I'm considering a geoexchange retrofit on an older home in PA and am concerned that geo may not mate up with my proposed HV system. I really have very little choice given the contruction and size of the home...HV is the best option and will dictate the rest of the project.

But as geo can only heat air to ~120 degrees, will my handlers (~four 3 ton units) be running forever? Will this negate my wintertime savings? Are there better options (like conventional compressors)?

Thanks!

If by HV you mean High Velocity (Unico or similar) air handlers, then yes, geo is compatible with it as far as I know - Unico is compatible with conventional heat pumps so it should work fine, actually better, with geo heat pumps.

Hi Engineer,

Would you expand on the high velocity systems working better with geo heat pumps? My house is a retro fit that will need some zones done by high velocity. Right now I favor a contractor that uses SpacePak (if brand matters), however, another contractor said high velocity does not handle heat that well.

Thanks,

Unico is all about substantially reduced trunk and runout duct sizes. They do this by both increasing the static pressure and velocity of the air. Nothing is free, though. Increasing pressure and velocity costs more blower power. They emphasize that their systems dehumidify more than conventional systems...well, yeah, if you cut CFM per ton you get better dehu. That shift costs SEER / EER. They also move fewer CFM in heating mode. That raises leaving air temps a bit (though again at a cost in increased blower power and reduced COP). During heating season faster moving air increases possibility of uncomfortable drafts, so it is in their interest to ensure that faster moving air is a bit warmer than that from a conventional heat pump duct system.

All that said, I can see where Unico is ther perfect answer in retrofit situations where low ceilings and other constraints render conventional ductwork unfeasible. My parting comment is that the smaller ducts and higher velocities reduce margins of error - Manuals J, D, S, and T had best be strictly adhered to.

12 tons might in fact be correct given the size and age of house and windows. I imagine it would test out about as well as a screened pool enclosure on a blower door (infiltration) test?!?!?

Given the first cost, complexity and operating cost, of 12 tons of Unico (or any other system) in your shoes I'd look long and hard at what could be done to reduce load before signing up for what could easily be $100k of HVAC

Look at replacement windows, doors, caulking, adding insulation, radiant barriers etc before deciding on the system. Your load is not set in stone.

Amen to that. Get that load down with better windows and insulation.

Thanks for the responses, guys. Replacement windows are not an option...we have over 97 individual frame casements in the house, some combos of which are over 48 square feet in size. Add to that we are in a historic district, which would require new wood windows be fabricated at great cost. I'm afraid I'm stuck with doing the best I can with what I have window-wise...film and some exterior storms. The engineer seems to feel the system is sized a bit light at this point. What I have is a great system specced out, but the drilling costs are breaking my budget. I may have to settle with regular heatpumps.

Run away! Run away!

Unless I had an extended family of 4 generations in the same house or 8 kids it is hard to imagine taking on 5500 SF and 12 tons of HVAC spread across 4 systems.

Better you than I.

Happy to try to help you out, but good luck with that.

The Hysterical Preservation bunch need to either get a clue or start writing big checks

I'm well aware I live in the structural equivalent of a Hummer.

Also well aware I could live in a hermetically sealed home 1/3 the size, but I don't. I like old houses. I have to do what I can with what I can.

I am sealing as much infiltration as I can find, insulating where I can (the home is really not that bad insulation-wise) and my use of heat and A/C is as light as you can get (heat to 65-68, cool to 78-80). The home is governed by a computer system that will communicate with HVAC, security and lighting controls...in essence the system will know when we're not home and when we are...when we're asleep and when we're awake, etc....making proper adjustments to temp.

Over time we will improve things. As I rebuild windows, they will seal better. As I renovate, insulation will be upgraded. Doors will seal better, ceiling boxes will be airtight, CFLs and LEDs will reduce thermal pollution.

So why not do all that first? Thanks to a favorable contractor relation, I have the opportunity to install a Unico system this fall at material cost plus labor. Add to that my oil bill...which this year would amount to about $12,000 (1/5th of the new system cost).

So I get that the margin of error is slim on these systems and any modification to the house will shift loads enough to impact. I'm used to this after years of living with steam heat (props to the Dead Men), and I'm working with the engineer to keep the future in mind.

How long have you been in the house? If you know your heating fuel usage over the last few years we can determine your heating load far more accurately than any calculation. What area of PA are you in? For instance, assuming that $12k of oil is about 2400 gallons, if you're near Scranton that would translate to roughly a 110,000btu/hr load on a design day. If you're near Philly it would be more like 130,000btu/hr.  This also depends on the efficiency of your steam system, of course. Mine turned out to be about 65% even though the steady-state boiler efficiency was about 80%.

If you can, I'd recommend keeping the steam system as a backup and for supplemental heat. I love steam systems for their comfort and simplicity - it's too bad that in many cases they aren't economically justifiable anymore. My radiators are works of art - no way I'd rip them out even though I only fire up the steamer a few times a year now.

I use about 2000 gallons yearly for heating the house (the rest is oil fired hot water). System tests in the high 80s for effeciency, but as always in the real world...

I'm directly in between Scranton and Philly. So given less fuel and further south I assume the 120,000 btu range would fit. (makes sense given oil's btu per hour and my average use of about 10 gallons daily).

And yes...the steam system will remain...mothballed at first and possibly prepped for NG conversion.

Well, very quick and dirty:

2000gallons*138,500btu/gal*.75=208MMbtu for the season.
I'll guess 6000 degree days per year, so 208MMbtu/6000=35000btu per degree day.
For a 5 degree design temp (again just guessing, don't have my references handy) that's a design load of 35,000btu/24*(70-5) or 95,000btu/hr.

Expressed in tons of cooling this is only about 8, but in heating mode especially at worst case conditions most heat pumps won't come close to their nominal tonnage as others have noted. Still, 12 sounds pretty high. Since you are thinking of multiple units anyway, you could always start smaller and add another unit if it becomes necessary.

I'm glad you're keeping the steam. I like having the peace of mind knowing that I all I need to do is run my oil burner (which I can do with a car battery and inverter) and I can have heat with no other controls, pumps, etc. To run my heat pump during a power failure I'd need a 10kW generator at least. You'd probably need a 15-20kW unit. You must be in coal country - you could always convert your steamer to coal and use that for backup - then you'd really have some historical interest!

At 95,000 BTUh, it would be about 10 tons heating.