Help with strap on aquastat

Redbeard

Hello, I am attempting to wire a strap on aquastat to my performance 96 gas furnace, but have no idea what I am doing. Any help would be greatly appreciated. 

neilc

what are you trying to control with the additional stat?
or, if it's the burner, what is or isn't working now?

Redbeard

I don't have anything hooked up currently. I want to attach the aquastat to the pex pipe connected to Exchange. I want the furnace fan to only come on when the wood boiler is on and the propane when I do not have the boiler going.  

neilc

so if the wood fire dies down, the propane boiler kicks in ?
or are there more parts involved?
you mention furnace fan(?)
is there a master sketch you have that @EdTheHeaterMan or someone else can mark up ?

Redbeard

Yea. So if the outdoor boiler dies down the propane will kick on. 

neilc

don't hold me to this,
and let's get others' opinions , , ,
but I think this can be as easy as making the 120v line to the propane boiler,
the aquastat will pass 120 to the boiler when the woodfire gets below ???*,

but, what about wood fire circulators, and isolation valves?
you'll need to break them also, and shut isolation valves,
right?
gonna need more relays, and,
it's over my head

do you have that sketch showing each boiler, circs, and valving?

Redbeard

Ok. From what I have read I should only have to connect the aquastat to the fan control I guess. Maybe. So when the furnace needs to kick on the aquastat will make only the fan kick on but if the water isn't hot coming from the boiler it will allow it to bypass and kick the propane on

neilc

turning on the propane is easy enough, and it can run its circ(s),
but you gotta stop the circ to the woodie, and valve off,
and I don't see that in my mind's eye,
s k e t c h

Jamie Hall

I think there is more to this than we have here.

What is this? Are we talking about a hot air furnace which has, somewhere in its ducting, a heat exchanger off a wood burning boiler? Or is this a hot water system with a wood burning boiler and a propane boiler? And what fan exactly? Air circulation? Burner?

Redbeard

Hot air gas furnace 

Redbeard

Im probably explaining this terribly. I have a performance 96 gas furnace. My outdoor wood boiler is new. I have the heat exchanger placed above the furnace and the pex from the outdoor boiler connecter. Now I need to put the aquastat on the pex and connect it so when I have the boiler running only the fan will turn on in the furnace
@Jamie Hall

neilc

I did not see that in my mind eye,

how will you control excess heat from wood boiler,
or will you just dump heat into the house?

instead of breaking furnace 120v, you break the 24v w call to the heat,
and relay swap between fan on, and fan auto(?)

better question,
why?

Redbeard

Basically so if I forget to add wood to the boiler or run out of wood the furnace will switch to the propane to keep the house at the specified temp. 

Redbeard

Right now if I fire up the outdoor boiler I'll have to set the thermostat to fan only and if the wood runs out I'll have to change the thermostat to heat myself. I just want to set the thermostat to specific temp and have it switch between the fan (when the aquastat determines the pex line is hot when the boiler is running) or the propane heat 

STEVEusaPA

Redbeard said:

I don't have anything hooked up currently. I want to attach the aquastat to the pex pipe connected to Exchange. I want the furnace fan to only come on when the wood boiler is on and the propane when I do not have the boiler going.  

You can’t attach a strap-on to pex.

Redbeard

@STEVEusaPA I actually just read the a min ago. It will have to be copper pipe so it can get the correct temperature is what I read. 

metrosilo

Wouldn’t it make sense to add a staging controller and have that as my master temp controller. If one stage (wood boiler) can’t keep up or maintain temp, second stage (propane gas furnace) starts up. 

Redbeard

That's basically what I thought the aquastat would do from what I have been reading

Redbeard

@metrosilo from the reading I have been doing I though that basically the thermostat would send the signal down if the house was below the temperature. The signal would go to the aquastat and if the aquastat says the boiler pipes are hot then the furnace would just kick on the fan. If the boiler pipes are not hot the signal would then go to the furnace and tell it to kick on the propane and fan.

Jamie Hall

It's not going to be quite simple as I think you hope it will be. That blower motor which I take it is the one you want to control is an ECM motor, and it is controlled by the furnace control computer. There is no handy dandy way to get at its control computer to just turn it on and off, so unless you are willing and able to reverse engineer that control system and figure out where to tap into it,,, which I assuredly do NOT recommend! You will need to come up with a plan B,

Which, I would suggest, is to leave the furnace control system to fan on rather than auto.

Now you have another non-trivial problem which has been hinted at above. How do you propose to control that wood boiler, and what provision have you made for when it gets carried away and over fires -- which it will. For safety, you need to have a fail safe dump zone which can absorb the entire output of a full load of wood with the draught open.

Redbeard

I guess something like this that I just found but for my specific furnace.
ratio

ratio Member Posts: 3,306

October 2020

Assuming that the air handler is dumb (no fancy fan control board on it), move the G wire from the thermostat to the G terminal (fan call) on the air handler. Move the W wire from the thermostat to the W terminal (heat call) on the air handler. Move the R wire from the aquastat to the W terminal on the air handler. Move the close-on-temp-drop terminal (IDK which one it is for sure) on the aquastat to the G terminal on the air handler. Set the thermostat to 'gas/oil' so that it does not energize the fan during a heat call.

During heating season, with no G call from the thermostat, the aquastat will turn on the fan when the pipes heat up enough (& give you post-purge for free!). During cooling season, the thermostat will control the fan directly.

Redbeard

I guess this video says what I want it to do but doesn't explain the wiring

https://youtu.be/ZKBG9zmWEAY

Jamie Hall

There is a remote chance that the wiring changes you quote above may work, @Redbeard . Problem is, of course, your furnace and air handler is not dumb, and there's a very good chance that they won't work, and some chance that they will blow the furnace control board.

Nothing like YouTube to solve the world's problems.

jad3675

I don't think this is any different than having a thermostat set up with 'auxillary' heat. I have an ecobee setup at the rental house - heat pump is set as primary; if it can't keep up the thermostat switches to gas. There's a cut over set to 35F - anything below that outside and it defaults to gas. 

John

EdTheHeaterMan

The problem I find with the Furnaces that are equipped with ECM blower motors is that when you power the blower only from the thermostat "FAN ON" switch, you only t the gan to operate at about 10% to 20%. the concept is if there is no call for heat or air conditioning, the only reason to circulate the fan is to filter the air. Air filters work best at slower speeds. So using the aquastat ro power G on the furnace will not get you the needed blower CFM to move the wood stove heated air.

In order to get the fan to move air at 80% to 100% speed is to trick the furnace into air conditioning mode but make sure the AC compressor does not operate. Here is one idea that may fool the control.

Make the CPU on the circuit board think there is a call for cooling by connecting Y, G, and R just like a thermostat would do. This will cause the fan to operate at the cooling speed. At the same time disable the condensing unit contactor with a set of normal closed contacts that open on a hot wood stove high temperature condition.

Sequence of operation would then be
1. Thermostat operates the furnace normally (@ say 65°) when ever there is no heat from the wood stove. by way of the normal thermostat wires shown in green on the diagram.
2. When the wood stove water temperature was above 150° the strap on aquastat would make on temperature rise.
3. Then a triple pole relay could be activated to connect R, Y, and G to bring on the AC system to make the fan operate at AC speed (higher CFM)
4. At the same time, the Common wire from the triple pole relay would use the NC contact to open the common wire to the AC condensing unit. (I selected Common for this in case of a 2 stage condensing unit)
5. As long as the wood stove was making heat, the blower would operate to circulate air thru the duct coil and the heat will enter the home. (The home could get to higher temperatures as a result... getting a balance of when to add wood burning to achieve the desired room temperature will be a skill that you will need to practice)
6. In the event of the wood stove fire going out, the strap on aquastat, would switch to the cold position, the relay would de-energize, then the thermostat for the Gas Furnace would take over and maintain the lower thermostat setting of 65°

Jamie Hall

That might work, @EdTheHeaterMan . May I make one suggestion to whomever does the fiddling, though? Document everything. Not only exactly what was done, but exactly why and what the objective was. That way, five years down the road with a new tech., you would have fighting chance of figuring out what's going on.

Redbeard

Thank you everyone for your input. I don't trust youtube much which is why I came here to find a solution. Also I apologize for the late reply my 15 month old stole my phone and hid it. 

EdTheHeaterMan

Redbeard said:

Thank you everyone for your input. I don't trust youtube much which is why I came here to find a solution. Also I apologize for the late reply my 15 month old stole my phone and hid it. 

As it should be! God bless the little one!

Regarding the last post with the wiring diagram. This is not my first choice in solving your design with a wood stove and furnace. The standard method that would be employed by a DIY or even a wood stove manufacturer does not take into consideration that you have an ECM blower motor on a 2 stage furnace. Your setup presents a challenge that is not common, So I don't think you will see anything like it on YouTube.

But this is what I like to do... figure out how to get things done, that are not in the original design or intention of the engineer at the furnace factory.

After looking at the diagram, if you have any questions. feel free to chat me up here or you can Private message me by clicking on my name.