For new northern MI cottage near Petoskey should I get 80% efficient furnace

[MidtoupperMichigan]

I have a 662 sq ft 2 bed 1 bath cottage we moved to a new 5 block tall foundation and I am going to replace the existing propane unit standing in center of cottage with a natural gas forced air furnace. I plan to locate the new furnace in the crawlspace. I was told that an 80% furnace since its not high efficiency will not have to worry about condensate. I do not have a floor drain in basement but could get a condensate pump but that is one more thing to add to the high efficiency puzzle. I plan to winterize the cottage for some of the winter weeks and or months in years to come. So I like the high efficiency option but the 80% efficiency furnace may not be used all winter if we winterized the vacation rental cottage and I dont have to worry about damaging a high efficiency furnace if i want to winterize the cottage? So should I get a more expensive high efficiency furnace or just get a more basic 80% ? I will have an ac/unit in conjunction with the furnace. The existing furnace standing in center of living room that I am replacing has a metal chimney liner but I am guessing it is not 3 lined chimney liner as probably code is now I am guessing so i will have that cost too. Also, should I consider a 2 stage blower and or can i change the blower to a 2 stage blower if it does not have one? I am looking at a 1 year used Goodman 4500 btu furnace but am open to opinions and tips.

 

[Dana]

Before deciding the equipment it's best to run the actual heating/cooling load numbers. I'm guessing that unless it's super-insulated & ultra-tight with almost no window area to speak of 4500 BTU/hr wouldn't cover even half the heat load of the place. Does Goodman even make them that small? A 45,000 BTU/hr furnace would be crazy-oversized for the likely heat loads of that place, and "heating" the place would be the opposite of "comfort"- it would cycle on/off at less than a 50% duty cycle even when it's in negative double-digits outside, overshooting the thermostat setpoint with a hot flash followed by the extended chill. For comfort you don't want the output to be more than 1.4x the heat load at the 99% outside design temperature (which in your case is roughly 0F- only 1% of the hours in a year are colder than that) so that when it's actually cold out the on-cycles are twice as long as the off cycles, and the room temps don't overshoot.

For a quickie DIY Manual-J-ish estimate of the loads, try LoadCalc, & or CoolCalc. With either, make the most aggressive assumptions possible for air-tightness & R-values or it could end up ridiculously oversized.

I suspect with an insulated foundation and reasonably air tight insulated walls and ceiling you'd be within range of a 15,000 BTU/hr Dettson Chinook. Most other ducted gas/propane furnaces would be too big. If you're interested in air conditioning a 1.5 - 2 ton cold climate mini-split ( or a pair of 3/4 - 1 tonners) could also heat the place at a lower operating cost than a condensing propane furnace (but a bigger up-front cost for the equipment.) With a heat pump solution you get rid of the flues and flue penetrations, and don't have to worry about combustion air, etc. But if it's going to be used in the dead of winter it's important to get a cold-climate specific unit, something that has a pan heater in the outdoor unit for automatically managing the potential build up of damaging defrost ice in the bottom of the unit.

eg: At 70F indoors a single ductless 3/4 ton Fujitsu -9RLS3H can deliver 15,000 BTU/hr @ +5F outdoors, 11,000 BTU/hr @ -15F, and runs ~$3-3.5K (full turnkey, professionally installed) in my neighborhood. There are ducted options, but they are fewer and more expensive, so the floor plan matters.

The first order of business is to get a realistic load number for the "after insulation upgrades" version of the house. Only then is it possible to come up with the most reasonable & right-sized options.

 

[Sponsor]

 

[MidtoupperMichigan]

Before deciding the equipment it's best to run the actual heating/cooling load numbers. I'm guessing that unless it's super-insulated & ultra-tight with almost no window area to speak of 4500 BTU/hr wouldn't cover even half the heat load of the place. Does Goodman even make them that small? A 45,000 BTU/hr furnace would be crazy-oversized for the likely heat loads of that place, and "heating" the place would be the opposite of "comfort"- it would cycle on/off at less than a 50% duty cycle even when it's in negative double-digits outside, overshooting the thermostat setpoint with a hot flash followed by the extended chill. For comfort you don't want the output to be more than 1.4x the heat load at the 99% outside design temperature (which in your case is roughly 0F- only 1% of the hours in a year are colder than that) so that when it's actually cold out the on-cycles are twice as long as the off cycles, and the room temps don't overshoot.

For a quickie DIY Manual-J-ish estimate of the loads, try LoadCalc, & or CoolCalc. With either, make the most aggressive assumptions possible for air-tightness & R-values or it could end up ridiculously oversized.

I suspect with an insulated foundation and reasonably air tight insulated walls and ceiling you'd be within range of a 15,000 BTU/hr Dettson Chinook. Most other ducted gas/propane furnaces would be too big. If you're interested in air conditioning a 1.5 - 2 ton cold climate mini-split ( or a pair of 3/4 - 1 tonners) could also heat the place at a lower operating cost than a condensing propane furnace (but a bigger up-front cost for the equipment.) With a heat pump solution you get rid of the flues and flue penetrations, and don't have to worry about combustion air, etc. But if it's going to be used in the dead of winter it's important to get a cold-climate specific unit, something that has a pan heater in the outdoor unit for automatically managing the potential build up of damaging defrost ice in the bottom of the unit.

eg: At 70F indoors a single ductless 3/4 ton Fujitsu -9RLS3H can deliver 15,000 BTU/hr @ +5F outdoors, 11,000 BTU/hr @ -15F, and runs ~$3-3.5K (full turnkey, professionally installed) in my neighborhood. There are ducted options, but they are fewer and more expensive, so the floor plan matters.

The first order of business is to get a realistic load number for the "after insulation upgrades" version of the house. Only then is it possible to come up with the most reasonable & right-sized options.

 

[MidtoupperMichigan]

Wonderful information! The other thing I figured out is that I will also be heating the crawlspace as I will be having water and sewer pipes down there and probably my water heater too so that is the same size sq ft but only 45" walls and then probably another 10" inbetween the beams of height so only 55" tall so only 4.5ft tall room so the crawlspace is about half of the upstairs wall height so I figure sq ft is 662 plus 331 that I will be heating about 993 sq ft. And I will look into your other information and tips too. Thanks for the help and thinking and experience on this.

 

[Dana]

There is no need to actively heat the crawl space if the crawlspace walls are air sealed & insulated, as long as the floor isn't insulated.

The temperature in the crawlspace will roughly split the difference between your first floor's floor temperature (say, 65F in an 70F room), and the deep subsoil temperature in your area, which is about 45-50F, so figure about 55F, give or take. If you're only heating the upstairs to 50F (for freeze protection) the crawlspace will drop to about 45F.

The square footage of the house is still only 662', not 993', even with the half-basement/crawl.

Heat load is not proportional to the square footage of the floor, it's about the amount of above-grade exposed surface area. The crawlspace is at least partly below grade (= less loss per square foot of wall area) and has conditioned space above it, not an insulated roof, and has minimal or NO window area. I suspect you have less than 20" of above grade exposure on the foundation (?).

Here's a rough I=B=R WAG at what you're looking at for heat load, assuming you only install 2" of EPS (R8) on the CMU block foundation (R2):

Assuming a 21' x 32=' footprint you have a perimeter of about 100 feet, and with 4' below the floor, 8' above a total 12' height, but only 10' or so above-grade. Assume also a 15% window& door/floor area ratio for a total of 0.15 x 662= ~100 square feet of U0.5 windows & doors (solid exterior door, clear glass double-pane windows).

So there is 100' perimieter x 10' height= 1000 square feet of above-grade wall, less 100 square feet of window & door leaves 900 square feet of insulated wall (assuming you insulate the foundation too). A 2x4/R11 wall runs about U0.1 or "R10 whole-wall", so the wall losses at 70F indoors, 0F outdoors (a 70F difference) will run:

U0.10 x 900' x 70F= 6300 BTU/hr

The window & door losses will run:

U0.50 x 100' x 70F= 3500 BTU/hr.

Assuming at least 8" of fluff in the attic the ceiling U-factor is no more than U0.040, so the ceiling losses come in at:

U0.04 x 662' x 70F = ~1850 BTU/hr.

Add it up and it's 11,650BTU/hr

Throw in another ~3000 BTU/hr for below-grade wall loss + crawlspace floor loss+ ventilation/infiltration loss and it's coming in at about 15,000 BTU/hr. If that's where the numbers come in, the ASHRAE recommended optimal equipment size would have 1.4 x 15,000= 21,000 BTU/hr, which makes a 45K furnace pretty ludicrous- are you expecting it to drop below -100F sometime soon? (Didn't think so.)

With better insulation and windows it'll be less.

At current IRC code minimums for insulation & l0w-E windows it'll be more like 10,000 BTU/hr, with optimal equipment sizing being roughly 14,000 BTU/hr.

 

[MidtoupperMichigan]

Wow, that is amazing information. The cottage sits just above the 42 line in the map. I do have a forest blocking wind from the north and a couple larger homes blocking wind on sides as bonus. Just saying, its not on a hill overlooking Lake Michigan for instance with crazy wind so that is somewhat helpful. Im amazed at the size of BTU's for that space. I like the thinking on the crawlspace not needing to be heated. Im assuming thats why I would not insulate the ceiling of the crawlspace. Should I put my artisian well pressure tank in the crawl space and should I put the artisian well pump in the crawl space? The other option is to put those items in the garage which is right next to the artisian well but garage is not heated but garage is on a slab so that ground temperature will help that if I insulate that well too which I want to do. Im wondering about condensation off the cold water artisian well pump and or pressure tank and I dont have a floor drain in the crawl space. But I could put in a surface of the cement floor back up pump if they make such a thing. Anyway, I see that if I do a good job of insulation, that i will need less btu's of furnace heat which is going to save heating costs and cooling costs. I dont mind a mini split but there is 2 bedrooms and 1 bathroom and then the living room, dining room and kitchen will be "open concept" mostly so I would wonder how I would heat and cool those 4 different spaces with a mini split. I would imagine I would need multiple mini splits so i dont have guests complaining about not warm enough in a room or too hot in a room etc.

 

[Fitter30]

First check with your electric company see if theres any rebates. Crawl space ever have a mice problem if so no flex duct. Might want to think ptac ( motel unit) heat pump with electric heat. They do make a duct package for ptac.
Two units one for beds.

 

[Dana]

Putting the tanks and pumps in an insulate crawl space is FAR better than putting them in a slab-on grade unheated garage. Garages are impossible to make air tight, and will run MUCH colder when the north wind blows than the crawlspace.

Multiple mini-splits leads to gross oversizing and inefficiency, which is why right-sized ducted mini-splits are a better solution. The "ductless head in every room" approach is only good for the installation contractor. If it's multiple heads served by a single compressor it's even worse, since there is refrigerant going through all heads when any of them is calling for heat/cool, and when the head is grossly oversized for the room load even that passive (no blower) heating & cooling can over heat /overcool the space.

From strictly a cooling point of view a single ductless mini-split in the open space is usually enough, unless the doored-off rooms have a lot of west facing window areas taking massive afternoon solar gain. Even with the mini-split in the main space, the humidity in the house drops every where in the house, and there is at least some sensible cooling through partition walls.

But from a space heating point of view it's tougher, unless the house is super-insulated. The 1% design temps in northern MI are in the low to mid 80s, a 10F higher than the ideal comfortable room temperatures, so even the doored off rooms will normally be within 5F of the main space (unless they have a lot of unshaded west window) but the 99% temp is 70F or more below comfortable room temps, and the differences in room temps can be 15F or more with the doors closed. Installing right-sized electric baseboard for the calculated room loads can cover the difference, but use a lot more electricity for the amount of heat delivered than a ducted mini-split. Turning the baseboards off on any unused room can still be pretty economical if the main space is heated with a ductless though.

 

[Dana]

First check with your electric company see if theres any rebates. Crawl space ever have a mice problem if so no flex duct. Might want to think ptac ( motel unit) heat pump with electric heat. They do make a duct package for ptac.
Two units one for beds.

All PTHP switch over to resistance heat only somewhere in the mid 20sF, which means they're no more efficient than baseboards in winter in northern MI, but can be reasonably efficient during the shoulder seasons in that location. From an upfront cost perspective 3-4 PTHPs isn't going to be cheaper than one mini-split for the main space and right-sized baseboard heat for the doored off rooms.

A 2" rat slab over the ground vapor retarder offers pretty good mouse-mitigation, but hard-piping is a better solution overall if going with a modulating heat pump. Flex might make sense for an 80% gas furnace with a big air handler.

Dettson flexible skinny "Smart Duct" system used with the modulating 15KBTU Chinook gas furnaces (+ Alize mini-split for AC) that can be run inside of 2x4 partition walls works great in new construction or full-gut rehabs.

 

[MidtoupperMichigan]

First check with your electric company see if theres any rebates. Crawl space ever have a mice problem if so no flex duct. Might want to think ptac ( motel unit) heat pump with electric heat. They do make a duct package for ptac.
Two units one for beds.

I have thought of the motel unit style combined ac/heat. Brand new crawl space so hopefully I insulate and seal off correctly to not have mice problem. Other cottages in the area dont seem to have a mice problem so may be hopefully good. But good points on not flexible duct work. I imagine they like to eat into the flexible duct work since its warm... I am thinking if I did a hotel style unit, I might need 3? One in each small room and one in the main area. That seems a bit much but I guess with some duct work then it could be 2.

 

[Taylorjm]

I have thought of the motel unit style combined ac/heat. Brand new crawl space so hopefully I insulate and seal off correctly to not have mice problem. Other cottages in the area dont seem to have a mice problem so may be hopefully good. But good points on not flexible duct work. I imagine they like to eat into the flexible duct work since its warm... I am thinking if I did a hotel style unit, I might need 3? One in each small room and one in the main area. That seems a bit much but I guess with some duct work then it could be 2.

I think the cost for the electric heat from those units would kill you being in the way northern michigan. I don't think the heat pumps would work in the mid winters up there, or they wouldn't put out much heat if they did. If you have natural gas available, there's no reason to consider anything else. Natural gas is the cheapest form of heat, so I'd find a way to use that for heating the home and hot water.