heat exchanger placement in the furnace

[bob32racing]

My local plumber installed a water to air heat exchanger in my cold air return duct. Im running water 24/7 through my dhw heater and heat exchanger. Now my problem is when the exchanger is hot my furnace will not shut off due to the temp limit sensor. The furnace continues to run to try to cool itself down, but it doesnt cool down since the water is constantly running through it. Is there anyway around this? If i put a zone valve in on a stat and set it at 70 degrees, wont the furnace still run a while after it reaches 70 degrees, until it cools itself down? Possibly making it 72 degrees or more by the time it cools off. Or am i wrong?

 

[NSMaple1]

Not a furnace guy here, but I am thinking the supply plenum would have been the proper place for it.

We don't know anything much about your piping, but if your circuit is all in series, putting a zone valve in will stop everything flowing. So you should also stop & start your pump while closing & opening the zone valve. If in series. Which will also stop heat from going other places you need it.

 

[jfriesner]

Not trying to be a donkey here but its called a "cold air return" for a reason. My furnace says right in the manual not to exceed cold air return temperatures of 90F. That hot air passes over lots of sensitive parts that can get cooked by 130+ degree air coming off your exchanger. The exchanger should have been placed in your supply plenum and preferably last. What i mean by that is if you also have an electric plenum heater, AC coil, etc...in your supply plenum as well, then you boiler exchanger should be placed after those things as to prevent it from messing with them as well. Another plus to that is if your boiler fails, for whatever reason, your secondary heat supply can back feed heat through the exchanger to prevent freeze up. Where its at right now if it fails and your secondary heat kicks on its only going to be able to back feed the boiler heat at the temp of your returning cold air, which wont be much heat.

Hope this helps. I would definitely fix the underlying issue of where the exchanger is placed rather then trying to find a temporary fix for a permanent problem.

 

[panolo]

Have never heard of one in the cold air return. Got to believe that takes some efficiency out of it as well. Mine is right above my AC coil in the plenum.

 

[Jakers]

Have never heard of one in the cold air return. Got to believe that takes some efficiency out of it as well. Mine is right above my AC coil in the plenum.

this is the proper placement for the HX. To the OP, I'd call another, better professional to fix the other guys F-up... Or maybe just do it yourself so you know where to point the finger if something doesnt work properly. its a simple install in most duct work

 

[bob32racing]

My original plan was to have the hx on the supply side of the furnace. Then i would constantly run water through it 24/7 during the winter months. Thats how my other house is set up, and has worked for 15 years. However the plumber that installed the hx on my new house placed it in the return vent just above my furnace. So i cannot run water constantly because the furnace sucks the hot air across the switch and will not shut itself off, since its trying to cool itself. So im stuck, plumber says to put a zone valve before the hx only to allow water to flow when the heat is needed. Well in theory that should work. However say the valve closes when room temp has reached 70. Now how long is it going to take to suck the heat out of the hx? Will it take a couple minutes and still raise the temp of tthe house while the furnace is trying to cool itself?

 

[jfriesner]

My original plan was to have the hx on the supply side of the furnace. Then i would constantly run water through it 24/7 during the winter months. Thats how my other house is set up, and has worked for 15 years. However the plumber that installed the hx on my new house placed it in the return vent just above my furnace. So i cannot run water constantly because the furnace sucks the hot air across the switch and will not shut itself off, since its trying to cool itself. So im stuck, plumber says to put a zone valve before the hx only to allow water to flow when the heat is needed. Well in theory that should work. However say the valve closes when room temp has reached 70. Now how long is it going to take to suck the heat out of the hx? Will it take a couple minutes and still raise the temp of tthe house while the furnace is trying to cool itself?

I would not try to make what you have work by adding more things to your system. Move the heat exchanger to where its suppose to be.

 

[Jakers]

I would not try to make what you have work by adding more things to your system. Move the heat exchanger to where its suppose to be.

yes, this

 

[Jakers]

My original plan was to have the hx on the supply side of the furnace. Then i would constantly run water through it 24/7 during the winter months. Thats how my other house is set up, and has worked for 15 years. However the plumber that installed the hx on my new house placed it in the return vent just above my furnace. So i cannot run water constantly because the furnace sucks the hot air across the switch and will not shut itself off, since its trying to cool itself. So im stuck, plumber says to put a zone valve before the hx only to allow water to flow when the heat is needed. Well in theory that should work. However say the valve closes when room temp has reached 70. Now how long is it going to take to suck the heat out of the hx? Will it take a couple minutes and still raise the temp of tthe house while the furnace is trying to cool itself?

this plan will work but simpler is always better. moving the HX to the proper spot will be more efficient in the long run and less complicated. the HX in your configuration could easily leave you waking up cold if the valve failed which they do (from reading many failure stories)... if you have to cool the HX down and warm it up every time the metal will also develop leaks sooner due to all the expanding and contracting. then theres also the side of the delay to heat output which wouldn't take very long and the delay to cool off. cool off could be a couple minutes and could maybe raise the house temp a degree or so. with the HX moved the water could flow freely and would keep things hot and ready for the next fan cycle. the ducts would always have some heat in them from natural convection so start up time would be instant for heat and stop when the fan shuts down with no cool down time. Win Win Win... Cheaper, Simpler, More efficient

 

[Mustang71]

Typically water coils go on the supply just like air conditioner coils. But depending on the system for air condition we put them on the return.

If you had an air handler which is basically just the blower part of the furnace then you could put your water coil in the return. But with a regular furnace there's a hi limit switch that kicks the blower on and shuts the gas supply off if the furnace over heats. You can bypass that limit switch but I wouldn't recommend it because if you forget to put it back together and run your furnace on the gas supply there's a risk of fire.

You currently are not filtering the air going through that water coil and that can cause it to plug up.

 

[Jakers]

Typically water coils go on the supply just like air conditioner coils. But depending on the system for air condition we put them on the return.

If you had an air handler which is basically just the blower part of the furnace then you could put your water coil in the return. But with a regular furnace there's a hi limit switch that kicks the blower on and shuts the gas supply off if the furnace over heats. You can bypass that limit switch but I wouldn't recommend it because if you forget to put it back together and run your furnace on the gas supply there's a risk of fire.

You currently are not filtering the air going through that water coil and that can cause it to plug up.

another excellent point

 

[NSMaple1]

My original plan was to have the hx on the supply side of the furnace. Then i would constantly run water through it 24/7 during the winter months. Thats how my other house is set up, and has worked for 15 years. However the plumber that installed the hx on my new house placed it in the return vent just above my furnace. So i cannot run water constantly because the furnace sucks the hot air across the switch and will not shut itself off, since its trying to cool itself. So im stuck, plumber says to put a zone valve before the hx only to allow water to flow when the heat is needed. Well in theory that should work. However say the valve closes when room temp has reached 70. Now how long is it going to take to suck the heat out of the hx? Will it take a couple minutes and still raise the temp of tthe house while the furnace is trying to cool itself?

Why did he put it there? Did you tell him to? If not it's his screw up to fix. Either way get the hx where it's supposed to be. You could develop more problems leaving it there and trying to jimmie up a work around.