if you wanted to heat (and cool the house) a 1400sq.ft shop, your house floors and forced air in your house would it be better to use hydronic air handlers to do the forced air and just run a big water to water heat pump?

or would it be better to buy a water to water heat pump for the shop, one for the house floors... and water to air for the house?

any down side to using hydronic air handlers

You could accomplish what you’re suggesting by using either a triple function geothermal HP that produces both forced air heating & cooling and full capacity hot water for in-floor radiant heating as well as DHW.

You could also use a liquid to water or DX to water geothermal HP with hydronic fan coil units to have both heating & cooling with in-floor radiant still an option as well as DHW.

Either way has its pros & cons. It all depends on what you want to do, how you want to accomplish it – and how deep your pockets are!

SR

its just the way the ducts are done in the house at this point.. i have two 25kw forced air units that look like air handlers would fit right where they are with out much duct work...

There is also some efficiency loss in going through an extra heat exchanger.

Typical air handlers use more than an order of magnitude more electricity to move the heat than using hydronic radiation/cooling panels would. Air handlers with ECM drive motors reduce that to being only 3-5x the power use. With an ECM drive air handler you can run them low speed low-temp for high efficiency on the heat pump when the heating loads are, low, which is more comfortable than the wind-chill of tepid air associated with lower-end heat-pump forced air.

From a heating comfort point of view hydronic panels/floors/radiators are WAY ahead of forced-air (far less noise, no wind-chill), and won't drive air-infiltration the way even the best-designed ducted air does.

From a cooling point of view, if you have high latent-loads you can't get away with cooling panels alone- you have to have something to force the condensation, and provide for disposing of it to get the humidity down.) At moderate sensible-loads a chilled floor or wall can feel pretty nice to bare feet, but at high cooling loads a super-chilled floor can feel pretty cold, even with beads of sweat on your brow. Depends on just how cold you need to run the floor to get the heat out- 60F feels fine on a hot day, 45F always feels cold in bare feet. If the indoor air dew points are above the temp of the floor, it'll condense, but even at near-condensing temps for extended periods could encourage mold growth in carpet or rugs on a chilled floor. (Probably not a problem in AZ or NM, but it could be in the gulf-states.)