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Question DetailsAsked on 10/27/2017

can a home in Michigan have 2 furnaces? or will 2nd furnace be considered secondary heating?

my home has an addition that sits on a cement slab. approx. 1000 sf with 18 ft vaulted ceilings and has no ductwork from current furnace. Is it feasible to add a 2nd furnace for this space or will it be considered secondary heating?

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Secondary heating is not an officially defined term that I have seen anywhere - is commonly inferred to include added in-floor loop heating (especially if not connected to the primary heating source), wood stoves, gas fireplaces, space heaters, electric blankets, etc.


Unless this was built as a 3-season room or summer room (in which case if connected to the main house likely has serious moisture condensation issues in the winter), I wonder why it never had heating at least (and maybe A/C depending on what part of the state you live in) installed when it was built ? In most areas this would not have been to code unless this room were isolated from the conditioned space.


Assuming you mean the 1000SF is the addition size (not the entire house) adding a second heating unit to heat it would be common for that size (especially with vaulted ceilings, which tend to be - I hate to say it - heat wasters) and would still be a Primary heating unit, at least as I see it. Though why whether it is primary or secondary system makes a difference I am not sure, unless you have a lease or condo association or such which prohibits secondary heating units (presumably for fire risk reduction purposes).


Anyway, you should have a well-rated Heating and A/C contractor or an Architect run an ACCA Manual J and Manual S calculation on your house (which determines the total heating/cooling load and the sizing of unit(s) to meet that demand - that would tell you if the existing unit could also heat the addition if ducting were run to it. That would tell you whether you need to either replace your existing unit (if older or past its reliable life) with a larger one, or add a second unit for the addition - which would have the advantage that if one unit fails (other than due to loss of power or fuel) you still have a hesting source in the house - though over the long run it is more expensive to use two units.


Then he would have to look at how ducting (Manual D calculation) could be sized and run from the present unit (or a replacement for it) to the addition - or if a new unit in a different location could economically tie into both the old system and new addition ducting effectively, and what it would cost. As you probably suspect, slab-on-grade construction is harder to retrofit ducting to.


In fact, in your area, I am surprised that you have a slab on grade - that is pretty unusual in northern states - is it possible (if you bought the house with this addition already built) that it has underslab / inslab heating which you do not know about ? Or was it a DIY house and the owner cut some corners - like lack of frost-protected foundation and heating and who knows what else ?


You also have other options for the new area


- electric heating - though usually a LOT more expensive to run unless used only when the area is in actual use - like for sunrooms or Florida rooms or dens or such which are normally kept much cooler when not in use,


- a dedicated hydronic (hot water) baseboard heating system,


- wood stove or gas fireplace or similar point-source radiant heating system, though for that size area and high ceilings, would result in a lot of lost heat high up unless you have recirculation fans in the vaulted ceilings


- though I don't like them, under-flooring heating loops (electric or fluid-filled) on top of the slab


- even a high-mount forced hot air unit heater (though normally limited to only one room unless open floorplan) - which is common for high-ceiling rooms because properly sites it can provide heat and also circulation of the air trapped high up in the ceiling area.


- even (though I like these even less) in-wall heaters - electric or gas. (My recommendation - if you go that route - do NOT use unvented units of any sort even if your local building officials have not recognized the risk and banned them - use only sealed-firebox direct-vent fuel-fired heaters which get their combustion air from outside (so they are not using heated air for combustion) and direct-vent the exhaust outside.


If the existing furnace has enough heating capacity (unlikely with that large an addition anyway), but duct runs to the addition are difficult to arrange, sometimes a scavenger or "robin hood" type through-wall fan system is installed (though rarely for that large an area) to steal warm air from an adjacent room with excess duct capacity and push it into the "cold" room, using natural air return through doorways back to the main "conditioned space". This sometimes works, especially in warmer climates than yours, especially if airflow to the room the heat is being scavanged from is maximized.


Your decision will likely hinge on personal preferences on heating sources, up-front capital cost, what usable space the new furnace would have to take up (unless you tack on an insulated utility closet to the outside of the house), and installed/fuel costs versus costs of running ducting and possibly upsizing your current unit. And bear in mind any solution should look at not only heating (and A/C if needed), but also heat distribution in those high-ceiling areas, because just putting the heat into the room(s) without providing circulation to at least partially recover the warm air rising to the ceilings will be throwing a fair amount of $ out the window. The circulation issue also has to consider excess moisture removal - both at the ceilings and at windows.

Answered 1 year ago by LCD




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