Isotherm Water Heater Expansion Valve Leak or ?

[Geoff H.]

In an earlier thread I mentioned that my 6 yr old Indel Isotherm (10 gal) water heater seems to be the source for a good amount of water in my bilge. Someone responded that this was probably normal, the expansion valve just doing it's job.
But I'm not sure. This valve leaked a few yrs ago and I replaced it. It's leaking again, exactly like it did before the last repair, and way more than I think it's supposed to, (over a cup overnight) and probably has been doing so for awhile.
A few facts:
1) I leave the heater off except when I plan to take a shower, turning it on before bed, and off again before I get in the shower in the morning.
2) The heater is mounted on it's side, so the valve, along with all other fittings, sits under water 'pressure' constantly, (as opposed to if the heater were standing upright, where I assume the valve would not have water sitting against it -- the point or question is, does mounting the heater on it's side contribute to pressure valve spitting, dripping, etc.?
I don't really have a choice regarding this positioning, there's not a place suitable for an upright installation.
3) I've manually engaged the valve a number of times, letting it "blast" water, in case there was some debris caught there, but that doesn't help.
4) I called Indel in Fla. on Friday to discuss, but got VM and no call back; I'll try them again on Monday.... but in the meantime, does anyone else out there have any experience with this problem.

I've got a pan sitting under the valve now (a cup wasn't big enough - it just ran over overnight.) But I sure would like to solve this and get rid of the running water!!!