Replacing furnaces. How do I select the correct BTUs?

[dpickard]

Well, after spending $9,100.00 this week to rebuild the transmission/cooling units/radiator core, I had the rear furnace go out today. When it rains, it pours!

They are the original units..13 years old now, and I have spent a fruitless day at Camping World trying to get replacements.
(I figured while I am at it, I'll just go ahead and replace both.) No luck. Not a stock item.

They are Hydroflame Excalibur 8520-II's.

I did some online research and learned that Hydroflame offers 16k, 20k, 25K, 31k and 34k BTU replacement units. They all supposedly fit in the the same framing dimension, so this is my dilemma:

Should I go with the same amount of BTUs I currently have, or would it be ok to step it up a notch or two.. I was thinking I would like to go with at least a 31k unit if its practical and wouldnt require any changes to the current setup.

The price differential is only about $58 per unit.

Does anyone have any experience in this sort of thing who could advise me? I would be very grateful for your help.

Dianne

 

[Jeff]

Dianne:

If you get too large a furnace for the ducting in your coach the overheat switch will continuously cycle as the temperature rises in the combustion chamber. We have two Hydroflames IIIs in our coach, one 20k for the bathroom/bedroom  with three outlets and a 31k for the front with four ducts.

These two seldom both run very long in temps down to 15 degrees keeping our 39' Tradewinds warm.  If you increase the size are you planning on adding ducts? You may end up with furnaces cycling on and off all the time if they are too big. Was your coach warm enough with the original units?

 

[dpickard]

Jeff,

Yes, for the most part, it was warm enough. They ran a fairly long time when it was really cold, so thats why I thought getting a bigger unit would be prudent. Perhaps it was due to the age of the units. Maybe they werent in such good shape. I dont know. Overall I'd have to say yes, they were adequate.

 

[Gary RV_Wizard]

What Jeff says is correct, but I suspect you could increase the size somewhat without causing a problem. It depends, though, on how good the airflow is in your existing ducts. 

Since you say the old furnaces ran a fairly long time (did not short cycle), I would guess they were getting plenty of air circulation and therefore a modest increase in btus would probably be OK. A larger furnace will usually have a larger fan as well. Check the amperage draw spec for each furnace model - it indicates the fan size. Also make sure you can supply that many amps to the furnace, i.e. existing wire size and fuse rating.

 

[dpickard]

Thank you both for your responses. There sure is a lot to consider. I think I will leave well enough alone and go back with the same size I have.

 

[172ndLIB]

The general rule is 1000BTU for every foot of length of box.  So if you have a 27 foot you can use either a 25000 or a 31000.  In my rig I have a 35000 because its 33 feet long.

 

[Karl]

Diane,
It appears you have two 20,000btu units. You could probably go up one size (25,000) without too much concern, but I would question going up to a 31,000. Unless you add another outlet or two, the additional heat may cause problems with anything near the outlets. Some time ago I had the plastic suction hose from my carpet cleaning machine laying on the floor in front of the kitchen outlet when the furnace was running. Came back to find it partially melted and looking like a snake with severe bowel obstruction ;D 

 

[dpickard]

OMG Karl....thats a hoot!!

Thanks for the information.