Weird request from engineer. opinions?

[Salesdog]

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.

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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?

 

[Dana]

With an off grid home the last thing you want is a geothermal heat pump (or any hydronic solution) and active HRV with pre-heat. The amount of battery and panel it takes to make even modest amounts of space heating viable over a couple of cloudy days is enormous.

Spending the money on higher performance building envelope to a level where point-source heating (no pumps or fans) is usually a better bet, and even at -20C freeze-up is no longer a problem.

How much of this house and solar + backup or geothermal system is already built?

 

[Sponsor]

 

[Salesdog]

The house is built, Solar is ready to go online. Equipment is onsite and starting install in the mechanical room.

My question is more in regards to combining a geo preheat on an erv with a Geo loop designed for GSHP. I have never seen it. I have seen a seperate horizontal Geo field for the ERV preheat but never a combined loop.

 

[Salesdog]

With an off grid home the last thing you want is a geothermal heat pump (or any hydronic solution) and active HRV with pre-heat. The amount of battery and panel it takes to make even modest amounts of space heating viable over a couple of cloudy days is enormous.

Spending the money on higher performance building envelope to a level where point-source heating (no pumps or fans) is usually a better bet, and even at -20C freeze-up is no longer a problem.

How much of this house and solar + backup or geothermal system is already built?

Its a passive house, it has 2' of insulation and is built of solid wood 12" thick. It loses less then 3 degrees overnight with the power turned off atm.

 

[Dana]

Assuming the windows are comparably high-performance, a house like that can usually be heated with single ductless cold climate air source mini-split using about the same power or even less than a ground source heat pump & hydronic floor, depending on how much power your pumps are chewing through. The hydronic heated floor is overkill- it's probably never as much as 2C above room temp even when it's -25C outside. (Just sayin'...) What is the local 99% outside design temperature?

A house like that also doesn't usually need glycol in the system to prevent freeze-up.

I'd be inclined to skip the pre-heat entirely- it's not buying you much comfort, and it's not buying you ANY energy use reduction if it becomes the critical point driving a requirement for glycol in the system.

 

[Fitter30]

30% glycol with inhibitors is the minimum concentration for the inhibitors to work properly less than the 30% I've seen the water turn to a corrosive mixture. Burst temp -14c, slush temp .4c. Just wonder why did the engineer use a horizontal geo field verses boring holes and wouldn't the ground temp be higher? Is there any backup heat or electric in case system failure other than the hydrogen?