Hello, I'm planning to break ground next Spring on a new Timber Frame vacation home in Northern Michigan. Structure will be Superior Wall pre-cast concrete lower level (walk-out) and SIP walls/roof. Home to be 48' x 32' cape style. Lower level to include 500 sq. ft. garage. Total living space about 2,550 sq. ft. Well and septic. Considering a condensing boiler for domestic hot water and primary heat. Rinnai offers a combination unit that I would consider, however they don't have a dealer in Northern Michigan. Looking at a "Buderus" boiler as they have dealers in the area. I would use an air handler with coils to heat and cool space. Since this will be a tightly constructed, highly insulated home I'm also planning to use a heat exchanger. I would like to entertain comments on the subject of using a combination boiler for heating and domestic hot water. Thanks!

you can heat and cool the house with air source minisplits for a much lower initial investment and lower annual costs.

Posted By Bob I on 13 Sep 2013 01:48 PM
you can heat and cool the house with air source minisplits for a much lower initial investment and lower annual costs.

Maybe, or maybe not. It depends on whether its on the gas-grid, and what the actual heat loads & floor plans are.  MI residential retail gas prices are quite a bit lower than in the northeast, but residential electricity rates are comparable.  In a N.MI climate you might average a COP of 2.8 with best-in class ductless, but you'd need to hit 3.5 or better to be at parity with condensing gas, and I don't see that efficiency average happening in that climate.

If we're talking propane, different story. The much higher fuel cost of propane would be sufficient incentive to go with a mini-split solution if the floor plan and total heat load make that reasonable.

Outside design temps in northern Michigan are in the low to mid negative single digits F, but cold snaps into negative double-digits aren't rare.  While it's POSSIBLE to design a 2500' house that would be within the -5F output of a couple of mini-splits, it's by no means a done deal until you've done the heat loss calculations.  In either case, the mini-split selection that actually works at the temperature extremes (a handful of degrees colder than the 99% design temp) are likely going to the Fujitsu Halcyon XLT-H series or the Mitsubishi H2i Hyper Heating series.  If the design heat load is under 30,000BTU/hr it'll probably be cheaper to install mini-splits than a combi boiler with cheapest possible radiation, but if it's over 40,000 BTU/hr not so much, but the difference in operating cost five years of heating with propane in a house with a 40K heat load can buy quite a bit of mini-split.

Gas prices will change over time (where did I just read that?) whereas minisplit fuel, supplied by a PV system, will be a fixed price for the life of the system.

I would use an air handler with coils to heat and cool space. ... I would like to entertain comments on the subject of using a combination boiler for heating and domestic hot water.

I would certainly price it with separate units too. Ie, a more conventional air furnace or mini splits and a domestic hot water heater. You might even find open loop geothermal a reasonable option (although the part-time issue doesn't help). Some geo heat pumps have two heat exchangers to heat air and water. Wood heat might be a good assist for air source heat pumps (assuming no nat gas) in the coldest weather.

I'd design a vacation house with a frost proof foundation and a system to blow the pipes out. Both so that it could be easily left completely unheated.

Posted By Bob I on 13 Sep 2013 04:15 PM
Gas prices will change over time (where did I just read that?) whereas minisplit fuel, supplied by a PV system, will be a fixed price for the life of the system.

Good luck with that!

In net-metering states you might get grandfathered in on existing systems, but unless you're building in the next year or so, expect the net metering deals to change substantially over the next few years, especially in states where third party ownership of rooftop PV is expressly allowed (MI is one of them) and growth rates in roof top PV installations are exponential.

If you're talking PV off-grid that's an even more expensive proposition- the lifecycle per kwh cost of the storage alone is daunting.

Getting to Net Zero Energy under a net-metered grid-tie will have a financial rationale in some circumstances, but I didn't get the impression that was anything like the design goal here.  Getting a house to Net Zero in a N-MI climate with $IPs may have some difficult financial math hurdles to clear.

Gas prices won't stay at the current low, but it would take a significant and sustained increase in the price in MI without a corresponding increase in electricity pricing to be able to state that the lifecycle cost of heat from a mini-splits would be cheaper than gas.  With some price inflation on gas they might become fairly equivalant though.

Not that I'm a big fan of combi boilers or other gas burners- there are other reasons to forgo the boiler for heat pumps, but the error bars on future energy prices are large, and over the past 5 years condensing gas would have been substantially cheaper to heat with than ductless at the MI averages, without a good narrative for how gas prices are going to double.  Wellhead pricing in the $4/MMBTU range is sustainable (barely) from a producer-profitablity point of view and that's the current price (well above the $2 low of a few years ago.)  A sustained price of $5 would make more known reserves profitable, but that would only yield about a 10-15% bump in the residential retail rate, and burnt in a condensing boiler would still competitive with 16 cent electricity in a mini-split. At $7-8/MMBTU at the wellhead, it would flip and mini-splits could be cheaper, assuming electricity stayed in the 15-16 cent range.

Basically, even at a COP of 3 you'd be getting 10,000BTU  of heat into the house per kwh or 100kwh/MMBTU, which with 16 cent electricity is $16/MMBTU. With 15 cent electricity it's $15/MMBTU.  At the current delivered residential price average a buck or buck-ten annually , in a 95% efficiency combi boiler that's only $12/MMBTU.  While there are states with 12 cent or cheaper electricity, MI isn't one of 'em, nor is there much reason to expect that MI power will drop 25% in price any time soon.  There's substantial room for the retail price of gas to grow before it's more expensive than a mini-split there.

In different electricity and gas markets (and climates)  the numbers stack up differently, but this is how it shakes out in MI- can't make the case for parity, let alone cheaper operating costs there.  In IL & IA the spread is a bit closer, and in WA mini-splits are definitely cheaper with 9 cent electricity and realistic COPs in the 3.2-3.5 range competing against buck-twenty gas.

Thank you all for your responses. Very thought provoking. A little more info; natural gas is not a viable option at this time, so I'll be using LP. I'll be paying the electric company $10,000 to get power to my sight (half a mile away). I looked into off grid solar with a back up propane generator, but decided against it.

I did consider mini-splits, but thought since i'll need ductwork for HRV and domestic water heater regardless, that maybe a dual purpose condensing boiler could make sense. I did't consider the cost of propane.

I need to reconsider the mini split system. But first I suppose a heat loss calc would make good sense. Any suggestions where I could get one? I have architectural drawings on PDF and site plan available. I don't have the knowledge to do this myself, so any ideas where I could get one done at a reasonable price?

Thanks!

You haven't talked about how much time you intend to be there, but it changes all the numbers significantly.

since i'll need ductwork for HRV

HRV ducting is not as extensive as HVAC ductwork, and a Cape style is easier yet. Supply air to the bedrooms, exhaust it from the kitchen and bathrooms and put a big fan in the great room to circulate. Use ductless minisplits. Of course, the great room will have a sealed wood burning stove, right?

For rooms where the door is likely to be closed (bedrooms?), put in a supply and a return or at least baffled through-wall grills. Otherwise you are pressurizing the room, which is bad for moisture in the walls and bad for heat loss.

Posted By Boyne on 14 Sep 2013 01:53 PM
Thank you all for your responses. Very thought provoking. A little more info; natural gas is not a viable option at this time, so I'll be using LP. I'll be paying the electric company $10,000 to get power to my sight (half a mile away). I looked into off grid solar with a back up propane generator, but decided against it.

I did consider mini-splits, but thought since i'll need ductwork for HRV and domestic water heater regardless, that maybe a dual purpose condensing boiler could make sense. I did't consider the cost of propane.

I need to reconsider the mini split system. But first I suppose a heat loss calc would make good sense. Any suggestions where I could get one? I have architectural drawings on PDF and site plan available. I don't have the knowledge to do this myself, so any ideas where I could get one done at a reasonable price?

Thanks!

HRV ducts are tiny- too small to hook up even 20,000BTU/hr air-handler & coil into.

You can run you own heat load calc using I=B=R methods and a spreadsheet, which works better than low end heat-loss software when building tight, higher-R houses instead of leaky code-min.  You have to do your own estimates for the assembly U-factors, or find sources on how to calculate "whole-wall" R-values, but there's little rocket science to it- if you passed seventh grade math it's doable.

Thanks again for your inputs!

I'm going to pursue the mini-split system with HRV for my vacation home.

I'll try calculating my own heat loss using I=B=R method, but plan on getting a few quotes from the pro's and compare notes.

This has been a most helpful forum with very constructive dialog.

THANK YOU!

"Of course, the great room will have a sealed wood burning stove, right?" Yes, the lower and main level will each have a sealed wood burner. Fortunately I have a close relative in the business which will be a great help in selecting equipment.