Will this tankless electric hot water installation work as I expect?

Question

I am planning to install a small electric tankless/instantaneous hot water unit in my bathroom.

My question is only related to the connected water pipes. Can I connect the incoming cold water pipe directly to the equipment and keep continuously under pressure the tankless electric hot water equipment? Then I would like to connect the shower tap to the output part so that I can control the water flow by opening/closing the water by the output side.

I could see many installations of tankless electric hot water equipment where the input flow is controlled this way. But in my case, this installation type will not work because I would like to keep working on the current hot water pipe system as well. So the head of my shower must connect to the wall tap, as it is now.

Can cause any problem if I keep continuously under pressure the new tankless electric hot water equipment?

This is what I would like to have:

                                 tankless
                               instant heater
                               in  the shower
                                   |   |
             boiler tank in        |   |
              the basement         X   X manual valve
                 |  |              |   |  open/close
                 X  X manual valve |   |
                 |  |  open/close  |   |
                 |  |              |   |
--> cold water --|  |-> hot water -|---+-+
                                   |     + -- mixing valve -- comfortable shower
--> cold water --------------------+-----+

Answer 1

You're working under a flawed assumption!

From your comment:

made to have constant pressure: not really sure. I have seen more instead equipment where the input is controlled then the output. That means, the equipment is only under pressure while I am having a shower. Not continuously! That's why I asked it. – zappee 6 hours ago

1. If the pressure going into the water heater is controlled by the shower valve, that means the heater is installed between the valve and the shower head.
   • This would, in fact, remove pressure from the heater when the valve is closed.
2. If the heater is installed between the shower valve and the shower head, then you're taking a shower at the full heating potential of your instant heater.
   • ALL water (both hot & cold) go through the shower mixing valve, then that mixed water goes through the heater and is heated to "full hot", comes out the shower head and, likely scalds you.
3. If you can use the shower mixing valve to control the water temp coming out of your shower head, then your heater MUST be installed prior to the mixing valve, and is, therefore always under full household water pressure.

Therefore, your premise is flawed and your understanding of how the showers you've used is incorrect.

This means that you have absolutely nothing to worry about.

You're thinking that showers are plumbed like this:

---- hot water ---+
                  +--- mixing valve --- water heater --- FULL HOT shower
---- cold water --+

When, in fact, they're plumbed like this:

--- hot water --- instant heater ---+
                                    + -- mixing valve --- comfortable shower
--- cold water ---------------------+

or like this:

--- cold water --+-- instant heater--+
                 |                   + -- mixing valve --- comfortable shower
                 +-------------------+

---

Original, less detailed, answer

You've asked the same question twice:

Can I connect the incoming cold water pipe directly to the equipment and keep continuously under pressure the tankless electric hot water equipment?

Can cause any problem if I keep continuously under pressure the new tankless electric hot water equipment?

To answer that question:

Every hot water heater, electric or gas, tanked or tankless, is ALWAYS under pressure. If not, no water would ever flow through it.

Usually, the pressure is as supplied by the city, sometimes it's as you supply it from your well, or a pressure reducing valve (or a pressure increasing pump, if needed), but they're always under pressure.

To know for certain if your instant hot water heater can take being pressurized all the time (and I cannot fathom how it couldn't - it's designed to run at "normal" house water pressure when running, so I'm not sure how it wouldn't take that pressure when water's not flowing through it), consult the owner's manual. Since you haven't shared the make/model with us, we can't answer that for you.

If your question is really about how you're planning on plumbing it, then please ask a new question specifically asking about that.

Answer 2

The proposed connection of the tankless WH in parallel with the boiler would not be a proper connection to the hot water side of the shower valve. The direct connection from the boiler would have much more flow rate than the flow through the small tankess water heater.

This would prevent virtually any flow through the tankless water heater and it would probably not even turn on. These tankless WHs only turn on when the flow exceeds a certain flow rate. Water only flows through a tankless WH if the pressure at the inlet exceeds the pressure at the outlet. By connecting the outlet of the tankless WH to the boiler hot water line you would insure that there there is no pressure difference.

EDIT

The current arrangement where the inlet to the tankless WH is fed by the cold water line is one correct way to plumb this. It is the most efficient way because it does not use any boiler hot output. When the shower hot valve is opened the first water to enter the WH is the water in the pipes in the house which is at ambient temp of the house. At some point the cold pipe is filled with water from outside the house which may be very cold. When this enters the tankless WH it will try to meet the temp setpoint by reducing flow rate.

If the inlet water is very cold, then WH will give minimum flow and the output will be less than the setpoint. With a very low flow shower head (say 1 gal/min or less) it could possibly be usable for someone who was willing to accept that in order to conserve energy, but if the inlet water wass very cold it might not.

If conserving water and shortening time for hot water to arrive at the shower is the goal, then one would use the boiler hot output to feed the tankless WH. This is assuming that this particular model is capable of accepting hot feed. When the shower is first turned on after a long period of non-use the tankless immediately turns on and heats the water (which has been sitting in the hot line and is now at ambient temp of the house) as well as it can. Flow rate would be medium to low.

Soon the hot water from the boiler output gets to the tankless heater and the tankless heater (if it has output temp control) would increase flow rate to maximum because it is meeting the temp output setpoint.