Adding Hydronic Loop Before HX?
Good evening yall, I’ve done tons of reading here and figure it’s time I ask for some help.
I’m interested in adding a secondary loop into my boiler to run a hydronic unit heater for my garage. The confusion on my end is where to add it. The system wasn’t plumbed with this system in mind so fitting it in could be tough.
comments/questions
- Internal pump is set to highest speed, I assume to overcome loss of HX. Manual notes any single zone systems with flow rate over 4 gpm require use of internal LLH.
- Is addition of second loop feeding before HX and returning afterward acceptable?
- Otherwise, adding secondary loop with closely spaced tees after HX would suffice but would always suffer from loss at HX.
Comments
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Why is there a plate HX on your boiler? Doesn't look like a tankless unit in combi config.
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Thats a Bosch can you post the model #. Do you know if the internal LLH plug is still in or bypassed? You could install two closely spaced Ts and be fine off the main loop you will need an additional pump in the hydro air loop. The boilers natural heat curve should keep up with the hydro air. What setting is the output at and do you have outdoor sensor installed?
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I am trying to understand the system …. First question ? Why the reason for the heat exchanger ? My first thought antifreeze , snow melt ? But it is radiant….
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Why is there a heat exchanger?
Is there oxygen barrier pex in radiant loops?
Is this somehow an open system?
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Cut two closely spaced tees into the supply coming off the boiler above the flow switch in the vertical pipe. Take the supply to the garage heater off the top tee the return from the garage goes in the lower tee. You will need a circulator in the return pipe coming from the garage heater pumping into the boiler supply with a flow check valve in the pump. You will need a 3 way valve on the radiant side of the hx to control the radiant temp as you will want the garage to get high temp water and keep the radiant at low temp with the three way valve.
This arrangement will not be text book but it will work. To change it to be text book will require a lot of repiping
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sorry everyone I see I left out some critical detail. It is my assumption HX is to isolate very old radiant floor, likely non oxygen barrier pex. Bosh boiler is not a combi (PN is on schematic but it is KBR 21-3)
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KBR21-3, I assume the LLH is not being used simply because there is no pump external to the boiler, is this good logic? I could open it up and look. I did note there is no tag indicating the seal was at one time broken but this may have been before they added HX.
Would you put in closely spaced tees before or after HX? Before gives me the hottest water obviously but I would need to tee in just above the low level fill as someone mentioned, before pressure tank.
Outdoor sensor is installed, Pump is at highest setting and I havent checked output curve but last night it was at 141 degree supply temp at 34 degrees outside according to the controller. 2 digit display read 155 but the manual isnt clear in the difference between these two. Seems like they conflict.
All of this makes sense minus the 3 way valve. Why would I need to throttle supply to the HX? I assume you mean I would be increasing supply temp to account for loss in unit heater on design day? In reality I dont expect all 3 zones and unit heater to run at same time very often.
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I agree with Ed: Closely spaced Tees just before HX. Although, if i am understanding your diagram correctly, the supply to the garage needs to be the T that is closer to the boiler and the return from garage needs to closer to the HX (this is different from Ed's post?).
You may have to adjust your outdoor reset but I think you can wait to see if your floor radiant loops end up too hot, before making decision on 3 way mixing valve. I don't think you will need one.
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I think this agrees with Ed's answer. Just for the sake of my understanding, why not after HX? And by just before, do you mean between expansion tank and HX? not a lot of space to work with there. I don't know convention on expansion tank location but it seems like closely spaced tees and only a few feet of 1" copper between them it shouldn't really matter before or after. I need to research more on the supply vs return tee placement as well as that part I cant quite wrap my head around.
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I made the assumption that your garage unit would be a hydro to air with fan. Those are usually designed to see higher temp intake. You'll get more BTU output with a higher temp, won't have to run it as long. Quicker response times for garage which will only be as needed?
The vertical section of pipe for the closely spaced Tees makes most sense unless you plan to do a lot of repiping.
Yes, you could place it after the HX, but you may have issues with slow response times in the garage.
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Youre correct, the system is currently designed for 120 on radiant so I figured I was at the mercy of that regardless without total redesign. The as needed factor of garage heating is really throwing my brain for a loop from a design perspective. It feels like this loop should be deprioritized for that fact however it will perform poorer when actually needed ha! The way I see it, hydronic systems are more around design day criteria and less about 50 most days and 70 when I'm working 6 days a month. Before HX means it will work best I want it warm, After HX will mean Ill have some control issues when I'm just attempting to keep it at 50 degrees. At 50 I do get more BTUs though due to temp difference. I could really mess with it and put garage heater on priority setting with the idea it really shouldn't have to run much at 50 but when I want it 70 I really want performance. The reality is my thermostat in the house only calls for heat max 8 hrs/day based on 3 years of nest data. 4.5 hrs average.
I still haven't ruled out a stand alone NG unit heater. Just wanted to avoid the annoyance of venting and I already own a hydronic that should be very close to the size I need.
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"The reality is my thermostat in the house only calls for heat max 8 hrs/day based"
This is important information which lets you know you have the heat load capacity to put the new loop prior to the exchanger. I don't know what your new garage unit or the pump you plan to use, but you can design in the option of being able to adjust the water flow and air flow to ensure a reasonable return temp (100 to 120F?) back to the HE from that loop so it doesn't totally zap your heat delivery to radiant loops. I imagine you will only run the garage to 70F for 3-4 hrs when you want to work there?
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exactly, sometimes more duration in garage but rarely and if garage is warm im likely out there not inside ha. Worst case I can also mess with mess with zone priority settings as a crutch. I plan to run 35k btu unit heater slightly over pumped as to recoup btus from lower supply temp. The modine hydronic manuals have been very helpful. While not the unit heater I own, I figured I would design around it because of all the helpful info they provide and the fact theirs is relatively cheap if I had to buy one it would be selected for it.
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