Another HVAC recommendation request

[ZX3ST]

Hey all, been lurking a while now and finally have something to post about. Some help please for the new guy?

Garage details:
Detached, 28’x32’x9’, 2x4 construction
Batted fiberglass insulation, R13 in the walls, R19 in the ceiling
Completely finished with 1/2” drywall
2 8’x10’ insulated doors, I estimate R7 but not 100% certain
No windows, 1 solid man-door
If it matters, soffit and eave vents, planned ridge vent upon new roof in the Spring

Central Missouri climate, nothing too extreme here but we do have rather high humidity. Temps can hit single digits and low 100s but typically don’t stay there longer than a couple days. The building currently has a conventional natural gas furnace that was converted to propane. Conventional metal ducts straight down the center of the ceiling.

I’d like to explore the option of swapping to electric heat. Propane is expensive and this thing seems to burn through it way too quickly (no pun intended). Plus the deliveries tend to tear up the yard. It’d be nice to set the thermostat to 35* in the winter and leave it without having to monitor the tank level. Natural gas is not available in my area.

One of my friends gave me an old (but working) electric furnace he ripped out of a rehab home he was working on. I could potentially just put that one in wanted to consider other options.

I’ve read some discussions on minisplits and thought that maybe it’d be a clean option to get a bit more overhead clearance from the duct removal. AC would be nice but it’s not a requirement, I’d be more interested in the humidity control I think.

The garage is fed by a 100A (240v) subpanel branched from the house’s main 200A service. Low power requirements would be preferred, as the house is already all-electric (furnace,dryer,stove,water heater).

What would you do?

 

[Richard Cranium]

Welcome to the forum, I can't help you on the hvac, But did want to welcome you.

 

[zmaxmotorsports]

What size is the current furnace thats in there? For the cooling side Id say a 1 1/2t -2t would be plenty.

 

[finn]

Need more ceiling insulation.

I don't see propane being more expensive than electric.

How much do you pay per gallon for propane and per kWh for electricity?

 

[tomroblee]

We really need more information.

It shouldn't take much heat (or fuel) to keep a decently insulated and draft free 900 sq. ft. building at 35 degrees in your neck of the woods. I'm guessing that 10,000 to 20,000 Btu should be adequate in even the coldest weather. If you are wanting to quickly heat the garage to 70 degrees when you are working out there, you may need 60,000 BTU+.

Are you presently keeping the garage at 35 degrees 24/7? How much propane are you using?

A modern high efficiency propane furnace could be 95%+ efficient. An older oversized converted propane furnace could well be operating at less than 50% efficiency. High propane consumption could also be caused by leaks in the propane lines or a lot of air infiltration in the building.

An electric heater or furnace should be nearly 100% efficient. One that is sized just to keep your garage at 35 degrees will probably require 30 amps or less (240 volt) and probably won't overload your system. Your free electric furnace may draw much more current, and may overload your system.

Any air conditioner, heat pump, or dehumidifier will help with excess humidity---but only when it is running. A dehumidifier may be the best solution if you aren't concerned with the temperature of the garage.

 

[theoldwizard1]

I’d like to explore the option of swapping to electric heat. Propane is expensive ...

In most part of the US. electric resistant heat cost more to operate than propane.

I’ve read some discussions on minisplits and thought that maybe it’d be a clean option to get a bit more overhead clearance from the duct removal. AC would be nice but it’s not a requirement, I’d be more interested in the humidity control I think.

A good quality mini-split heat pump is much more efficient than resistance heat and probably more efficient that a propane furnace. Certain much more efficient than a central A/C unit added to a furnace.

Yes, the good ones are NOT cheap ! Depending on how much you use it, you might be able to recover the addition cost within 10 years or less.

 

[HoosierBuddy]

In most part of the US. electric resistant heat cost more to operate than propane.



A good quality mini-split heat pump is much more efficient than resistance heat and probably more efficient that a propane furnace. Certain much more efficient than a central A/C unit added to a furnace.

Yes, the good ones are NOT cheap ! Depending on how much you use it, you might be able to recover the addition cost within 10 years or less.

^^^^^^This

If you think you're propane is expensive...wait until you see your power bill if you try to do this with electric resistance heat.

A good minisplit system will use a small fraction of the power that ER heating would AND you can use it to cool with in the summer.

Phil

 

[ZX3ST]

Welcome to the forum, I can't help you on the hvac, But did want to welcome you.

Thanks!

What size is the current furnace thats in there? For the cooling side Id say a 1 1/2t -2t would be plenty.

Not sure. I'll do some digging on that when I get home tonight. It must be fairly large because it heats from freezing to 60* in a hurry!

Need more ceiling insulation.

I don't see propane being more expensive than electric.

How much do you pay per gallon for propane and per kWh for electricity?

Going rate for propane right now is about $1.50/gal plus $20 delivery. Average over the last 12 months is closer to $1.80. I think my tank is around 100gal but will get some measurements tonight. Maybe I would get better usage with a larger tank and less deliveries?

Winter electric rates (if I'm reading correctly) are $8 plus
$0.0858/kWh first 750kWh
$0.0573/kWh after 750kWh
https://www.ameren.com/-/media/missouri-site/Files/Rates/UECSheet54Rate1MRES.pdf

Are you presently keeping the garage at 35 degrees 24/7?

Nope, only when I'm playing in the garage. I'll usually set to about 60* 30min before I go out there. It's off otherwise.

How much propane are you using?

I know this isn't very helpful but I'd guess 2-3 tanks in a season? I haven't used it at all this year so I don't have any good recent data. My last propane company was taken over by another that has not handled the transition very well and I don't have any of my old bills to check.

 

[brownbagg]

hotel unit heat pump

 

[justinjoyal]

A 18k BTU heat pump mini-split is what you need.

 

[ZX3ST]

UPDATE:

I'm having trouble finding efficiency ratings, but the furnace sticker says input 75,000 BTU/HR.
It's a TempStar (International Comfort Products) NTC6075FBA1. I'm guessing this isn't exactly a high-end model.

The tank has 120 (water) gal listed on the nameplate. I think I heard somewhere they only fill to 80%? So my 100gal estimate is at least in the right ballpark.

 

[tomroblee]

From what I can find your furnace is about 15 years old and is rated at about 80% efficiency. That means that your heat output is about 60,000 BTU. The efficiency and BTU output could vary somewhat depending on how the conversion from natural gas was handled and whether or not the furnace is in proper adjustment.

Your furnace is probably a reasonable sized for the manner in which you use it. (i.e., turning on the heat only when you are working in the garage). If you were going to heat the garage 24/7 something in the 25,000 to 30,000 BTU output range might be more appropriate unless your garage doors were very drafty or were opened many times per day.

Based on the propane costs (including delivery charges) and electric rates you quoted, electric resistance heat would be cheaper to operate. Propane prices seem to vary quite a bit. A couple of years ago they were in the $5 per gallon range during a cold weather shortage. I would suspect that you could purchase propane at a cheaper price if you had a larger tank and had it filled during the summer months when there is lower demand.

You read the electric rates correctly---but stopped before the mention of at least two "riders". Utilities use riders to bill for significant expenses that may vary considerably (such as fuel costs and purchased electric charges). The riders allow the utility to vary it's billing rates from month to month without having to go through the process of a full-blown rate case before the state commission. You need to go through a mathematical exercise to determine how significant these riders are. To do this, recalculate a recent winter billing amount by using the charges and rates you quoted in you post. Then subtract the amount actually billed by your utility. The difference will be the rider charges. Divide the rider charges by your total electric consumption to determine the rider charge per KWH. Where I live the rider charge is almost as much as the quoted high usage rate.

It's still hard for use to understand how you use your garage. You say that you are using two or three tanks of propane per year, so I'm guessing that you use at least 200 gallons per heating season. A gallon of propane has about 92,000 Btu. If you use 200 gallons of propane per year, your 75,000 BTU input furnace would be running about 245 hours. I'm guessing that it will run steadily for some period of time on a cold day until the garage warms up to your thermostate set poing (60 degrees?). After that (an on warmer days) it will cycle on and off quite a bit. This possibly means that you are heating your shop 500 or more hours a year? If you have a standing pilot (rather than electronic ignition), the pilot flame could be using a considerable amount of propane over a year's time.

There are a lot of fans of mini split heat pumps. Your operating costs would be considerably less with one than either your existing propance furnace or you free electric resistance furnace. However, the cost of a system may be high. I see posts talking of $1,000 systems advertised on ebay. There is a currently another thread in this forum where someone is discussing HVAC options. He quotes a $6.000 price for a mini split system in an only slightly larger building. I assume this price includes high end equipment and all installation costs. It's possible that permit and inspection costs are high in his area.

It's impossible to tell whether you have sufficient electric capacity for your free electric furnace. A 60,000 BTU electric furnace (electric furnaces are about 100% efficient, so input and output BTU are the same) is going to require about an 80 amp circuit. (it will draw about 60 amps). If you furnace is larger, you may have more of a problem. I suppose that it is possible for an electric furnace to have multiple heating elements, and you might be able to disable one or more to "de rate" the furnace?

You mentioned how it would be nice to leave the thermostat at 35 degrees. I believe that you can buy low temperature thermostats for propane and electric furnances that will go as low as 35 degrees. I would be surprised if you could find a mini split heat pump with a thermostat that will go below 40 degrees. You might want to investigate this point if it is critical. I seriously doubt that you would require much propane or electricity to keep your garage at 35 or 40 degrees in all but the very coldest days of the year. My garages are attached, but never drop below freezing without any heat.

You could add an air conditioner (or heat pump) to you existing propane or newly acquired electric furnace. I would question spending the money on a 15 year old propane furnace or electric furnace of unknow age or condition.

At other have said, you would benefit from more ceiling insulation and do anything you can to minimize heat loss from your doors. It's not unusual for half the heat loss in a garage to be from infiltration.

For information sake, this appears to be a manual that would cover your furnace:
https://c.searspartsdirect.com/mmh/lis_pdf/INSTL/L0502532.pdf

This is a link to a propane/electric cost comparison tool.

http://suhresgas.com/propane_vs_electric.htm

If you want to save money no matter how much it costs you, I would recommend a very high end heat pump (mini split or ducted system) with a propane or electric back-up for the very coldest days. If you are as tight as I am, the furnaces you already have look pretty good. Various sorts of air conditioners and dehumidifiers can be found at reasonable prices.

 

[ZX3ST]

tomroblee, thank you for that incredibly detailed response!!!

I'm not 100% sure, but I think the furnace uses an electronic ignition. And everything was supposedly leak tested when they filled the tank for the first time. (first time for me at least, this setup was here when I bought the house) I do seem to remember noticing the gas level going down when I wasn't using anything so I'll need to do some checking on that. Is it possible and/or economical to retrofit electronic ignition if it doesn't already have it? I would think there are UL listings to worry about here.

The garage gets used nearly exclusively on weekends. I'm not often in there during the week. However weekend usage is nearly constant, as in it's being used all day Saturdays and Sundays. I would usually just turn the thermostat down to around 40* overnight Saturday instead of switching off completely.

The 35* degree figure was just something I was considering to keep everything above freezing at all times. It's not necessarily a requirement, though it'd be convenient to melt anything from the vehicles overnight.

To your point on the "free" electric furnace, I had already thought I may need to disable one of the elements to reduce amperage.

My garage doors already sit pretty tight against the little rubber trim around the door frame. I'm not sure what else I could do with that, but I'll have to do some research on it.

Based on your estimated age, that's in line with how old I thought the house furnace and AC might be also. I wonder if there's merit in being able to finagle some kind of package deal with my local HVAC guy? Switching both buildings over to a heat pump might not (potentially) be a bad investment. Is 15yrs usually about the time one should think about replacing this equipment? How long are they usually expected to last?

Lastly, what kind of efficiency gains are there to be had going to a heat pump? 20%? 120%? I don't mind investments as long as they make sense. A ROI of 10+ years probably doesn't make sense for me with this house. Chances are that I'll be upgrading before then.

 

[tomroblee]

At this point I'm really stretching any expertise that I may have, and I still don't have

Your furnace appears to be about 80% efficient, and furnaces with that efficiency rating may or may not have electronic ignition. The manual I linked to an earlier post shows that all the furnaces covered by the manual have a spark ignition system. (A spark is used to ignite the pilot light, and the pilot light ignites the main burner.) This is no guarantee that you don't have a standing pilot light or some other sort of electronic ignition.

If you have a standing pilot light, it could be consuming 5 to 10 gallons (or more?) of propane per month. This would be a material percentage of your total usage. Electronic ignition conversion kits are available. A search on the internet shows a couple priced at about $300. A new furnace equivalent to yours (80% efficiency off brand) seems to be about $600. You could get a new much higher efficiency propane furnace for $800 to $1500.

If you are "using" propane while the furnace isn't running (and you have electronic ignition), something is wrong.

If you do a search on the life of gas furnaces, you will see answers ranging from less than 10 years to 30 years or more. There are lots of variables! I bought my previous home in 1978 and was told that the gas boiler was 15 to 20 years old at the time. I was still using the same boiler when I sold the house in 2011. Sometime in the late 1990's my neighbor replaced his furnace that his father had installed in 1938. People tend to quote 15 to 20 years for the life of a central air conditioner. The one in my previous home was 30 years old and still running when I sold the house. However, it was probably only half as efficient as a new one would have been.


I think that any new heat pump would save you some $$$. In your house, it won't save anything on the $8 per month fixed fee you pay or on the electric used for your stove, dryer, water heater, lights, TV, etc. A new heat pump (or air conditioner) would be a lot more efficient than a 15 year old one--if you have one. An expensive heat pump will save you much more in operating costs than a cheap one.

You might make an extremely wild guess of your present heating costs by comparing your heating season electric bills with the bills you get in the shoulder months (October and April when neither the furnace or air conditioner is likely to be running much, if at all.) If you don't keep your old bills, your utility can probably provide this information to you. Many utilities have this information online these days.

An inexpensive heat pump could save you 80% or more when the outside temperature is in the 50's. It may shut off entirely when the outside temperature gets below 20 degrees. The top of the line heat pump will save you even more when the temperature is in the 50's, and will probably save you 40% when the temperature gets to the singe digits.

 

[ZX3ST]

Thanks again for your insight. It appears heat/cool is the bulk of my energy usage so it'd be the low-hanging fruit to start with. I wonder if the propane will need to stay for backup heat when the temps get too low for the heat pump? My dad just mentioned we have someone in the family who does HVAC so I'll see if he can come out and look at it sometime.

A friend mentioned that the electric company will come do a whole house audit and provide suggestions for improvement for $100. I may start there. Thanks again!

 

[Falcon67]

I can heat 960 sq/ft with a 5kW electric. Takes a few minutes, but I have those minutes to spare. I have propane backup, have not used it in 2 years. And I have less attc insulation that you - only R13. I've been in Missouri, you should put some AC in there LOL. 15~18K should be plenty for that space.