Hi all,

Great forum here!

This is a question about further improvement to an already well-functioning GSHP system.  The question is will adding an outdoor reset system help out much more in the following scenario.  Don't have the KW usage at hand but our electric- for-heating portion of the bill runs between approximately $40-$90 month during the colder parts of the winter (includes well-pump power). 

Setup:

4200sf in 1st/2nd living space plus 1500sf garage/workshop
(living space is tubing in gypcrete/garage is tubing in slab). 
All exceptionally well insulated and sealed (w/dual-core HRV). 
We keep the living space @ 73degF day/night/winter/summer and the garage/shop @ 58degF during the winter.
Location: Montana (northern) climate  Design temp -35degF+20mph wind.

GSHP is one Econair GW471 (4 ton) w/DSH takeoff for DHW assist.  Buffer tank is 50 gal. water heater with heating elements as backup (never needed/powered off).  It does have cooling capacity through forced-air cooling coils that are in-use not more than 2-3 months of the year.  Duty cycle at -25degF is approx 60% on.

System is groundwater sourced with 1 supply & 1 recharge well.  Groundwater temp is constant 60degF.  Fortunately, there are no water fouling/scaling problems to speak of with a LSI index of -0.1 (no nasty water chemistry for the core either).

Hydronic hi (buffer tank) temp is 92degF which is plenty for worst condtions encountered so far (-25degF/25mph wind).  I found that when it is warmer outside (say 50degF) we can run the tank at 77degF.  I don't think we could go much lower as the room temp is set at 73degF.

Which leads to the main question:  With such a limited range between the required high and minimum low buffer tank temps will an outdoor reset unit be worth it?  There are no direct numbers in the Econair engineering chart to work with so we have to interpolate.  For example our EWT is beyond their chart for heating but I think the COP would improve to some extent (using Econair's 50degF EWT chart entry should increase COP from 5.5 to 7.3 for at least some of the heating season).

I was looking at the Tekmar 256 unit as it provides all the functions required during heating season but am concerned that it might not operate quite right within the limited buffer tank working range (using auto or fixed differential) and the pre-programmed curve may not be optimal as it appears to be primarily designed for boiler systems.  I did read somewhere there was a firmware change in 2010 to the Tekmar 256 unit to better accomodate heat pumps but I cannot verify this.  Also I wasn't thinking of using an outdoor reset on the cooling side as the season is so short.

Any thoughts are welcome,

Thump

Congrats on your very efficient house and very efficient radiant system. I don't think you can increase the performance very much giving your nicely low buffer tank temps. You might as well lower the buffer tank temp in the fall and in the spring by a few degrees manually if you want to push efficiency even further.

Thanks doc,

We're always looking for ways to lower our "footprint". Even small improvements can add-up over time. The cost to purchase and install an outdoor reset is only $160 plus DIY labor so the return on investment should be ok even with modest yearly energy savings.

Our question remains: with only approx. 15 degF of buffer tank differential available can the Tekmar #256 operate properly?

Thump

Sure it can. Keep in mind that the circulation pumps will run longer at lower temperature. Make sure you have an efficient one.

A buffer tank with the lowest (assuming heating) possible temperature make sense. But if you have a high mass radiator and no buffer tank, then I think the efficiency increase happens automatically.

It works with only a 15* separation between the 2 setpoints!  I am using a Tekmar 256.  Had it in for about 2 weeks - it works great!

What is your total budget of whole-house with HW and HVAC and all high winter total billing , then lowest month...?
WHAT A GREAT RESULT FOR HEATING !

Nice to see distribution ~ the 90's, but with Tkm I will like hearing about the 0 degree day and max required entering the radiation.

is the 4200 over say 2000 basement walk out finished ?

We have only had 1994 down to 27 below in NE and Warren are by Youngstown, and one spot near the border to WVA that year by Marietta

When I get a chance I'll go back and look up the manual J budget numbers that were used. Several programs were used to run the numbers and cross-check. The actual construction and techniques we used seem to be better than the calcs indicated. We have a lot of windows (total of 49) ranging from 4 sq. ft to 48 sq. ft and they all are the *Serious Windows 925 series* plus a couple of Serious Windows french doors (all solar rejecting type). The windows are NOT in any particular solar orientation and there are several overhangs/porches shading some windows. The construction is wood frame 2x6 with 1" poly-iso foil foam-board wrap on the exterior (staggered joists over the sheathing and all taped) carried up to the truss cords. Wet-blown cellulose for walls, 17"+ stabilized cellulose for flat ceilings (with energy heels), 3" Corbon foam + 2" air space + 17" netted cellulose for all cathedral ceilings, 11" netted cellulose + air space for the 12" floor joists, 6" of Corbon in rim band (backed up by the floor joist cellulose), 6" EPS foam panels (4'H x 12'L) for crawl space perimeter stem-wall insulation. Corbon was used to boost insulation of wall top-plates and seal all exterior penetrations.

The 4200 sq ft heated living space is almost all 1st and 2nd floor with a small (240 sq ft) basement mechanical room. The rest is a "semi-finished" crawl space. One of the reasons for our good transfer is the 9" spacing of tubling in poured gypcrete (with a little portland added to mix) and all the living areas being floored in commerical tile or stone using thinset. The garage/shop areas have the same walls and windows with 2" XPS foam down 4' inside the stem walls and under the slab.

92 degF hydronic was fine with -25deg nights that's as cold as it's been so far. As to the hydronic distribution efficency question we use one Alpha constant-pressure pump.

I'll be adding the Tekmar outdoor reset in a few months....we'll see if that improves things a bit more.

Thump

Thump the manual J has fudge fact.or and is rather generic. Total usually represents peak load and may easily be 20% higher than actual usage. It does not account very well for high eff. Building practices or improvements, but it do keep a contractor outta trouble.