forced air hx thermostat dilemma

[rkusek]

I finally got my first taste of wood generated heat (via forced air HX) last week. It took me way too long to assemble the primary manifold near boiler, secondary manifold in basement, and get the HX properly installed in the plenum. Purging the air out of 700' of underground PEX and fixing a few small black iron leaks took a few days of trial and error as well. Good news it is done and puts out heat. I will be able to carrover almost all of my wood supply to next year which should give me a year cushion to be able to maintain a supply of well seasoned wood. The EKO really loves some dry walnut & red elm that I've been giving it. Dry wood is really the key it seems.

I still haven't found the best way tie my current thermostat into the wood heating solution. I have been just shutting off the heat pump completely and letting fan run continuous which requires me to turn it off when it gets too hot. At night I let it run in heat pump mode so the hot water HX is further warming the air going out even more allowing it to run less to keep house warm. I did also open the breaker to the heat pump itself one night and that seemed to work fine too until the wood ran out and the water got cold.

I know at a minimum I need an aquastat or snap disc sensor on the secondary manifold coming into the basement. I suppose this could then be wired in series to a 2nd thermostat. So when the water is hot enough AND my 2nd thermostat calls for heat I could use the thermostat output to power a relay tied directly to the air handler blower and HX circulator. This output of this relay would probably drive the existing blower relay at whatever the coil voltage is (haven't checked that yet). The NC(normally closed) side of my new relay would be the existing feed to the blower relay while the NO(normally open) side would come from the output of my 2nd thermostat. I have a LUV baseboard 120v thermostat (says not for anything other than baseboard electric) with a simple dial that I want to attempt to try in this configuration. It should work fine since I am only using it to trigger the blower & circ and not doing anything with the existing Lenox wiring. I would set my existing thermostat 3-4 degrees lower than the hot water thermostat and I doubt the heat pump thermo would ever run when the water is hot enough at the aquastat. Even if it did it should not hurt the existing Lenox equipment the way I am hooking it up. Not sure how the built in humidifier works but there may be a way to make that function as well when the wood heat thermo is operating too. Would need to check if the existing thermostat wire has any free wires too.

Now while I am pretty sure the above paragraoh should work I also wondering if I can find a new thermostat that will do everything in one package without costing a bundle. I've seen posts where someone was going to try a multistage thermostat (like Honeywell 8320 3H/2C) using the wood option as one of the stages. Reading the installation manuals I'm not so sure this will work in a wood boiler setup. The Honeywell instructions indicate that if you bump it up 10 degrees it will immediately fire the 2nd stage rather than waiting a specified amount of time like I'm envisioning it should. Hopefully next winter I will have some Arduino monitoring mechanisms that may provide for this directly but I need to get a failsafe approach working now. Anyone see a problem with my plan or have you had sucess using a multistage thermostat to do everything. The geek in me thinks it would be cool to simulate one of the cool $300-400 thermos (outdoor temp, humidity, room temps, boiler temps, stack temps,storage, etc.) on a web page that could be accessed from anywhere (home, work, vacation, easy chair using my blackberry). That probably won't happen anytime soon however.

 

[hokiefan]

Hello there huskers.

I set up my system with a honeywell 8320 thermostat, I will be ading some type of snap disc control to my system to keep gas heat from kicking in when temp is more than 5 degrees below what the thermostat calls for.

I have a owb setup as the first stage, my gas pack is the second stage, it works well except in the morning [ I have mine programmed to drop to 65 at night and set to 69 in the day time] the gas pack kicks in to raise the temps faster.

I like the thermostat but without a aquastat or snap disk I can't be sure when the gas is running or not.

I would like to know what you decide as I will be addind something in my system to keep the gas from kicking in as long as my water temps are high enough in my HX.

 

[rkusek]

It sounds like your setup may be what I was concerned about using the Honeywell 8320. I don't believe the 8320 will allow you to specify when the 2nd stage should be activated. It sounds like it uses the 2nd stage to bring it up to temp faster which may be what you are after. I would rather let the wood boiler do it all unless of course the water is not hot enough. I don't believe there is a way to tell the Honeywell that. I think the 8320 is mainly designed for a multistage heat pump with 2 internal compressors. 1st stage is one, 2nd could be both, 3rd would be the backup heat. I guess if you can program a delay time between 1st and 2nd stages (say 5-10") then it might work. Only problem is waiting the 10" when you know the wood boiler is out. I was hoping someone with some backround on these thermostats could shed some light.

 

[pybyr]

This is what I use to turn my air circulator blower on when my water-air HX has come up to temperature:

http://www.patriot-supply.com/products/showitem.cfm/13525

(Johnson Controls a419)

I have the sensor bulb tucked and zip-tied into one of the "u-shaped" ends of one of the HX's tubing rows.

This control is wired in parallel with the pre-existing temperature switch for the blower in my oil fired warm air unit. That way, the blower activates automatically if either the oil unit or the wood-heated water-air HX have heat on tap.

Actuation of the heat from the wood to the water-air HX is controlled by way of a "zone control" run by the house thermostat-- when there is a call for heat, it'll turn the circulator on, and then the Johnson a419 will turn the blower on after- and only after- the HX is warmed up.

What you _might_ be able to do is to put some sort of temperature sensing switch at your wood boiler, which powers a relay to feed the "wood zone control" and to shut off the heat pump, when, and only when, there's heat at the wood boiler/ storage; when there is no heat at the boiler/storage, then the "wood zone control" would be de-energized and the heat pump would be re-energized.

You _might_ be able to have a single house thermostat serve both the heat pump and the zone control, but you'll probably need an isolating relay between the thermostat and the controlled devices so that the heat pump's control circuit and the wood zone control are not electrically tied together.

Anyways, that's some food for thought for you.

 

[Medman]

I am using a Honeywell IEQ thermostat with four heating stages to control my system now, in conjunction with relays and snap-discs. The Honeywell logic is fixed; stage 1 for x minutes, which is my w-a hx circulator, and fan at low speed. If house temp is not rising after x minutes, stage 2 - ramp up fan to high with circ on. Again, after x minutes, stage three - first back-up elec. element on in air handler, fan on high. After x minutes, second back-up elec. element on, fan at high. After z minutes, if still calling for heat, turn off and begin again.
Time for x and z is not changeable. Outputs of the thermostat are relays, and in my system feed larger relays to handle the circs and elements.
After two years I am unhappy with this system and am looking to build my own controller to run things - there simply is not enough control with this setup for all the variables to be considered when heating from wood boiler/storage with backup heat. I may keep the Honeywell thermostat to provide the initial call for heat, but interface to the controller for all of the outputs.

 

[dogwood]

Medman, when you build your own controller, what additional variables would you be trying to account for that your current system is not addressing? I was going to get the Honeywell IEQ that you suggested in a previous post. But if you're dissatified, I'll go another route.

Pybyr, clever idea using that Johnson Control with sensor on your w/a hx, and parallel wiring it. Mind if I copy it?

Like Huskers, I am trying to figure out how to avoid a second thermostat . That Johnson control will help simplify things. I'd like the thermostat to tell the circularor when to pump hot water from the boiler or storage to the w/a hx over my furnace. It will also need to tell my back-up Lenox forced hot air furnace when it needs to turn on if no hot water is availble for heating. Anyone suggest a thermostat to do that, and be able to turn on and off the central air in the summer. The Johnson control Trevor suggests should solve the problem of telling only the furnace's blower to turn on, and solve the problem of it turning on prematurely before the hx is up to temp.

Mike

 

[pybyr]

Pybyr, clever idea using that Johnson Control w. sensor on your w/a hx, and parallel wiring it. Mind if I cop it?



Mike

As they say, imitation is the sincerest form of flattery- go for it. The handiest thing about that unit is that you can try different settings to see what really works best for a point at which to turn the blower on.

 

[dogwood]

Trevor, what do you have yours set at now? I'll start there and then play around with it. Think I'll order it when I get home. And then locate the right circuit to tie it into in the furnace blower. Thanks

Mike

 

[Tennman]

pybr, I was just fixin' to buy a Honeywell aquastat to keep my fan from running continuously. Your Johnson Controls Temp Control appears to do the same function with the addition of a digital display. As a non-HVAC guy I sure like this better than the Honeywell aquastat. I am correct in understanding that it does the same thing as an aquastat but programmable with a digital display. How does it power itself? Does the thermostat provide the power or 24 VAC from the furnace. This appears to do everything the Honeywell device does and far more. I'll have my HVAC guy install it, but want to purchase now if if functions as the aquastat is intended to. Is the sensor a thermocouple? So why didn't you just tie wrap to the supply line into the HX? Really like this option over what I had intended to purchase.

 

[feelingasi]

i have forced air oil heat as my "backup". i have not purchased oil in two years. i also have central air which leaves the "primary " tstat in play year round. i simply installed another stat (programmable) next to the original one. the "wood" one is set at 70 deg when we're home and 65 when we're not. the existing stat OIL is set at 60 deg. i simply isolated the fan ckt with a relay and used the n/c side to lock out power to the burner/control ckt when wood was in use. i also have a "system" aquastat that will shut down the boiler if the temp drops below 120 deg. it is hooked up with a 3-way switch that allows me to manually select weather or not i want the low temp cutout in/out of play. my fan does not run except when there is a call for heat and it does not care if the heat comes from wood or oil. its all a matter of understanding how relays work and how to get them to perform one function while disabling another, "interlocking". my oil heat and oil driven fan will only run if the temp in the house drops below 60. if the house ever gets that cold its cause i was away and the wife didn't feed the fire. there is no need to run your fan constantly, if anything your helping your house cool down and only adding to the amount of wood you burn. i made a descision a couple years ago that if the house gets below 65 its cause of somebodys lazyness, and if somebody is cold for a couple hours its thier own damn fault. EFFORT= WARMTH.

 

[Medman]

My dissatisfaction with the thermostat is not the unit itself - it works well, controls the AC and humidifier, etc. It just does not have a programmable time interval between stages and you cannot set the temp offsets. It has worked well the past two years and I plan to leave it in the system, sending the heat call signal from the thermostat to a separate controller which will monitor the boiler and storage and supply heat appropriately. I will leave the control for the backup elec. elements connected to the Honeywell, so that if the controller fails there will still be heat available to the house.

One thing that makes my setup more challenging is that I want to run my forced-air fan on the lowest speed all the time, summer and winter. I have a variable speed air handler with an ECM fan, so the Honeywell takes care of the fan control during heating, cooling, humidification, and ventilation modes.

The additional inputs and outputs I want to control include a separate temp sensor for one large room in my house that has a northern exposure and does not benefit from solar gain in the winter. I would like to monitor the temp in this room so that the system can begin to compensate for dropping temps in the house before the sun goes down. I also want to be able to charge storage and draw from storage independent of the boiler, monitor storage temps, and have a remote display in the house showing status of the boiler and storage, which is in an outbuilding 140' away.

 

[pybyr]

pybr, I was just fixin' to buy a Honeywell aquastat to keep my fan from running continuously. Your Johnson Controls Temp Control appears to do the same function with the addition of a digital display. As a non-HVAC guy I sure like this better than the Honeywell aquastat. I am correct in understanding that it does the same thing as an aquastat but programmable with a digital display. How does it power itself? Does the thermostat provide the power or 24 VAC from the furnace. This appears to do everything the Honeywell device does and far more. I'll have my HVAC guy install it, but want to purchase now if if functions as the aquastat is intended to. Is the sensor a thermocouple? So why didn't you just tie wrap to the supply line into the HX? Really like this option over what I had intended to purchase.

Here's a link to the datasheet/ instructions for the A419.

http://cgproducts.johnsoncontrols.com/MET_PDF/125188.PDF

There are some variations of the model that can run on 24 volts, or some (like mine) that run on 120VAC; the unit then has a separate set of switched contacts for switching the load (in my case, the blower motor).

I don't recall if the sensor is a thermocouple or thermistor- it comes with the unit.

I deliberately put the sensor on one of the "u-loops" of the water-air HX on the side of the HX opposite the hot water supply line, as I did not want the blower to start until the HX was actually warm and getting warmer (which, due to thermal mass, has a lag time compared to the commencement of the flow of hot water into the HX).

What temperature you set your A419 for is going to vary, and I set mine by trial and error so that the blower would turn on at the point at which the "u-loop" of the HX became distinctly warm to the touch.

 

[dogwood]

Trevor, I think I'll use the A419 to turn on the "Fan Only" circuit on my thermostat. I think that's a 24v circuit. Since I have 120v right by the HX to power the A419, if I purchase the 120v model, there's not any reason it couldn't switch on the 24v circuit, is there?

Mike

 

[Tennman]

Sounds great. It just dawned on me what you mean by "u-loops", got it. And I guess you're getting enough surface contact off those small tubes its working fine. Sounds like a good idea, since I was just planning on attaching to my big inlet tube. Thanks I was just about to place an aquastat order. This is perfect timing.

 

[rkusek]

My dissatisfaction with the thermostat is not the unit itself - it works well, controls the AC and humidifier, etc. It just does not have a programmable time interval between stages and you cannot set the temp offsets. It has worked well the past two years and I plan to leave it in the system, sending the heat call signal from the thermostat to a separate controller which will monitor the boiler and storage and supply heat appropriately. I will leave the control for the backup elec. elements connected to the Honeywell, so that if the controller fails there will still be heat available to the house.

One thing that makes my setup more challenging is that I want to run my forced-air fan on the lowest speed all the time, summer and winter. I have a variable speed air handler with an ECM fan, so the Honeywell takes care of the fan control during heating, cooling, humidification, and ventilation modes.

The additional inputs and outputs I want to control include a separate temp sensor for one large room in my house that has a northern exposure and does not benefit from solar gain in the winter. I would like to monitor the temp in this room so that the system can begin to compensate for dropping temps in the house before the sun goes down. I also want to be able to charge storage and draw from storage independent of the boiler, monitor storage temps, and have a remote display in the house showing status of the boiler and storage, which is in an outbuilding 140' away.

Medman,

Any progress on your new homebrew controller? I have issues like yours and cannot just simply just interrupt the call for gas heat like others have suggested. The plain Jane thermo I have now will force on the electric elements "to catch up" when you turn it up say 5 to 10 degrees above the set point. The heat pump also uses the backup elements to "defrost" itself and I wouldn't want to mess with that circuitry either. I also would not want the house to freeze up if the aquastat was showing the water was hot enough but for some reason (circ failure, etc.) the HX doesn't put heat.

I believe the only reliable "failsafe" approach is to have a 2nd thermostat set to the "desired" room temp and leave the existing heat pump tstat set a few degrees lower. Unfortunately, my wife can detect a 1-2 degree change in temp in a heartbeat. I will place a relay on the fan ckt to isolate the "heat pump activation" and "wood activation" defaulting to the heat pump request. The heat pump request will also stop my HX circulator. If something went haywire with the aquastat, my HX would just keep "stealing" heat from the heat pump effort which is not very warm to begin with. This should not be that big of deal since the wood Tstat would have another chance to do the job on the next call for heat anyway. If for some reason (cooler water, circ failure, extreme cold temps outside) the temps fall enough for the heat pump tstat to activate it would catch itself up. Damn!! As I am saying this I remember last winter during bitter cold, it would run for extended periods in heat pump mode avoiding activating the electric elements as long as it was able to maintain the set temp within a degree or two. May need to rethink this portion. My humidifier appears to have it's own separate controller (mounted on air handler) hooked directly to outside temp sensor. I hope it still operates whenever fan is on independent of the existing heat pump tstat. My tstat does not have any humditiy control on it.

 

[rkusek]

pybr, I was just fixin' to buy a Honeywell aquastat to keep my fan from running continuously. Your Johnson Controls Temp Control appears to do the same function with the addition of a digital display. As a non-HVAC guy I sure like this better than the Honeywell aquastat. I am correct in understanding that it does the same thing as an aquastat but programmable with a digital display. How does it power itself? Does the thermostat provide the power or 24 VAC from the furnace. This appears to do everything the Honeywell device does and far more. I'll have my HVAC guy install it, but want to purchase now if if functions as the aquastat is intended to. Is the sensor a thermocouple? So why didn't you just tie wrap to the supply line into the HX? Really like this option over what I had intended to purchase.

Here's a link to the datasheet/ instructions for the A419.

http://cgproducts.johnsoncontrols.com/MET_PDF/125188.PDF

There are some variations of the model that can run on 24 volts, or some (like mine) that run on 120VAC; the unit then has a separate set of switched contacts for switching the load (in my case, the blower motor).

I don't recall if the sensor is a thermocouple or thermistor- it comes with the unit.

I deliberately put the sensor on one of the "u-loops" of the water-air HX on the side of the HX opposite the hot water supply line, as I did not want the blower to start until the HX was actually warm and getting warmer (which, due to thermal mass, has a lag time compared to the commencement of the flow of hot water into the HX).

What temperature you set your A419 for is going to vary, and I set mine by trial and error so that the blower would turn on at the point at which the "u-loop" of the HX became distinctly warm to the touch.

Trevor,
I'm finally getting back to doing this. Think I'm going to use the A419 on the secondary manifold in my basement fed by the underground pex coming from the pole barn. My primary loop is in the pole barn with the boiler, storage (not complete), and barn heater. I may run this underground loop 24/7 at first but eventually only when something in house is calling for heat, or to maintain 140 degrees at the A419, or possibly periodically (to be determined) to prevent freezing.

What I really need to know is if it is necessary to have another A419 on the HX loops like you talked about. If I have 140 degree water to start with, the HX circ should only take a minute or so to heat up the coils. I'm thinking of going the 2 thermostat route anyway with an identical honeywell 5000 (non-programmable). Can anyone verify modern thermostats by default wait a predetermined about of time before they start the blower fan? Likewise when the burner or heat pump compressor shut off (when satisfied) the fan usually runs for a few more minutes.

 

[pybyr]

Trevor,
I'm finally getting back to doing this. Think I'm going to use the A419 on the secondary manifold in my basement fed by the underground pex coming from the pole barn. My primary loop is in the pole barn with the boiler, storage (not complete), and barn heater. I may run this underground loop 24/7 at first but eventually only when something in house is calling for heat, or to maintain 140 degrees at the A419, or possibly periodically (to be determined) to prevent freezing.

What I really need to know is if it is necessary to have another A419 on the HX loops like you talked about. If I have 140 degree water to start with, the HX circ should only take a minute or so to heat up the coils. I'm thinking of going the 2 thermostat route anyway with an identical honeywell 5000 (non-programmable). Can anyone verify modern thermostats by default wait a predetermined about of time before they start the blower fan? Likewise when the burner or heat pump compressor shut off (when satisfied) the fan usually runs for a few more minutes.

I assume that the 5000 is a household thermostat- I'm not sure if they have a time delay.

From my understanding of the systems I've worked on (and remember, I am not in the trade- just a persistent amateur), delays on fans seem to be a product of sensors placed in a location that may have a thermal-mass-induced "lag time" compared to the firing of the core system of the unit (result= fan turns on later, and also turns off later).

I used the 419 on the HX coils simply because I don't want to be blowing cold air around, even for a short while, when there is a call for heat, and also because I do want to extract all feasible remaining heat from the water-air HX even after the circulator has stopped runnng. I'm not saying that my way of doing it is the only way-- but I can verify that it seems to work quite well in practice. Also, in my case I am using my preexisting oil-fired warm air furnace blower to push the air through my water-air HX, and I do want to retain the operability of the oil furnace as a fall-back, so using the thermal sensor on the HX was a good way to achieve that (i.e. the furnace can kick on its blower itself, or the 419 can kick on the blower).

Hope that helps.

Trevor

 

[rkusek]

I see your point. I think I will start with one A419 in the manifold that will determine if I have 140 degree water available. If the 2nd tstat I get doesn't handle the fan delay portion I can always add another A419 like you suggest which would be more accurate than a fixed time frame anyway. Is everyone with a buried insulated pex setup only circulating when their is a call for heat or 24/7 keeping everything hot? I believe Heaterman suggested the latter would be best for me but just wondering what others are doing. My tstats will be identical so I hope by keeping the wood set 2 degrees higher than the heat pump I should be able to maintain temp using only wood heat.

 

[Tony H]

I use the two thermo setup like feelingasi and it works ok the only thing I need to add is a shutoff so the wood therm does not keep the fan running for the the entire time it is calling for heat and the boiler is not providing any heat.
I have the controller in the EKO running the circulation pump so it runs as needed based on the status of the boiler meaning it runs 24/7 if the boiler is putting out heat and when the boiler cools down the pump shuts off.

 

[rkusek]

I use the two thermo setup like feelingasi and it works ok the only thing I need to add is a shutoff so the wood therm does not keep the fan running for the the entire time it is calling for heat and the boiler is not providing any heat.
I have the controller in the EKO running the circulation pump so it runs as needed based on the status of the boiler meaning it runs 24/7 if the boiler is putting out heat and when the boiler cools down the pump shuts off.

Sounds like an aquastat or that Johnson Controls A419 with digital readout will work for you also. How close together do you set your thermostats? Does the wood always keep your gas or electric furnace from coming on? How far through the night will a load of wood last in 20 degree weather?

 

[soupy1957]

When it's all said and done, my observation in our home, which has a forced hot air system already installed, is that I lose temp on the air I'm moving around with the Manual Fan on.

I've wondered if insulation around ALL the plenums and not just "some" of them, might contribute to keeping the air hot that I'm transferring around the house. Don't know. Just pointing out that in my attempts to utilize the current plenum system to balance the heat in the house, I have learned that I still end up with about 65ºF down below grade in the Family Room, and about 75ºF on the upper floor (one floor up from the wood stove).

Good luck keeping the "heat" in your transfer of air, (without it cooling off)! I've thought it might be a good idea to have some sort of trigger mechanism for the furnace fan so that I don't have to be manually turning it off and on, but I've not found a solution to that yet, short of finding a thermostat on the market that has some sort of automated fan setting apart from my A/C and Furnace. I wonder if they make a 3 catagory sensor trigger like that????

-Soupy1957

 

[Medman]

I'm late coming back to this discussion - I've been busy wringing the last few days from sailboat season.
I am currently in the design stage for my controller; I have decided to go with a PLC solution that controls all functions of HVAC. I am leaving my current setup (snap-disc sensors and relays) in place as a fail-safe if the PLC is offline.

I currently circulate water (above 140*) all the time. When tx calls for heat the plenum hx circ energizes and heats the coil in the plenum. With the new controller water will only circulate in the underground loop when there is a call for heat from the DHW tank or the plenum hx. I will build in a sensor to delay fan start until the underground loop is hot. I don't know how much down time the underground loop will have, but I'm hoping to save a few btus by not circulating water if it is not needed. Freeze protection may be necessary forthis loop; if so I will build that into the program.

 

[rkusek]

Keep us updated on your progress. Even though this seems so simple at first glance it doesn't take much to complicate things. I think I too will set up my failsafe portion using relays and aquastats keeping the UG loop hot all the time at least for now. I would hope my insulated pex would be fine no matter how cold it got but who wants to take that chance. I know my weak spot would be where it comes up through the floor in the corner of the pole barn. Everywhere else it should be at frost line (42" here I think).

 

[rkusek]

When it's all said and done, my observation in our home, which has a forced hot air system already installed, is that I lose temp on the air I'm moving around with the Manual Fan on.

I've wondered if insulation around ALL the plenums and not just "some" of them, might contribute to keeping the air hot that I'm transferring around the house. Don't know. Just pointing out that in my attempts to utilize the current plenum system to balance the heat in the house, I have learned that I still end up with about 65ºF down below grade in the Family Room, and about 75ºF on the upper floor (one floor up from the wood stove).

Good luck keeping the "heat" in your transfer of air, (without it cooling off)! I've thought it might be a good idea to have some sort of trigger mechanism for the furnace fan so that I don't have to be manually turning it off and on, but I've not found a solution to that yet, short of finding a thermostat on the market that has some sort of automated fan setting apart from my A/C and Furnace. I wonder if they make a 3 catagory sensor trigger like that????

-Soupy1957

Sounds like your using your "fan" setting to circulate heat from your main level wood stove. I don't believe you will find a tstat that can do that. You should be able to hook a mechanical timer into your fan to accomplish what you want to do. I will post one if I can find it.

 

[Medman]

Keep us updated on your progress. Even though this seems so simple at first glance it doesn't take much to complicate things. I think I too will set up my failsafe portion using relays and aquastats keeping the UG loop hot all the time at least for now. I would hope my insulated pex would be fine no matter how cold it got but who wants to take that chance. I know my weak spot would be where it comes up through the floor in the corner of the pole barn. Everywhere else it should be at frost line (42" here I think).

That is exactly the spot I had trouble with, so I wrapped the pex here with heat trace and put the sensor outside of the insulation a bit so that it would not freeze again. The one time it did freeze was after we had been away a week during Feb. A few minutes with the hair dryer fixed the frozen spot, with no damage.