Posted By tigerfan6 on 03 Aug 2010 08:18 PM
Dana1,
This is off topic for the Geo board, but what kind of slab edge insulation approach would you suggest in a termite prone area that requires visible concrete for inspection?  Are there any non-foam based products, like maybe a vermiculite or perlite based product that would be better in these situations?

Most houses that I have noticed around here have no visible termite barrier (doesn't mean it's not there, I just haven't noticed) and all have the slab edge exposed, probably for termite inspection.

There are vermiculite and perlite based insulating concretes, maybe that could be used to coat the edge of the slab.  Have you ever heard of something like that?

There are a couple of manufacturers of "ant & termite resistant" borate-loaded rigid EPS out there  (eg PerformGuard), and flashing the edge insulation with copper that exends under the studwall plate as a capillary break also goes a long way toward termite-proofing.  If that's not good enough for your local code folks and they insist on an inspection zone of bare concrete, most don't require more than a 2"  or 3" strip of exposure (which would still be a manageable heat gain/loss along the gulf coast.)

I've never looked into perlite/vermiculite concrete- I'd imagine it's K value would be under R1.5/inch, but even if it's only R1/inch, that would be ~10x better than standard concretes.  IIRC there are people who mix EPS beads into the mix for non-structural "rat-slab" applications that run about R2/inch.

A couple times an inverter (I'm assuming VFD) driven compressor is mentioned in relation to a ASHP supplimental system.  I always thought these systems were great because of their unique ability to turn down (unload) and match a widely varying load, and related high efficiencies.  But one post specificaly mentioned that due to the reduced in-rush these systems were great for running on a generator.    This contradicts engineering bulletins I've read on emergency backup for critical cooling applications (hospitals, banks, data centers... etc).   Due to the non-linear power draw from a VFD, a VFD load can quickly overheat and damage a generator.  If I remember correctly they recommend that a VFD should not be the only load on the generator and should generally be less than ~50% ?? of the total connected capacity at any given time.  I can't remember where I put the bulletin now... so I'm uncertain on the numbers.   Since, my water well is a constant pressure system run by a VFD, this particular issue is relevant if I ever wanted to size/buy a generator that could run both my gshp and my well pump at the same time. 

Anyone out there have experience with running VFD's on a generator ?

I work with VFD's and soft starts. The generator is no problem for a VFD as long as the generator is regulating the voltage normally, i.e, if everything else works, the VFD will work just fine. VFD's are often used on larger motors on oil rigs just because they reduce the inrush to usually 100 - 150% of motor FLA, depending on the load and how fast the motor must reach speed setpoint. The non-linear load you mention is due to harmonics, which are compounded by the higher impedance typical of a generator. Long story short, there are things you can put between the VFD and the generator that will keep them both happy if your VFD driven load is a high percentage of total load (line reactors, harmonic filters), you can use a more expensive VFD with a more sophisticated "front end" which produces less harmonic distortion, or you can manage the load such that the VFD is only about 1/2 the load on the generator and there is other linear load running. If you were running a VFD air conditioner in a house with a refrigerator, freezer, a couple of lights, etc, you will probably have no problem. You can't even hear the generator load up when a VFD driven motor is started if the start up ramp is five seconds or so. A softstart is also an excellent way to reduce motor starting load on a generator.

I should have read your entire post before responding. Your well pump is VFD driven and your GSHP is not, therefore your well pump produces a non-linear load with about 100% current distortion and your GSHP produces a linear load (0% distortion). If both motors are the same power and at FLA (doubtful) you'd have 50% current distortion. IF you put a line reactor in front of your VFD you'd have 25% at worst. Most likely, your well pump is sized less than your GSHP, also, on the VFD your well pump, which is typically a centrifugal pump, is going to draw less amperage if it is running slower than 60 hz, which is almost certainly the case and will reduce this concern. A 10% reduction in speed of a centrifugal pump reduces current (and therefore power) to 73% of full load amps, so if your well pump is modulating at all, your exposure to this issue is lessened because the current draw of the non-linear load is less. Bottom line, if you have a problem, limit the speed of your well pump if you can. Problem is you probably won't know if you have a problem until breakers start tripping, which is no big deal in a home after a hurricane but is a big deal in a hospital using critical back up power.