Duct size may be an issue even with a 5 ton. The short coming however is ususally return air which is easier to add. If entering water is 32F, what is leaving? I'm still not confident in the loop size, what is the diameter? An under sized loop would create poor recovery, let's look at leaving water temp, and consider upping loop size before changing heat pump if the Delta T is off.
J

Leaving water temp is 27F. 

The loop is 1 1/4" SDR11 & is now rather protected against change as  its under 18" of ice and 10' of water.

Did your old system recover quickly with a 2°C setback?  If so, you have likely have enough duct, but may need aux heat to raise the supply air temp.......or was the 4+ hour recovery with aux heat?  How much "thermal mass" to you need to warm in the morning?  Have you checked the supply air vs. return air temperatures.

Can you do something like closing the register in the bedroom at night to let it be cooler than the rest of the house?

Just to add to a previous point, I have a Daikin split system with a dehumidifaction cycle that works very well.  It tends to cool very slightly slightly.

Bruce

Leaving water sounds right.
Are you saying it takes 4 hours to recover 4 degrees with aux. coil?
J

Are you saying it takes 4 hours to recover 4 degrees with aux. coil?

   - No.  The 4+ hours is without aux.  

   - The current system is a time-based upstage control & includes an outside temp sensor that prevents aux unless outside temp min is breached.  As an experiment I adjusted the outside temp controller to enable aux at any temp.  Therefore the aux came on in the morning.  The result was quick recovery, memories of fossil fuel, mmmmmm.

Did your old system recover quickly with a 2°C setback? 

   - Yes.  But it was a 454HP hemi propane furnace.

How much "thermal mass" to you need to warm in the morning?

   - Huh?

Thanks for all your inputs & questions guys.

Are you heating drywall on studs on a wood floor system, or plaster on brick with concrete floor system?

As you can imagine, the the former will heat up (and cool off) a lot quicker than the latter.  I live in a building with brick, plaster and concrete.  Thermal mass can be a mixed blessing.

If you system is hanging in at the design temperature without aux heat, I think your capacity is right and it seems the recovery problem is solved by aux heat.  If you do much air conditioning in the summer, the extra ton could create some humidity problems as engineer suggested.

I would like to hear from one of the pro's here if adding a ton of capacity will increase your supply air temp to permit a faster recovery....or will you need to move more air at the same temperature, which gets to a duct size issue.

Bruce

Another ton will require more air. Newer units with ECM blower allow some flexibility with CFM settings, but in general, an extra ton needs another 400 CFM of air. Failing to provide adequate airflow will raise air temperature in the short term, but this comes at the price of increased head pressure at the compressor discharge, higher compressor current, decreased efficiency and shorter compressor life. Unit may trip out on high head pressure.

The building is a wood frame, wood sided, drywall, wood floor bungalow sitting on a concrete walls / concrete floor walkout basement - all sitting on a rock that goes all the way through to the west coast of Australia.

So the answer might be stick with the "slighly undersized - pretty much rightsized" unit and instal a switch somewhere that overrides the outside temp control - so if a quicker setback recovery is desired the switch can be thrown and allow aux to come on?

That's the thing, your system appears to be designed okay for your needs, but by interupting the aux. coil you are interfering with it's recovery abilities. Please do not take this as anything other than observation; you can't have it both ways- speedy recovery with right sized unit=aux. coil. As you can see, we contractors are diveded as to the benefit of set back on geo. I understand set back for you is a comfort/preference (if mama's happy....), not an economy issue. That said, turn the aux heater on and you can make Mrs. jambsi happy, have decent recovery time, and still save money over a fossil system. Insist on recovery without aux coil and your heat pump will be oversized. If your contractor is willing to do that for you at no charge....great! If you would have to pay for the upgrade, I suggest you pay the extra $100/yr for aux heat.
Good Luck,
J

Thanks for all the education & experience.