Is a PTHP a HVAC option for a super insulated structure?

Question

Is a Packaged Terminal Heat Pump a good HVAC system for a structure that is super insulated or passively insulated assuming the unit is sized and fitted to meet the needs of the structure.

There is not really much mentioned about PTHP and very well insulated buildings. Every publication out there dealing with well insulated structures seems to be all mini split ductless or small duct high velocity hvac systems. Are PTHPs worth considering in place of the other just mentioned options? Below is a potential list of advantages and disadvantages that I came up with for considering a PTHP system, it seems to have some pretty good advantages. Why are we not seeing more of these in well insulated building projects, am I missing something, sans the larger whole in the wall I would think these units would work well where a mini split wall unit would go?

Advantages

1. more reasonable pricing than ductless split systems

2. Usually offer inlet fresh air vent, that could be worked into a low cost fresh air vent with a exhaust vent wired into the fan thermostat circuit.

3. Ususally offer standard 24VAC thermostat control, perfect for smart home integration.

4. Reasonably efficient depending on which models selected with a range of 10-13 EER.

5. Usually has back up electrical resistance heat as a 2nd stage heat option for colder climates

6. Available in under 1 TON sizes as small as 7000 BTUs

Disadvantages

1. Potential an issue since it will require a large about 43" x 16" hole to be placed in the wall, not sure if this would be a massive air / thermal transfer leak surface area even if a air tight seal is accomplished around the unit

2. Usually do not have a dedicated de-humidification / dry mode like many mini ductless systems do

3. Is not a efficient as mini ductless systems

Answer 1

The point of a super insulated home or a Passive house is that they rely on the body heat of the occupants plus the heat from the various gadgets in the home to keep them warm, with the insulation slowing the heat loss. A home built to Passive house standard will usually be fitted with hydronic heating (probably as the new owners are not sure that it will do what it says on the tin.) And the heating may never be used. Of the 37, 000 Passive houses in the world, 20, 000 or so are in Germany where 92% of the owners are satisfied with their economy. A true Passive house is almost air tight, with a heat recovery ventilation system, fitted with a in duct heating coil to warm the incoming cold winter air, should it be 3.5C or more lower than the room temperature. You can see from the above that having a large hole in a wall, albeit sealed with a steel sheet, where a vast amount of heat can escape will defeat the whole point of carefully installed insulation.

Answer 2

After narrowing my focus from PTAC to PTHP I found an answer. PTHP can be a practical option for well insulated structures, it really depends on the intended use and the size of building. For smaller open space plan like offices studios, this kind of system appears to work effectively as noted from some answers on Green Building Advisory to a similar question of Heating a small house with a PTHP

I'm planning on doing this. I only found one other guy who has done it, but he's happy:

"Because the house is so compact and highly insulated, we eliminated the central heating system. The air source heat pump, which is like one found in a hotel room both heats and cools. Heat in the lower level is provided by small baseboards in the bedrooms and an electric heater with a fan in the bathroom.

We tracked the performance of one unit from February 1st 2009 to February 1st 2010. The cost to heat the unit came to $185.24 which averages to $30.87/month or $1.03/day. The square footage of the unit is 1150. A separate meter which tracks electric used by the heaters showed 2795.5 kilowatts used during the six month heating season of mid-October to mid-April."

Mini-splits will cost $2500 and up installed, but a PTHP is only about $700. That amount will take a long time to recoup with just a 30-50% better COP.

Yes, it is a "hole in the wall", but so is a window. So, if installed properly, the added heat loss is about like a 26" x 14" window.

A small, superinsulated house is a perfect application since the output of a PTHP is 12,000 btuh, and that should match the house pretty well.

What about when the outside temperature is below 25F? PTHPs have backup resistance heat if necessary, but we ran a yearly simulation that showed that resistance heat was needed for only 4 hours for the entire year in a 1200sq.ft. house.

A PTHP also has a "fresh air" option, that just might save you another $500-$3000 on an HRV

More discussion at http://greenbuildingindenver.blogspot.com/2009/08/mini-splits-or-ptacs-a...

Answered by Kevin Dickson, MSME Posted Fri, 10/22/2010 - 10:32