Was wondering. Purchased this home a couple of years ago.  2500 square foot, 8 year old home with a 2 stage 3T FHP and a 50 gallon buffer tank supplying 2 infloor radiant zones and a indirect water tank.   It is set to only run at 2nd stage.    My neighbor said that the same installer had his set the same way and was told that is because the Heat Pump for heating only and is recharging the buffer tank (110F), when running finishing the floors and Hot Water (100F).      My question is, should I start using the 1st stage feature?   Is there any advantage to this or should I leave as is?   Heat is heat right and it is coming from 3 x 175 foot wells.   FYI I do not use the geotherm for cooling.    Thoughts on this would be appreciated.

Thanks

A very happy "heat from rock" homeowner

If it can meet load some of the time in stage one it makes sense to use it. It may run more efficiently since heat exchangers become oversize for the rate of heat transfer so become more effective. The loopfield itself may respond with higher EWT since first stage loads it less.

A downside may be that longer runtimes put more hours on pump(s) and pumps won't use any less power at lower tonnage.

There may be manufacturer specification tables for your particular model and operating conditions. Compare part and full load data.

I'm surprised that you have an 8 year old two stage water to water FHP.
I wasn't aware they had one at that time. Do you have the model # for that unit?

<!--[if gte mso 9]> Normal 0 false false false MicrosoftInternetExplorer4 <!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0in; margin-bottom:.0001pt; mso-pagination:widow-orphan; text-autospace:none; font-size:10.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} @page Section1 {size:8.5in 11.0in; margin:1.0in 1.25in 1.0in 1.25in; mso-header-margin:.5in; mso-footer-margin:.5in; mso-paper-source:0;} div.Section1 {page:Section1;} --> <!--[if gte mso 10]> I just checked with my son and he said there is a WT049 on a sticker along the side..   Would that be the model?     He said there was a sticker with a plumbing and heating contractor's name.    I could call and ask.   I never thought they unit may not be as old as the house.  There is an electric boiler there as a back-up and perhaps the unit is only a few years old...  not sure.  We moved in Dec 2008.  Appreciate your help.    

That was hard to read.

I did Google FHP WT049 and came up with that being a 4 ton 2 stage water to water FHP. Specs in a brochure suggest it runs about 10% more efficiently in heating mode in low vs. high stage. Your mileage will vary.

It is unfortunate that whoever bought the unit paid for but is not using both stages, but it may be that this unit in high stage is more efficient than a single stage unit of comparable capacity. The manufacturers tend to put their best foot (as well as heat exchangers, compressors, etc) forward in their premium dual stage models.

FHP did not have 410A 2stage units available until 2004 or there about so it's not as old as you think.
As heat loads vary throughout the year I don't see how a 2 stage unit would be more efficient running 2nd stage all the time. In the dead of winter it probably wouldn't make much difference.