Considering replacing water heater with tankless

[raist101]

We are currently considering replacing our electric water heater with an electric tankless model and I have a few questions.

The electric for the new unit is not a problem, I'm only worried about issues with the plumbing since we currently have an issue in the house where turning on any cold water (faucet, flush a toilet, etc.) causes our hot water to "go cold" until the cold water is shut off (toilet fills, faucet is shut off, etc.).

The house is one story, on a slab, about 45 yrs old, the water heater is in the attic, the lines throughout the house are mixed 1/2" and 3/4" copper (mainly 3/4") under the slab, the main supply is 1" PVC which steps down to 3/4" where it enters the house, and there are no backflow devices anywhere in the system (except for one I installed before a hose bib outside).

Questions:
1) Is there any way to isolate why the hot goes cold as described above?
2) Could this cause problems with a tankless system?
3) There is no thermal expansion tank, check valve or vacuum relief valve currently on the supply line, should I install a tank, check valve and/or vacuum relief valve? With current unit, with tankless unit?
4) Since the water heater will be the high point of the system, would I need to do anything special to purge it?
5) Anything else I may have overlooked?

[joed]

1. Do you have any single handle control faucets or tub controls? Probably one of them is bad and allowing the cold water flow into the hot lines.
2. Tankless won't fix the problem. identify the problem and then consider a tankless.
3.Not unless yo have a PRV or back flow preventer on your main line.
4.Leave the power off until you have cold water coming out the hot taps. After the plumbing is connected turning on the main supply and opening a hot tap will purge it of air.

[suemarkp]

Are you sure the electricity for the tankless won't be a problem? Most need 80 to 100 amps and generally have two or three 40A to 60A circuits feeding them. About the only house that wouldn't need an upgrade is one that has mostly gas appliances and a 200A service.

For your other questions, I agree with joed.

[raist101]

1. Do you have any single handle control faucets or tub controls? Probably one of them is bad and allowing the cold water flow into the hot lines.
2. Tankless won't fix the problem. identify the problem and then consider a tankless.
3.Not unless yo have a PRV or back flow preventer on your main line.
4.Leave the power off until you have cold water coming out the hot taps. After the plumbing is connected turning on the main supply and opening a hot tap will purge it of air.

1. No - everything is 2 handle, could old 2 handles cause this as well? Some of them are original to the house.
2. Didn't think it would fix the problem, wondering if the problem could cause an even bigger one with the tankless if we don't get it sorted out before installing the tankless
3. thanks
4. thanks

Are you sure the electricity for the tankless won't be a problem? Most need 80 to 100 amps and generally have two or three 40A to 60A circuits feeding them. About the only house that wouldn't need an upgrade is one that has mostly gas appliances and a 200A service.

Positive electric will not be a problem ;-) Right after we moved in, we had to upgrade the house from 200A to 300A service (highest residential available here without a second pan - or them changing the transformer $$$$$). We couldn't get a permit for a subpanel in a detached shop without it - at the time everything in the house was electric and old (translated, energy wasters pulling huge amps), we have since replaced some of the culprits so the load is not even close to the capacity we have. We're at about 60% capacity right now - adding the extra 50A (80A - existing water heater's 30A) required by the model we are looking at will only put us at about 77% - and this is just by doing paper calcs., load only hits this when I'm running a saw in the shop with a dust collector, the dryer's running, the stove and oven are on, the A/C is running full tilt, every tv/light/fan in the house is on, etc.