A way to temper geotherm loop

Where are you? If you're north, that seems pretty darn good. What number are you looking for?

LP condensing boiler would work. You could either run the boiler to heat up the geo loop. Or you could install the boiler to feed hot water coils in the duct work and run them when the temp is below cop or balance point.

A single house that has 5 Geothermal heat pumps?

I have at least 2 houses with 2 WA HP's. Both owners say they have never used the supplemental heaters. If they get turned on accidently they might call because they smell the dust burning off and think something is wrong. Both of these are pump and dump well water @ 50-55 degrees. That 3 to 8 degrees must make the difference.

More houses with only a single zone WWHP might need supplemental heaters on occasion. The 2 zone houses have oversized capacity with the logic applied that in AC season one unit may cover the requirements.

Would slowing down the blower deliver air temps that are more tolerable?

NG preheat of inlet water would really make that ModCon condense, max efficiency and give you some really hot air. IMHO

Does the loop flow thru the 5 HP's in a series connection? Out of the first and then into the 2nd etc.

In and out meaning source and sink loops, or in and out of the same HX circuit?

So the pumps are in series with the HX between them? Where does the expansion tank connect?

47 degree entering water temp is not bad during heating season. I would look at the manual to see what air temps are expected under those conditions. May be, all you have to do is slow down the fan slightly. Throw your gauges on the units and make sure the refrigeration cycle is sound.

I certainly would not add a boiler to the ground loop if you have 47 deg entering water temp. Also, at the end of the day, a Geo unit is not a furnace. The air will be cooler. Actually, the cooler the supply air is, the more efficiently the compressor will run due to decreased head pressure and increased mass flow rate. The desired supply air temp is always a balance between comfort, efficiency and of course, it has to be high enough to effectively meet the heat load at desired indoor temp.

It seems odd that with 47 ODT an air to air HP will deliver comfortable TEMPERATURE rise across the coil. The 47 temp is of the air/water from which the heat is extracted. Assuming the IDT is at 70 in either case, both machines have this 47 degree heat source. No, I realize if the ODT is 10 then the air/air HP is needing help. But the geothermal still has this 47 to work with.

Are you moving enough fluid thru the coil, outlet temp drop should show some drop. Or a refrigerant issue? Air flow too high?

If I am missing something please tell me, thanks.

I'd like to piggyback on the discussion. 47 deg is high. I would expect the design temperature for heating to be more like 32 deg. But for sake of the discussion let's say the heat pumps were sized based on say 50 deg loop for heating (so it is not heating sufficiently on a design day). So you can 1. replace the heat pumps (not practical), 2. Add a well or two (very expensive), 3. Install a boiler to "temper" the loop temperature.
Now thinking out loud - Using boiler you would have to heat the water from the below design temperature to the design temperature. Otherwise the heat would start going to the ground. Right? Otherwise pick up the entire heat add load on the boiler and shut off the flow to the wells. Please correct anything that I wrote that doesn't make sense.

I wonder if adding a well is the best answer.

I can't imagine that turning down the air flow would do anything but make the situation worse.

I know of a similar situation further north where the heat pumps are just not heating sufficiently on design days (<10 deg). Common piping, one pump, I would assume the loop design was more like 32 deg.