Thermostat to control boiler zone with fan control
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    Thermostat to control boiler zone with fan control

    As outlined in my new Garage Build thread, we added a boiler to heat the new garage with hydronic in floor heat. The Lochinvar boiler we went with has up to 4 zones (thermostats) that we can hook up to it. I buried an insulated PEX loop out to my pole barn where I have my main shop. So one end (with a pair of 1" PEX lines) comes up in my mechanical room in the new shop, the other in the pole barn. Today I am heating the pole barn with a forced air NG hanging style shop heater. I will likely leave that in place for the time being but am looking at heating the shop with a water to air heat exchanger. This is the one I think I will buy.

    https://www.amazon.com/Hydronic-hang.../dp/B077V63CM1

    So to operate something like this I will need a way to call for the boiler to kick on and generate heat as well as trigger the pump to pump glycol out to the pole barn. I will also need to trigger the fan on the water to air heat exchanger to kick on. I will run a thermostat wire from the boiler to the pole barn via conduit that will be shared with the fiberoptic cable for internet. Shouldn't be an issue mixing low voltage with glass in the same pipe. That will allow me to install a thermostat out there to trigger the boiler and pump however how do I trigger the fan? Am I going to need two thermostats? My thought is to put a probe on the return loop or in the heat exchanger somewhere to detect a temp warmer than say 80F and kick on the fan. This way the call for heat controlling the boiler/pump will start circulating warm fluid, once it gets to the heat exchanger it detects it or the return loop heating up and kicks on the fan. Once the air temp hits my target the boiler/pump cuts out but the fan would keep running until either the return loop or heat exchanger drops in temp below whatever it is set at. The only problem is that the fan would try and run all summer unless I set it to something like 110F or just turn it off in the summer which is likely what I would do. Might as well keep using the warmth in the exchanger for a bit even though the boiler/pump kicked off. The boiler will send 160F to the pole barn and I expect some heat loss but it will be very minimal the majority of the run is under the new garage which has a heated floor, and it is about 15' from the new garage to the pole barn where it would be in frozen ground. I think the PEX is rated to 1F loss per 100' Total run is probably just shy of 70'. So I expect to basically have 160F liquid.

    So, my question is if anyone has a setup like this and if so, how do you control both the boiler/pump and line voltage to the fan? Is there a single thermostat that will do this or am I better off going with two thermostats with a probe on the heat exchanger and a separate air thermostat to trigger the boiler/pump.


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    I didnít look real closely at the unit you linked...

    I used to sell these things years ago. These would have their own fan limit control in them just like a furnace (hot air) would.

    When the thermostat calls for heat the boiler will fire along with the circulating pump. As the heat exchanger warms up the fan limit control will sense the temperature and start the fan when appropriate. Then of course the opposite when cooling down.

    The specs didnít say if a fan limit control is included or not. That is what you will need for the fan to come on and turn off as certain temps of the heat exchanger.
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    Ok, I'll give this a shot, but I'll have to re-read your spec a couple times....

    Yes, you can do it with one stat.

    You will need a couple relays. Relay In a Box, by Functional Devices. RIBU1C - single pole, double throw. RIBU2C, double pole, double throw. Coil has connections for 24vac & 120.

    I would also use a temp sensor on the pipe to keep the fan from running till the flow is hot.

    Stat makes, pulls in fan contactor, but fan doesn't start because pipe sensor wired in series is still open.

    Stat also makes 2 pole relay at the boiler, one starts zone pump, other contact start boiler.

    Water gets hot, pipe sensor makes, fan turns on.

    Shop gets warm, stat opens.

    Boiler should stop when the stat opens.

    If you want the pump & fan to continue, you can use a time delay off relay, or you can use another pipe sensor to keep them energized (like a latching relay) until the pipe is cool.
    Last edited by rtgt; 09-26-2019 at 10:32 AM.
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    Quote Originally Posted by coaltrain View Post
    I didnít look real closely at the unit you linked...

    I used to sell these things years ago. These would have their own fan limit control in them just like a furnace (hot air) would.

    When the thermostat calls for heat the boiler will fire along with the circulating pump. As the heat exchanger warms up the fan limit control will sense the temperature and start the fan when appropriate. Then of course the opposite when cooling down.

    The specs didnít say if a fan limit control is included or not. That is what you will need for the fan to come on and turn off as certain temps of the heat exchanger.
    So they sell four versions. Well packages I guess is a better term.

    Water to Air Heat Exchanger alone then bundled with different combinations of things like a rheostat to control fan speed and a thermostat.

    The thermostat that they offer I looked up.

    https://www.amazon.com/LuxPro-Progra.../dp/B079GL1TQZ

    But the issue is this appears to plug into an outlet and then the fan plugs into this. Also the temp prob is pretty short and on the back of the thermostat so only senses air temp. Then it is programmable for weekdays and weekends for different temps. Yeah don't care about that. I normally keep the shop at 36f or so all winter and bump to 60f when working out there. It also doesn't appear to have an output to low voltage to power the boiler/pump. Because of these reasons I was going to skip the thermostat.

    I was looking at getting the bundle with the rheostat. Not that I feel the need to control the fan speed, maybe it would be nice, but for some reason it is cheaper to get the one with the optional rheostat than without.

    I guess you are touching on my question, I don't see anything that is a fan controller, I guess I could order one and see what happens when I plug it in. If the fan turns on then there isn't a controller. That is where I was thinking I could make one with a second thermostat with a probe on the heat exchanger to detect that the boiler is heating something and then it triggers the fan. In homebrewing I have a similar thermostat like this to control my fermentation process. It is a dual stage thermostat that can trigger cooling (turn on a fridge) or heating (turn on a heating element in the fridge) so that I can maintain whatever desired temp I want.

    This is what I used for that project and I could adapt one to act as a fan controller.

    https://www.amazon.com/Inkbird-Tempe.../dp/B0152LYY0I

    I asked the boiler guy what I should add for this and what he thought it would cost. He said that the water to air exchanger would be around $1000, plus a thermostat for the boiler/pump and misc plumbing. Total he was thinking around $1500. This one in the amazon link is $300-400 depending on options. For the same 100,000 BTU heat exchanger. Now it might not have the fan controller that you mentioned but that is what I am trying to figure out. Worst case, I don't see why I can't just make my own with the one above for $15. Doesn't need to be all that fancy, just no need to run the fan 24x7. I just didn't know if there was an all in one option out there for a thermostat that can read air temp and a probe for the heating element. Then call for heat with the low voltage to the boiler/pump and line voltage for the fan.


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    Quote Originally Posted by rtgt View Post
    Ok, I'll give this a shot, but I'll have to re-read your spec a couple times....

    Yes, you can do it with one stat.

    You will need a couple relays. Relay In a Box, by Functional Devices. RIBU1C - single pole, double throw. RIBU2C, double pole, double throw. Coil has connections for 24vac & 120.
    I might have one of these laying around I just don't remember what flavor of a RIB it is. My dad gave me one years ago when he never had a use for it but I haven't found one either. Now where did that go.


    I would also use a temp sensor on the pipe to keep the fan from running till the flow is hot.
    Yeah this is what I was thinking either on the return run or the heat exchanger. I was leaning toward the return only because if the boiler/pump shuts down, and the inlet pipe may stay hotter than whatever my fan cutoff temp is set to, it will keep the fan going long after the heat exchanger isn't producing heat. That is also why I think it might be better on the heat exchanger somewhere vs on the pipe. Once there is flow of 160f liquid the heat exchanger should get to temp quick. I also don't really care about what the temp of the pipe is to control the fan, the heat exchanger is heating the air being blown by the fan.

    Stat makes, pulls in fan contactor, but fan doesn't start because pipe sensor wired in series is still open.
    Can the pipe (or heat exchanger) take line voltage and how would I set it? I assume that would basically be another thermostat unless I wired in a thermistor in series.

    Stat also makes 2 pole relay at the boiler, one starts zone pump, other contact start boiler.
    The Lochnivar boiler has controller built in for 4 zones (pumps) so I run the Tstat to the boiler, that will control my pump.

    Water gets hot, pipe sensor makes, fan turns on.

    Shop gets warm, stat opens.

    Boiler should stop when the stat opens.

    If you want the pump & fan to continue, you can use a time delay off relay, or you can use another pipe sensor to keep them energized (like a latching relay) until the pipe is cool.
    I could do a time delay relay. I think that STC1000 thing I posted above will do that. I know there is a start delay that I can program in. I use that to keep from short cycling the compressor on my fridge in the fermentation process. So basically backwards but if I am sensing the temp at the heat exchanger I don't need to get that fancy. Lets say I set it to 80F. Well if the boiler kicks out because the shop hits the target air temp, the fan would keep blowing across the heat exchanger until that probe hits 80F. Same effect. I just figured why not keep the fan blowing, I just would want it set to a temp greater than what I would call for. I could go 110F because we rarely hit that temp in the summer around here but that is still quite a bit of usable heat which could be circulated in the shop. I can always shut it down in the summer. I do that today with the NG furnace.

    While I think your solution will work, I wonder if from a complexity standpoint it really isn't much difference than just doing two thermostats. One sensing heat at the heat exchanger and let it control line voltage of the fan. The other sensing air temp and controlling the boiler/pump.
    rtgt likes this.


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    Quote Originally Posted by sennister View Post
    So they sell four versions. Well packages I guess is a better term.

    Water to Air Heat Exchanger alone then bundled with different combinations of things like a rheostat to control fan speed and a thermostat.

    The thermostat that they offer I looked up.

    https://www.amazon.com/LuxPro-Progra.../dp/B079GL1TQZ

    But the issue is this appears to plug into an outlet and then the fan plugs into this. Also the temp prob is pretty short and on the back of the thermostat so only senses air temp. Then it is programmable for weekdays and weekends for different temps. Yeah don't care about that. I normally keep the shop at 36f or so all winter and bump to 60f when working out there. It also doesn't appear to have an output to low voltage to power the boiler/pump. Because of these reasons I was going to skip the thermostat.

    I was looking at getting the bundle with the rheostat. Not that I feel the need to control the fan speed, maybe it would be nice, but for some reason it is cheaper to get the one with the optional rheostat than without.

    I guess you are touching on my question, I don't see anything that is a fan controller, I guess I could order one and see what happens when I plug it in. If the fan turns on then there isn't a controller. That is where I was thinking I could make one with a second thermostat with a probe on the heat exchanger to detect that the boiler is heating something and then it triggers the fan. In homebrewing I have a similar thermostat like this to control my fermentation process. It is a dual stage thermostat that can trigger cooling (turn on a fridge) or heating (turn on a heating element in the fridge) so that I can maintain whatever desired temp I want.

    This is what I used for that project and I could adapt one to act as a fan controller.

    https://www.amazon.com/Inkbird-Tempe.../dp/B0152LYY0I

    I asked the boiler guy what I should add for this and what he thought it would cost. He said that the water to air exchanger would be around $1000, plus a thermostat for the boiler/pump and misc plumbing. Total he was thinking around $1500. This one in the amazon link is $300-400 depending on options. For the same 100,000 BTU heat exchanger. Now it might not have the fan controller that you mentioned but that is what I am trying to figure out. Worst case, I don't see why I can't just make my own with the one above for $15. Doesn't need to be all that fancy, just no need to run the fan 24x7. I just didn't know if there was an all in one option out there for a thermostat that can read air temp and a probe for the heating element. Then call for heat with the low voltage to the boiler/pump and line voltage for the fan.
    What I had in my mind was a hydronic controller - one that has a probe that is in the liquid flow from the boiler. Of course the heat exchanger would have to have a port for said probe.

    Thinking more it would be overkill for your situation.

    The LuxPro thermostats you linked would be a simple solution. It will be sensing the air temp adjacent to the unit. However it will turn the fan on and off with the change in the ambient air temp and not the temp of the liquid in the system which is what I was going for.

    There would be no way to accurately sync the system thermostat and the luxpro thermostat. But that may not be a big deal as long as you got them close. What may happen is the luxpro thermostat may kick the fan on before hot liquid is flowing through the heat exchanger.

    One other benefit of a true fan and limit controller is they are designed to kick the boiler burner off if it reaches the preset limit. In a situation where the boiler didnít shut down when it should the fan/limit controller would shut it down before the system got too hot and caused damage.

    So for my idea - you want something that will kick the fan on when the heat exchanger itself reaches a certain temp - then shut the fan off when the heat exchanger cools to a certain point. So something with a probe or sensor at the heat exchanger itself. It can and should act independently of the thermostat itself which is controlling the boiler and circulating pump.

    Iím trying to digest rtgtís theory of operation.
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    Quote Originally Posted by sennister View Post
    As outlined in my new Garage Build thread, we added a boiler to heat the new garage with hydronic in floor heat. The Lochinvar boiler we went with has up to 4 zones (thermostats) that we can hook up to it. I buried an insulated PEX loop out to my pole barn where I have my main shop. So one end (with a pair of 1" PEX lines) comes up in my mechanical room in the new shop, the other in the pole barn. Today I am heating the pole barn with a forced air NG hanging style shop heater. I will likely leave that in place for the time being but am looking at heating the shop with a water to air heat exchanger. This is the one I think I will buy.

    https://www.amazon.com/Hydronic-hang.../dp/B077V63CM1
    Before I comment on anything else, will your plan here even work well enough even if you can find the correct controls?

    What temps is your boiler heating water/glycol to?

    Typically, hydronc in-floor heating systems use water temps at 85-125 degrees.

    That exchanger you are looking at is designed to be connected to a system for standard hydronic baseboards. Those systems typically heat water to 160-185 degrees.

    If you are pumping water that is only 120 degrees in to that exchanger, you're going to have to wait an awful long time before your shop ever warms up.
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    Quote Originally Posted by coaltrain View Post
    What I had in my mind was a hydronic controller - one that has a probe that is in the liquid flow from the boiler. Of course the heat exchanger would have to have a port for said probe.

    Thinking more it would be overkill for your situation.

    The LuxPro thermostats you linked would be a simple solution. It will be sensing the air temp adjacent to the unit. However it will turn the fan on and off with the change in the ambient air temp and not the temp of the liquid in the system which is what I was going for.

    There would be no way to accurately sync the system thermostat and the luxpro thermostat. But that may not be a big deal as long as you got them close. What may happen is the luxpro thermostat may kick the fan on before hot liquid is flowing through the heat exchanger.

    One other benefit of a true fan and limit controller is they are designed to kick the boiler burner off if it reaches the preset limit. In a situation where the boiler didnít shut down when it should the fan/limit controller would shut it down before the system got too hot and caused damage.

    So for my idea - you want something that will kick the fan on when the heat exchanger itself reaches a certain temp - then shut the fan off when the heat exchanger cools to a certain point. So something with a probe or sensor at the heat exchanger itself. It can and should act independently of the thermostat itself which is controlling the boiler and circulating pump.

    Iím trying to digest rtgtís theory of operation.
    The luxpro I linked to only seems to provide the ability to switch the line voltage. Sure I could go with a relay in a box like what rtgt mentioned but to close a low voltage circuit back to the boiler. If I mounted the luxpro to the side of the heat exchanger it might all work fine because as the coils heated, it would heat the sides of the box but that might just cut off the boiler. The other problem with this is I would have to climb up a ladder to get to the heat exchanger to change the temp when I am working out there.

    This is why I keep going back to two separate thermostats like you also indicated. One to sense the temp of the liquid (to control the fan) and the other to sense the temp of the air (to control the boiler/pump). Unless someone makes a single thermostat that has two probes and can control line and low voltage. I could do something in the plumbing to actually sense the liquid temp. Again going back to homebrewing. I use thermowells in my kettles so that I can get temp readings of the wort during different stages of brewing. I then use a computer to control the electric heating elements. Same thing could be used to get a reading in the plumbing but I think it is overkill as I don't need accuracy to a 1/100th of a degree to control a fan. I am just thinking of sticking a probe in the heat exchanger somewhere. As long as the fan doesn't hit it, it should be fine. There are some thermowells in the installation right now but those are back at the boiler and too far away to control the fan. They are also before the splits to the zone pumps.

    I am sure the more expensive water to air heat exchangers do this with something integrated but some of those are in the $1000 range. It isn't that I am cheap but if I can accomplish the same thing with a $15 thermostat, why spend close to $700 more to have it built in. It would be one thing if it is was in my livingroom. This is my shop.
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    Quote Originally Posted by JimR View Post
    Before I comment on anything else, will your plan here even work well enough even if you can find the correct controls?

    What temps is your boiler heating water/glycol to?

    Typically, hydronc in-floor heating systems use water temps at 85-125 degrees.

    That exchanger you are looking at is designed to be connected to a system for standard hydronic baseboards. Those systems typically heat water to 160-185 degrees.

    If you are pumping water that is only 120 degrees in to that exchanger, you're going to have to wait an awful long time before your shop ever warms up.
    For the floor hydronic zones there is a mixer that will limit the glycol/water mix to 120F or so. For the pole barn zone it will be before the mixer so it can get full 160F but great question as I didn't mention it. I will also be adding another zone and put another water to air heat exchanger in my furnace plenum but that is for a later project. Doing the pole barn now.

    I don't have a great photo of how it is plumbed in the mechanical room but I can take one to show how he did it when I get home.

    I will also still have the NG forced air hanging furnace out there. Depending on how things operate in real world vs theory right now, I can at a worst case use hydronics to keep the shop at 34-36. Then when I go out there to work use the NG forced air to kick it up to 60F. It can do that just fine.

    The shop is roughly 32x32 insulated. Not sure of the R value. I also have a wood stove out there I use from time to time if I am going to be out there all day.


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    I have one of these for a homemade control system for the solar panels on my pool.
    https://www.amazon.com/bayite-Fahren.../dp/B011VGASLW

    It's lived outside for 3 years now and still works perfectly.

    There are multiple program modes but if you tape the temp probe to the water pipe on the heat exchanger you can set the unit to turn the fan on when it gets warm and shut the fan off when the water is cold.

    As for triggering the boiler loop you would need a second regular thermostat to control the space air temp.
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