I'm working on a passive house and the engineers are over complicating things as usual. However one item is bothering me and I cant seem to get through to them. Can anyone let me know their reasoning behind their design plans. We have a offgrid house with a Geo loop and heat pump serving a buffer tank and infloor heating via warm board. (which is then covered in acoustic board - compressed insulation - and then hardwood) The house has an ERV, which on the blueprints indicates a preheat coil on the air intake. The drawings do not show a filter rack or ductwork required to connect it properly. that is being sorted out and is fine. ------ However my issue is that the preheat coil is connect to the load side of the Geo heat pump. The air temperature here during winter months will consistently reach -20c, with 3c ground water acting as a preheat at a 30% propylene glycol mixture. So... the preheat coil is taking the cold water temp from the geo and then cooling it down rapidly and sending it back down in -c temperatures... does this seem like a good idea? it seems counter intuitive to me as you would be cooling your geo field while trying to take heat out of it via the heat pump to heat the house with the infloor heating... I asked why its not connected to the load side of the geo heat pump and the engineers replied that it was because of increased pumping costs to add glycol to the heating system... so the floor will have no freeze or burst protection... The second reason was the heat carrying capacity of glycol was not as efficient as water and would increase electrical consumption to run the system... The house has a solar panel wall but is not in correct orientation and WILL NOT produce enough power for the house so the engineers are concerned with saving all electrical costs. there is a hydrogen back up plant but can only sustain for 36 hours. Regardless of all that, I'm concerned about the cooling of the geo field. Im concerned about the 30% gelling up and possibly causing dead head and killing the pump. They haven't even specd a control system for it yet... What I want to propose is a fairly normal type of control for make up air units ect.. in this area. which use a 3 way mixing valve to mix supply water and return water to maintain a set air temp after coil. This would be done with a PID loop control with inputs on either side of the coil. Activated by a amp sensor on the disconnect from the ERV. The load side of the Geo would be connect to this supply water of the preheat cooil after the buffer tank. A freeze stat could be used to shut off the ERV at -5c so air would stop moving across the coil and we could maintain 18% gylcol concentration for -7c freeze protection to limit additional pumping costs and any heat transfer concerns. What do you all think? with problematic electrical the last thing i would want is no freeze or burst protection?