Sizing heat exchanger and circ. pump

[dspoon19]

Just got a new outdoor wood burning appliance and I'm getting ready to purchase a heat exchanger for my forced air furnace and was looking for some advice on sizing components.

My simple thought just says coil btu's should match furnace max btu's and pump should be slightly over sized then throttled back.

Can anyone help out and explain out calculations or give a general rule of thumb on sizing?

 

[MarkW]

I went in excess of my previous furnace rating by roughly 40% as the coil rating is only somewhat reliable at 180+. I wanted to be able to pull from storage below that temp.
I'm running an alpha pump so the flow is handled by the pump. It pumps through 2 water to air HE, ATM.

 

[dspoon19]

That's a good idea. I am doing a forced air install and have a 20x20 plenum I can put the heat exchanger in. I should probably just stuff the plenum with the highest rated BTU heat exchanger I can fit in there. I was not planning using storage and my capacity is only around 100 gallons. The more I have thought it over storage would be a good idea but I don't really have a good place to put it at this time.

 

[maple1]

Second the Alpha.

 

[dspoon19]

Checked those alpha pumps out. They look like a really good unit. Thanks for the heads up. I found tho the biggest heat exchanger I can fit is an 18x18 without doing some major mods to my existing plenum. It's only 120k btu. I will probably run a radiant loop on my kitchen/dining room floor.

 

[dspoon19]

I forgot to mention that I saw the inlet pressure of the alpha pumps to be around 4psi. Does the head pressure in the boiler provide this necessary inlet pressure?

 

[maple1]

Yes.

Well, maybe not. You've got an unpressurized OWB? You'd have to see what your pressure is where the pump would be - and are you sending boiler water thru the HX? Or do you have another HX (plate?) between your boiler & your heating system?

BTW, the 4psi also applies to the non-Alpha 15-58 3 speed.

 

[MarkW]

I ran mine in an un pressurized system all last winter. I do have 6 feet of water column above the inlet of the pump though so you would need to know where you're placing your pump. If it were high on the boiler outlet I would suspect you might get cavitation.

 

[maple1]

There's around 0.4psi/foot of column. So technically with an open system you'd need 10 feet of drop from the top of the system to where your pump is at the bottom to get 4psi.

 

[dspoon19]

Yes.

Well, maybe not. You've got an unpressurized OWB? You'd have to see what your pressure is where the pump would be - and are you sending boiler water thru the HX? Or do you have another HX (plate?) between your boiler & your heating system?

BTW, the 4psi also applies to the non-Alpha 15-58 3 speed.

Yes its unpressurized. I'd guess that I have 6 feet of head between the OWB and the proposed pump location. I can mount the pump a lot lower and try and get more head pressure on the inlet side. Yes the boiler water would be going directly through the HX that is in the plenum.

 

[dspoon19]

Also while I am on the subject of circulating water, anyone have any good recommendations as far as water treatment chemicals for an unpressurized boiler?

 

[MarkW]

Sounds very much like what I did last year. 16x18 exchanger in the plenum for main body of house and a 12x12 for addition. It kept the house and basement quite warm. I was able to maintain temps down to 130 on the storage.

 

[MarkW]

I don't recall what brand water treatment I purchased

 

[MarkW]

They're not cheap but its not the place you want to skimp

 

[dspoon19]

mark I see your local to me....how many cords you go through this past winter? I got 7 cords of 2 year seasoned ash and a cord of 2 year white oak that im hoping will get me through next year.

 

[jebatty]

I heat with 4 cords of pine, maybe my house is smaller, but on the other hand, winter temps get into the -30'sF with lots of days and nights below 0F. 4 cords of pine is about equivalent in btu content to 2.5 cords of white oak/ash.