage pump must exist or I could plumb in parallel assuming check valves exist?
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What? Turn the thermostat to your desired temp and walk away. That's how these work.
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If you move less air through the coil you will output less heat from the unit heater. You could do that either with a multi speed motor or with a motor that is capable of using an electronic speed control and use a relay to put the speed control in circuit or bypass it. I barely see any benefit to changing the output and I see no benefit to changing the water flow.
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its not really worth diving into why Im looking into this option but it revolves around unknown performance of the garage space. My plan is size everything for 70 degree and run at 50. If it doesnt work as well as I hope Im just making sure I have an out. Point is if I cant run hydronic efficiently it would make more sense to throw up an 80% Unit Heater. Seems like blower control is my best path here. I appreciate the help!
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And again, a mod/con at low fire will always use less fuel than an 80% unit heater. You said it's already in operation, right? Run it.
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A mod con set to max 150°. An air handler with a hydro coil and constant torque blower motor. A well insulated garage to be kept at 50°. I don't see an issue. The lower the set temperature, the more efficient. Off is 100% efficient. You're trying to reinvent the wheel. There are water temperature sensors on the boiler, correct? The boiler does modulate, correct? The boiler should see SWT and RWT, and do what its designed to do. Depending on your location though, making 70° on design day at 150° limit with a fan coil might be pushing it.
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One intersting option suggested here was to use the domestic hot water mode of your mod con for the air coil (fan coil thermosat triggers the DHW aquastat input). This lets you set higher temperature just for the fan coil plus if set for priority you can get full output from your boiler to warm up the place quick.
If you want to get fancy, you can go two stage. First stage just runs the fan with existing temp to maintain the space, stage 2 triggers the DHW mode for quick heat.
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You may also run in to issues with the size of your gas service if you try to design a piping system to run both the boiler and a gas unit heater and your other loads.
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Do you only go in the garage during heating season?
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is this a shop space the you will be working in? 70 is fairly warm, I run 66-68 for my work space. If it is more office work, 68-70
Also blower type heaters can be annoying to work around. The noise and cold floor hot ceiling effect.
Load calculation programs do have some built in fudge factor, as well as your inputs
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0
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