Questions about Hydronic Unit Heater for Garage

[superskaterxes]

So im looking to heat my garage to be able to work in it during the winter and i have explored several options but im not sure yet what i want. I have gas available but i dont want to have to run the vent setup for a blower unit and i heard horror stories about the moisture buildup in non-vent setups. Right now im using a Mr. Heater propane heater similar to this and it heats the garage up very well

electric is out of the question because its expensive and inefficient. Here are my garage specs:

22x22
3 walls and roof are insulated
BTU calcs come out around 22-25k which is what my propane heater puts out and it only needs to run for about 10-15 min before the whole room is hot.

I then came across the hydronic unit heaters which make alot of sense to use. I know they are designed for a boiler system but i cant find a reason why they would not work with a regular water heater.

http://www.modinehvac.com/web/Hot-Dawg/Hot-Dawg-H2O.htm#.VJsIoV4DYU


I currently have my water heater set to 120*F which is the minimum the hydronic unit can run at. My idea was to take the output of the water heater and feed it straight into the unit heater in the garage and then out of the heater into the rest of the homes hot water system.

the unit heater has approx a 14-20*F temp drop over the coils so if i were to raise my water heater to say 130-140*F i would still be getting 120*F at the faucet to avoid burns. I would turn the water heater back down to 120*F in the spring/summer/fall when i dont need the garage heated.

does anyone see a reason this wouldent work? it sometimes dips below freezing here but very rarely so i dont see the pipes freezing especially if they are in the garage. I have attached a picture to explain what i was thinking.

The only downside i can see is the pressure drop over the coils which is approx 4-6. Here is a link to the model i am looking at which has a chart on page 9. I am looking at the 45 size unit.

http://www.modine.com/download/1/1-500.pdf

ultimately i would like to be able to run a closed loop system with a coil running through the hot water heater but i will have to wait for my current water heater to die first =D

 

[anthony666]

how would you regulate the DHW temp when the unit is not running and not dropping 14-20 degrees across the coil ??

does your tank have enough recovery ability to make 20k btus an hour ??

does the unit have a flow switch to turn on the fan ?? if so, how are you gonna handle that when you have a dhw call but no heat call ??

lastly do you really want to bathe/shower/cook with water that has been pulled through a unit heater ?? any junk left over from manufacturing and any outgassing is gonna contaminate your entire DHW system

if your tank has the jam to make enough btus per hour to keep up with out blowing out your wallet, build a side arm to seperate the circuits, crank the temp and run your DHW through a mixing valve

 

[superskaterxes]

how would you regulate the DHW temp when the unit is not running and not dropping 14-20 degrees across the coil ??

does your tank have enough recovery ability to make 20k btus an hour ??

does the unit have a flow switch to turn on the fan ?? if so, how are you gonna handle that when you have a dhw call but no heat call ??

lastly do you really want to bathe/shower/cook with water that has been pulled through a unit heater ?? any junk left over from manufacturing and any outgassing is gonna contaminate your entire DHW system

if your tank has the jam to make enough btus per hour to keep up with out blowing out your wallet, build a side arm to seperate the circuits, crank the temp and run your DHW through a mixing valve

i figured i would leave it running all the time and just lower the tstat in the garage when im not in there (which isent very often =D)

i think the unit is controlled by a tstat so it will only turn on the fan when its called but otherwise water flows through it constantly.

i diddent think that there would be anything left over inside the unit but a good flushing should do the trick before first use no?

what do you mean by building a side arm? basically like a semi closed loop system for the HUH but only some of that hotter water gets mixed back into the DHW system so it doesent over heat the usable water?

 

[Randy in Maine]

How do you heat your house?

 

[anthony666]

a side arm is a heat exchanger

 

[superskaterxes]

How do you heat your house?

forced hot air


i thought about ducting into the garage but i think that would tax the system too hard and the tstat isent in the garage so it wouldent be able to regulate the temp there.

 

[superskaterxes]

a side arm is a heat exchanger

hmmm after a little google searching this sounds exactly like what im looking for =D

im going to dig into this a bit more but im sure ill be back with some questions.

thanks!

 

[anthony666]

very welcome

 

[superskaterxes]

heres another idea

what if i did a small tankless hot water heater and made a closed loop system inside the garage itself? i have seen some vent free units that are pretty cheap and it would save me on a ton of piping and complexity. i would have to install a re-circulation pump that i could turn on and off with a thermostat.

the only concern i would have is having enough pressure to keep flow over the HUH and if i would need a surge tank inline vs just having piping.

 

[yeldogt]

As Anthony pointed out .... most water heaters don't have a big enough burner to supply the amount of heat you will need .. some only have 23- 25 BTU's and they run at 80%. They are also not designed for this -- it will cause the unit to run much more than intended -- and rob the tank of HW when you need it most.

You are currently running a ventless heater ? -- and propane is a lot more $$ then NG.

I have an Empire through the wall unit in one of my garages. The unit is sealed combustion with a fan .. a bit more efficient as I only have propane ... and they run a tab bit quieter. They make others and are very easy to install.

You have NG -- get an Empire or the common Modine or similar garage heater -- they are not hard to install or vent. Do it correctly .. and once.

You will spend 1/2 as much or the same on what you are trying to do .. and it is not going to work .. cost more to run.

 

[keating]

What you have diagrammed there won't work, as you will only get hot water flowing through the heater when a hot water tap in the house is turned on.

You need a circulator, like a Taco or a March, capable of supplying the 2-4GPM the unit requires. The pump will take water from the hot side of your DHW tank, pump it through the heater and return to the cold side of the tank.
I'm not 100% sure, but I believe this violates a whole book of building codes.
Ideally, as mentioned before you would run a closed loop of glycol through a HEX, as having the heater turned off while you're gone on vacation for 2 weeks in February could result in a frozen core.
This solves 2 problems. 1) the glycol won't freeze and 2) if you developed a leak in the core, whether due to freezing, or other factors, you would only leak the fluid from the loop, a couple gallons, max, and not 50+ PSI of city water pressure dumping water, wherever the leak happens to be until a neighbour notices the water running out under the garage door.
Adding a HEX to the system means you need 2 circulators. One to circulate the tank water through the supply side of the HEX and a second to circulated the closed loop through the load side.
HEXs aren't 100% efficient, so you will lose some heat there
Also factor in the heat loss in the piping to the unit. Because you want to maintain as much heat as possible in your water (minimize extra load on your water heater), you want to insulate both the supply and return lines.

As for running it a 120 vs 140, crank up the tank temp and run a tempering valve to supply 120F water to taps in the house.

FWIW, the cost of the circulators and the HEX will cost as much or more than a professional NG install.
If your garage has an exterior wall and you have NG nearby, the overall cost for an HG HotDawg will be less than one of these things, properly installed (I don't even know what they cost, but it would have to be almost free, to be cheaper in the end.)