Retrofitting an old wood furnace

[dgeesaman]

Hello, first post here.

My home was built in '82 with a Royal Oneida wood furnace (and heat pump) at least that old. I'm contemplating modernizing the setup but I'm not very familiar with HVAC systems. So far my search has turned up some questions:
- My home is a ranch home with all HVAC ducting under the slab. Since my furnace is indoors off the garage, the hot air plenum goes up a foot, turns left, and then turns down into the concrete. This would be a down-flow setup, correct? I've researched a few modern wood furnaces and the EPA ones don't allow downflow ducting. The BK MP-80 appears to support it when properly configured. Are there clean / efficient wood furnaces that support downflow ducting?

- What's so wonderful about a $5k wood furnace? I can see how a $1k furnace from Lowe's may not perform well or last, but it's hard to imagine it requires another $4k to make something durable.

- We're near the end of 2015, and I've read that non-EPA furnaces are no longer permitted to be sold in 2016. My house is not located where we have tight emissions requirements, so while I don't want a smoke dragon it wouldn't bother me to buy a decent furnace that didn't quite pass EPA. Thoughts?

- Can I trust my local HVAC shops to know anything useful about these systems? How can I tell if they're competent to get it set up well?

 

[laynes69]

The problem with a downflow configuration is power outages. You have a large steel box full of wood, and no gravity flow can occur. You end up with an extremely hot furnace. I do not think the bk handles it either, that's a series configuration. In a series configuration, only the central heater can push down, but the ductwork off the woodfurnace must remain up top to gravity feed in case of an outage. A heat dump would pretty much have to be installed to safely remove heat from the woodfurnace in that configuration. Even then, it will be almost impossible to find a manufacturer that wold approve of that install.

As far as price, there's a huge difference between the quality of woodfurnaces. Some simple with a baffle, while others contain special insulation, stainless high temp parts, involve heat exchangers, good blowers, electronics, etc. There are middle of the market furnaces like the Tundra, and higher end furnaces like the Kuuma and Caddy. Lower end models like the Englander and hotblast will heat, but consume more wood and require greater chimney maintanence.

 

[dgeesaman]

The problem with a downflow configuration is power outages. You have a large steel box full of wood, and no gravity flow can occur. You end up with an extremely hot furnace. I do not think the bk handles it either, that's a series configuration. In a series configuration, only the central heater can push down, but the ductwork off the woodfurnace must remain up top to gravity feed in case of an outage. A heat dump would pretty much have to be installed to safely remove heat from the woodfurnace in that configuration. Even then, it will be almost impossible to find a manufacturer that wold approve of that install.

So the "issue" with downflow is a CYA situation for the manufacturers?

If that's the risk, it seems a practical solution is to install a robust power backup system with automatic switchover. I haven't jumped toward that project because my house has an underground feed from the street, and the street is a major road carrying power for a huge number of customers. But a furnace could overheat in just an hour or two I suppose.

As far as furnace choices go, I'm probably going to do at least mid-range, but spending $5k on a firebox is not happening.

 

[brenndatomu]

I'd say the difference between a $1k furnace and a $5k furnace is...
1. Initial build quality and functional design
2. Customer service after the sale (and this can be a big one if things don't work perfectly on start up, and they often don't. Wood furnace installs are often not as simple as most think)
3. Long term longevity/reliability
No, I wouldn't count on your local HVAC guy having the first clue about wood furnaces...heck, even half the stove shops are full of crap, especially when it comes to furnaces. Generally there is much more expertise available here, unless you just happen to stumble upon a gem locally.
IIRC, larger non-EPA fireboxes, like wood furnaces, will be available for another year and a half(ish). The furnace MFGRs cut a deal with the EPA for another 2 years to do RnD, May 15 2017 is the deadline for them.
Downflow installs are not approved with any wood furnace to my knowledge. You can install backup power, but even that is not 100% reliable, and not cheap to install or maintain. If you are dead set on doing things this way, I wouldn't even consider it without a bullet proof way of dumping the heat if needed. If power fails right after you load the firebox, things can overheat in minutes...not hours. This is the best (only) heat dump that I know of (and own)...simple, reliable. http://www.yukon-eagle.com/FURNACEACCESSORIES/SAFETYHEATDUMP/tabid/200/Default.aspx

How big is your house and what is your heat load like? A good mid range EPA furnace would be Caddy or Max Caddy

 

[STIHLY DAN]

You can have one made up easily, Its just a meltable link. Like on fire dampers.

 

[brenndatomu]

You can have one made up easily, Its just a meltable link. Like on fire dampers.

You can buy the fusible links from Yukon too...I think they are $5-10 each. 'Course you may know where to get 'em for much less. I've never seen 'em for sale anywhere...

 

[maple1]

So the "issue" with downflow is a CYA situation for the manufacturers?

If that's the risk, it seems a practical solution is to install a robust power backup system with automatic switchover. I haven't jumped toward that project because my house has an underground feed from the street, and the street is a major road carrying power for a huge number of customers. But a furnace could overheat in just an hour or two I suppose.

As far as furnace choices go, I'm probably going to do at least mid-range, but spending $5k on a firebox is not happening.

It's not CYA - it's safety. Plain & simple.

A backup power is an option, but I consider it only part of a solution & would not rely on just that or consider it particularly practical considering how much they can cost. In the middle of the night when everyone is asleep, or during the day when nobody is home, is not the time to find out the backup power solution isn't as reliable as thought when you have a fire in your firebox. I would absolutely use a heatdump also, if the ductwork won't allow for convection flow.

 

[laynes69]

Without a means of heat removal via gravity, or with a blower, it takes no time to damage a furnace. Melted electronics, burnt paint, etc. And if these don't follow clearances to combustibles, then fire can occur. Our old setup, just a half hour or so into an outage, I could sizzle water off the ductwork. The furnace however was much hotter. If a furnace is okay to run gravity, even then you open any access doors in the jacket and remove the filters to help assist flow if ducting allows it. With a downflow flow configuration, none of that can happen.

 

[brenndatomu]

If a furnace is okay to run gravity, even then you open any access doors in the jacket and remove the filters to help assist flow if ducting allows i

That's a good point on pulling the the filter. Gravity heat doesn't work very good through a filter. The 3 different wood furnaces that I am personally familiar with all had gaps that the filter didn't cover. Manufacturer said it was to allow gravity flow when the blower isn't running, and also (most importantly) during power failure

 

[dgeesaman]

Thanks for the information everyone.

I did a little research and found that at least the Blaze King MP80 is permitted to run in down-flow configuration, and the price is within reason. It's not fully EPA but it does have secondary air burn so I expect it will be miles better than my 40 year old unit. It does mandate the use of an exhaust relief kit, which my current furnace does not have. Even if I don't find $$ to replace the furnace, I'll look into getting the exhaust vent installed. For now though I think I'm not teetering on disaster because my system's heat control is poor and that's got me building and maintaining a small fire when I want heat - I'm a long way from fully loading the furnace and getting 8 hours of slow burn.

My air filter is on the inlet air side of the system. I do know my system uses the blower that's on the heat pump. We set the blower to "on" whenever the wood furnace has wood in it.

 

[dgeesaman]

In case anyone else is reading this, and would like current information:
I made a couple of phone calls today. The Kuuma is ok to use in downflow applications. He recommended the thermal dump vent in the case of power outage and overheating, and to consider the size/flow of the blower fan already in my system, and maybe choose the larger model to reduce the losses when my air pump was providing the heat.

Blaze King is out of the US market wood *furnace* business as of last week. They weren't selling many MP80s and VC120s in recent years and apparently their sales of the Apex is not enough to justify qualifying it to be sold in the USA. EPA regulations being part of that. So I didn't even get to discussing whether it could work in my application.

 

[BKVP]

I can confirm that we no longer sell any wood furnaces in the USA.

 

[1970roadrunner]

Thanks for the information everyone.

I did a little research and found that at least the Blaze King MP80 is permitted to run in down-flow configuration, and the price is within reason. It's not fully EPA but it does have secondary air burn so I expect it will be miles better than my 40 year old unit. It does mandate the use of an exhaust relief kit, which my current furnace does not have. Even if I don't find $$ to replace the furnace, I'll look into getting the exhaust vent installed. For now though I think I'm not teetering on disaster because my system's heat control is poor and that's got me building and maintaining a small fire when I want heat - I'm a long way from fully loading the furnace and getting 8 hours of slow burn.

My air filter is on the inlet air side of the system. I do know my system uses the blower that's on the heat pump. We set the blower to "on" whenever the wood furnace has wood in it.

I'm using an MP80 right now. They're well built units, but they don't have secondary burn technology.

 

[dgeesaman]

I'm using an MP80 right now. They're well built units, but they don't have secondary burn technology.

(broken link removed to http://www.blazeking.com/EN/furnace-mp80.html)
"Our 4 port secondary air burner system makes this a very efficient medium sized wood furnace"

Perhaps they updated it?

 

[1970roadrunner]

(broken link removed to http://www.blazeking.com/EN/furnace-mp80.html)
"Our 4 port secondary air burner system makes this a very efficient medium sized wood furnace"

Perhaps they updated it?

I don't think they've updated it. I compared your link with my unit, and it does indeed have the small holes on the interior of the door that they refer to as "secondary burn" (mine has a manufacture date of 2002). So I stand corrected!

When you mentioned "secondary burn" I envisioned the secondary burn tubes that you see in a lot of the newer units, that run the entire length of the firebox. I like my MP80, but it's an old school design.

Good luck with your search!