What do you think about the Tankless Water Heaters?  Never-ending hot water sounds great but it seems like many bells and whistles to maintain.
 
Yes and Yes.  Or maybe No and No.  It does sound great but much to maintain in controls and equipment.  The Tankless Water Heater is actually a boiler and a rather large one at that.  In fact in many cases BTU/Hr rating will be higher than the boiler (or furnace) that will heat your home.  My view, a tankless water heater is a solution looking for a problem.  The only real justification is “standby heat loss” attributed to the tank type water heater.  But with better tank insulation, that justification is fast disappearing and wasn’t even real if the water heater was within the heated envelope.  There the water tank was just losing heat to the house which is heated anyway.  And the boiler?  The hot water is anything but instantaneous as the boiler goes through its purge and firing cycle, maybe just to rinse a dish in the kitchen sink. Actually many of the Tankless Water Heaters have added a tank just for this purpose; while small, are we back where we started?
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Advice from here:  “Own your plan”
 
Obviously we favor owning and understanding your Mechanical Systems (that’s what we are all about) but what about your house plan?  There are the One-Stop Building Contractors offering the Turn-Key project.  “No Muss, No Fuss”, “We Design, We Build, Leave the Worrying to Us”.  But you are going to live in this house.  Is this your best option? 
 
A real case:  Skylight over the kitchen sink.  Nope; and now the metal roof is on.  Heat duct cut in directly under future “home” of the china hutch.  In other words that old adage, “Failing to Plan is Planning to Fail”.
 
Mechanical systems are no different.  Has your mechanical contractor looked at heat loads?  The HVAC industry has a good procedure known as Manual J for this very purpose.  Even a Manual D for duct sizing if using Forced Air.  Has your contractor availed himself to these resources?  Odds are, Not.  Rule of Thumb is the name of the game.
 
Hydronics, the situation is even worse with ½” tubing being the norm for in-floor heating.  Can ½” tubing be used?  Sure.  But not without doing the numbers and sizing accordingly.  What is the point of installing 300’ loops of ½” tubing and then run out of heat at 150’ at design conditions?  Not a good idea and the homeowner will not even be aware until the temperatures drop to Design Conditions and the house is cold. 
 
With a Manual J and flow rate calculation, these numbers can be known and above situation avoided.  At least ask the questions and maybe even talk with someone that knows the answers.   Rule of Thumb is the name of game.  Our advice:  Choose not to play.

What is a Mini-Split? Should I consider?
 
Well yes and no!  Or it depends, right?  A friend actually approached me with that very question.  He has a two-story, open architecture, house, actually three-story given the basement shop, all heated with wood.  (Biomass these days, right?)  Here, there might be two right answers.  If installed to supplement the wood stove then, yes the mini-split could work well.  But if it is to be the main heating system, maybe not. 
 
First of all, what is a Mini-split?  It is a heatpump for sure.  One might call it an air to radiant. Generally on the smaller side with the outdoor air unit and the indoor radiant unit or “head”.  Sometimes there might be several heads, but that’s the basic unit.  While there can be air flow across the inner head, there is no ducting so heat is very localized.
 
Efficiencies can be good in cooler weather to not so good in cold weather.  In cold weather there are two things acting against it.  There is not so much heat in the air and the indoor unit is calling for more heat and therefore higher temperatures both pushing down the efficiency.
 
Generally there is not a backup heating element as we have in the air-air and air-water so sizing the unit for worst case conditions is difficult.  But if a house been designed with a mini-split in mind, with an open house layout and with low heating loads, it can work.  To the mini-split’s credit, most have variable speed refrigerant compressors so that over sizing is not a major issue in efficiency, just cost.  With the mini-split, the main issue I see is the inability to direct the heat where you might want it, say the bathroom.  And I find the “head” mounted on the wall less than attractive.   
 
Back to our friends.  They installed the mini-split and are very pleased with it, using it in the fall and spring to knock off the morning chill in the kitchen while having coffee.  They might not even start the woodstove on days when the sun is out.  Even mention a savings of some fire wood for not much in electrical useage.
 
The mini-split also served another purpose. It replaced a build-in electric heater wall heater that was used to keep the house from freeze up if gone in the winter.  While the mini-split may not maintain temperature in winter, it can keep the house above freezing in our climate. 
 
Bottom line, the mini-split can add value by recognizing its limits and designing accordingly.

Geothermal Heat Pumps, are they worth the effort (and expense)?
 
First some terminology:  Geothermal is better termed “Ground Source”.  Why?  Few of us actually have a Geothermal feature such as a Geyser or Hot Springs to take advantage of.  But we do have ground or in some cases maybe a lake or a pond from which heat can be extracted. 
 
Properly designed a Ground Source heatpump can be very efficient, particularly when coupled with a low temperature heating system such as hydronics.  Possibly a Coefficient of Performance (COP) of 5:1 (5 units of heat energy for every unit of electrical energy to run the heatpump compressor).  To compare, an air to water heatpump might have a COP of 3:1 and air to air maybe a COP of 2:1. 
 
The COP for the air units are Overall Estimates as “Heating” must be supplemented by Restive Heat.  In the Air to Water this would be an Electrical Element in the water buffer thank.  This is essentially the same as the electrical element in your electric hot water tank.  This supplemental heating is necessary because the air-source heatpump’s heat output drops with decreasing air temperature, just as the heating load for the building increases.    But this does drive down the efficiency or overall COP of the air source heat pump.
 
The familiar Air to Air heat pumps operate on the same backup principle.  In this case there are electrical surface elements within the blower housing that cut-in to maintain temperature as the heatpump output drops due to lowering outside temperature. 
 
Of the two styles of heat pumps, the Air/Water heat pump will have the better COP because the heating temperature required will be lower than the Air/Air.  An in-floor radiant water system may only require 90F water to heat the house.  While for a Forced Air system it is not uncommon to require 120-130F at the heat pump heat exchanger.
 
The answer to the original question?  It depends! 
 
Properly Designed can be a very big question for the ground-source heatpump.  While I claim no expertise on designing these systems, there have been enough failures to use caution here.  In particular, the ground loops might be undersized to minimize construction costs but at the expense of the system.  The water to earth heat transfer through the relative thick HDPE piping is not a great heat transfer mechanism at best.  Or another issue, the hydraulics of the pumping loops have not been properly evaluated.  Here I do claim some expertise.  Pumping horsepower directly offsets against system efficiency and should be fractional horsepower requirements.
 
Heating Load is always an issue.  Minimizing heating load goes a long ways toward decreasing the need for extraordinary solutions.  Ground-source heatpumps being one of those.  Note:  See other discussions on minimizing heat loads.
 
Equipment Availability is the final issue.  There are a number of manufacturers of Water-Water heatpumps and many-many that manufacture Air-Air heatpumps but very few that offer the air-water heatpump which might be the best option in terms of return on investment.
 
What a discussion, right?

Heat Pump or Boiler – Which should I consider?

To a large degree, it depends on preference and what fuel options are available to you.  For instance if Natural Gas is available it is hard to argue with the boiler option.  Price of fuel is reasonable and regulated.  However if the fuel choice is oil or LPG, where price can vary with the world markets then maybe the heatpump is a consideration.  The heatpump can be more expensive to install, but they are very efficient in the use of electrical energy. And heatpumps have the added advantage of providing summer cooling.
 
The Answer:  It depends!  Preference, Fuel Costs and last but not least, Heating Load requirement.  Minimize the heating load by building tight and insulating well, then equipment is smaller (and less cost) and other options can be considered.