Downdraft Furnace Heat Exchanger Install?

[Hosted]

I have a downdraft furnace in my house. It brings the return air in the top of the furnace and blows the hot air out the bottom through my slab. I am getting ready to install the heat exchanger and wondered if anyone on here has done one of these before. I'm a little worried about installing it below the fan because when the fan was off, the heat from the exchanger would go up into the electronics on the furnace. I could put the exchanger underneath the furnace in the slab, but I'm trying to get away with not tearing out my whole furnace to put the exchanger in. If anyone has any input I would appreciate it. I can take pictures of what I'm dealing with if that would help.


Hosted

 

[BrownianHeatingTech]

I have a downdraft furnace in my house. It brings the return air in the top of the furnace and blows the hot air out the bottom through my slab. I am getting ready to install the heat exchanger and wondered if anyone on here has done one of these before. I'm a little worried about installing it below the fan because when the fan was off, the heat from the exchanger would go up into the electronics on the furnace. I could put the exchanger underneath the furnace in the slab, but I'm trying to get away with not tearing out my whole furnace to put the exchanger in. If anyone has any input I would appreciate it. I can take pictures of what I'm dealing with if that would help.

As you note, the heat from the heat exchanger could cause a problem. So, you need to not have flow through the heat exchanger, except when needed, and have the fan controlled by the temperature there.

To accomplish the first, you can either have a thermostat control whether flow is getting there (ie, turn on/off the pump or valve that sends water from the boiler) or, in the case of a constant-flow system like many outdoor boilers use, you can install a three-way bypass valve at the heat exchanger, and have that controlled by the thermostat (basically, it shunts water directly from the "in" to the "out" of the heat exchanger, rather than letting it flow through).

To accomplish the second, the best bet is to install an aquastat (Honeywell, or similar) or electronic temperature controller (the Ranco ETC is nice, and used by several folks here) on the heat exchanger piping. The controller triggers the fan whenever the heat exchanger is above a certain temp (typically 120-140F). That means the fan doesn't blow cold air, prior to the heat exchanger warming up, and also protects the furnace by cooling the heat exchanger at the end of a cycle.

Joe

 

[Hosted]

Joe, could you give me an example of a three way valve I would need? I have found lots that are temperature controlled, but am having a hard time finding one electronically controlled, which I'm guessing this one would need to be. I may be completely off on my thinking though. Thanks in advance.

 

[BrownianHeatingTech]

Joe, could you give me an example of a three way valve I would need? I have found lots that are temperature controlled, but am having a hard time finding one electronically controlled, which I'm guessing this one would need to be. I may be completely off on my thinking though. Thanks in advance.

You're probably finding three-way mixing valves, instead of three-way zone valves. It can be confusing, I'm sure, with all the options out there.

Taco is a fairly common brand of zone valve, and their 560 series would fit your application. (broken link removed to http://www.taco-hvac.com/uploads/FileLibrary/100-3.pdf) shows the valves, and how they are piped, on the second page.

Personally, I would recommend Caleffi valves, or Honeywell, rather than the Taco valves, simply because the Taco valves draw a lot more electrical power. However, the ultimate best choice may be determined based upon what is readily-available in your area (so you don't end up with trouble because a part fails and you need to mail-order a replacement).

Joe