This has been discussed here many times, so please look in the archives.
A tankless water heater compared to the tank type is a very different animal.
Tankless water heaters require a much larger gas supply pipe and propane regulator, as they have to heat the water flowing through them instantly. Consequently they require a much larger vent and combustion air supply, because they are burning so much more gas.
All tankless have a flow switch to turn them on and off, and many of them will not start with low water flow, as in an RV type faucet.
I have never heard of a Girard, and don't know its input rating, but it does not have a standing pilot, so you will have to provide it with 12 Volt DC power to operate.
After having looked at their website, and read their temperature rise, I wouldn't buy one. An input of 36,000 BTU is less than many household tank-type water heaters, and their own statement of temperature rise gives it away. They say 70 degrees max, (depending upon water flow,) so if it is cold outside, your water tanks is 40, and you are taking a shower, it isn't going to be a hundred! When your better half turns on the kitchen sink at the same time, you will freeze!
I have installed at least fifty tankless heaters in houses, without a cold water complaint. I use them as a way to heat the house and the water, but they are rated 199,000 BTU, not 36,000, which in my opinion isn't enough
G
