Tarm 30 with air handler (sizing question)

[twitch]

I recently received my Tarm 30 and am planning on using it with an air handler with heat exchanger attached to my existing hot air duct work. My question is on the sizing of the air handler.

The boiler output is 100,000 BTU/hr and my heat loss calculation using slantfin is about 37,000 BTU/hr. I'm not sure if I should match the BTU rating of the air handler to the boiler output or the heat loss. My existing hot air furnace is a Miller CFM series. Below is the link to the air handler the local supply shop has.

http://www.firstco.com/products/fwa_hw.asp

 

[Nofossil]

I recently received my Tarm 30 and am planning on using it with an air handler with heat exchanger attached to my existing hot air duct work. My question is on the sizing of the air handler.

The boiler output is 100,000 BTU/hr and my heat loss calculation using slantfin is about 37,000 BTU/hr. I'm not sure if I should match the BTU rating of the air handler to the boiler output or the heat loss. My existing hot air furnace is a Miller CFM series. Below is the link to the air handler the local supply shop has.

http://www.firstco.com/products/fwa_hw.asp

Size it at least close to the boiler output. That way you can raise the temperature of the house more quickly. Matching your peak heat load means that you won't have any reserve at all to raise the temperature of the house on a cold day.

Also, oversizing it would allow you to run the Tarm at a lower temperature, which is a little more efficient.

 

[Blevesque]

Also what is the size of your existing ductwork. Chances are it's pretty small, so you'll need to make sure the fan can handle the static. Let us know your duct size and we can let you know your static.

 

[twitch]

I have to main ducts running almost the entire length of the house, and they are 7" or 8" by 16". I'm going to run a flexible insulated round duct to the end of each of the main ducts. I'm not sure if two 8" round ducts will be big enough. Guess it depends on the output of the air handler, and may need to use 12".

Attached is a basic view of my existing ductwork. The feeder duct is from the existing hot air furnace. I'm going to be attaching the insulated duct to the garage end of the run on the left of the diagram.

 

[Blevesque]

You will have to go with the FWA60-HW. A 7" x 16" duct is only good for 500 cfm @ .75" Static. So you will need the bigger unit to overcome the static. That should get you close to your 100MBH, might have to play around with the GPM's. Also you will need to run 2 12" rounds from the unit to the existing ductwork. Hope that helps.

 

[pybyr]

if the box and blower of your Miller are still reasonably intact and reliable (and those were, if I recall, a pretty stout-ly built make) and if space permits, then leave your Miller in place, use the blower in it to move the air, and then call Ross Stephens at Nationwide coils to help you size a water to air heat exchanger to stuff on the outlet bonnet of your Miller in relation to your BTU needs, water flow, air flow and back pressure, and duct work.

that's what I am doing with my arrangement on top of my un-killable ThermoPride, with a 24x24 4 row water to air coil that cost me WAY less than an air handler, and lets me leave the oil furnace in place "in case".

 

[twitch]

That was my original plan, to use the existing furnace blower with a heat exchanger. The problem is that the output duct from the furnace is only 11 1/2" by 11 1/2 ". If I had an HX made that size, it would probably have to have a lot more rows to get to the correct BTU output, and I thought that would put too much restriction in air flow.

 

[BrownianHeatingTech]

That was my original plan, to use the existing furnace blower with a heat exchanger. The problem is that the output duct from the furnace is only 11 1/2" by 11 1/2 ". If I had an HX made that size, it would probably have to have a lot more rows to get to the correct BTU output, and I thought that would put too much restriction in air flow.

You can install a larger plenum under the floor, to fit a larger coil.

Joe

 

[pybyr]

That was my original plan, to use the existing furnace blower with a heat exchanger. The problem is that the output duct from the furnace is only 11 1/2" by 11 1/2 ". If I had an HX made that size, it would probably have to have a lot more rows to get to the correct BTU output, and I thought that would put too much restriction in air flow.

You can install a larger plenum under the floor, to fit a larger coil.

Joe

I'm sort of doing the same thing Joe says- a 24x24 4 row coil sitting on top of a 22x22 furnace bonnet, and then heading, via a big sheetmetal funnel/ elbow of sorts, into a 24x8 main trunk duct