I have just had installed zip hydroboil hs003 and the water pressure is low on using it

There are several potential causes of low water pressure with a tankless water heater like the Tempra 24 Plus:

1. Flow rate limitations: Tankless water heaters require a minimum flow rate to operate properly, typically around 0.5-0.6 gallons per minute (GPM). If the flow rate drops below this threshold, the heater may shut off or fail to produce hot water. In your case, the initial flow rate of 3.4 GPM is well above this minimum, but the drop to 1.6 GPM suggests that there may be a flow restriction or other issue causing the lower pressure.
2. Water supply issues: Low water pressure can also be caused by problems with the water supply itself, such as a clogged or damaged pipe, a water main break, or problems with the city water system. Check with your local water utility to see if there are any known issues or maintenance activities that could be affecting water pressure in your area.
3. Water filter or softener issues: The whole-house water filter and softener that you installed could also be causing low water pressure if they are clogged or not functioning properly. Check to make sure that the filters are clean and that the softener is operating correctly.
4. Plumbing issues: There could be issues with the plumbing in your home, such as a clogged or damaged pipe, a faulty pressure regulator, or incorrect pipe sizing. Have a licensed plumber inspect your plumbing system to identify any potential issues.

If you have already checked for these potential causes and are still experiencing low water pressure with your Tempra 24 Plus, it may be helpful to contact the manufacturer or a licensed technician for further assistance.

check pressure in pressure tank. should be same as water pressure. low pressure in pressure tank will cause issues.

Pressure inside tank builds up?
Follow same steps used for low pressure on following link
http://waterheatertimer.org/Low-hot-water-pressure.html

Pressure below 80 psi is fine
Gene

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If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

The pressure switch shouldn't be reducing pressure unless it is smaller that the water pipe. Pressure switches in water heaters are a safety device that activate when pressure builds up above a pre-defined limit. I'm guessing that the most probable cause of low water pressure is either not enough head into the water heater (this is the height of water pipe) or not enough mains pressure. Your best bet is probably to install an inline circulation pump with a flow sensor. This will automatically detect that water is being used and turn on to boost water pressure.

Probably the factory-installed heat trap nipples are clogged.
Open following links.
http://waterheatertimer.org/Low-hot-water-pressure.html
http://waterheatertimer.org/Low-hot-water-pressure.html#trap
Noisy whistling and humming and low pressure are common problem with heat trap nipples and inserts from factory.
Replace heat-trap nipples with pex-lined nipples, or remove heat trap inserts with screwdriver or needle nose... careful not to break insert so it falls into tank ... which can then get lodged in hot water line elbow or under-sink shut off.
Factory-installed heat traps are part of overall strategy so manufactures can claim more efficient water heaters, without requiring customers to any thinking.
Other strategies include foam insulation instead of fiberglass (so that wires cannot be replaced on electric units), reducing factory-set temperature from previous 130-135 to 100-120, and re-estimating downward the hot water use for average family.
So some of the strategies are useful, while others problematic or imaginary.
Call manufacturer to pay-for or provide new pex-line nipples. Warranty for fist year covers Labor and parts, and after 1st year, warranty covers parts
http://waterheatertimer.org/Water-heater-manufacturers.html
Save $10 a month in future replacement costs: maintain water heater so it lasts longer:
http://waterheatertimer.org/How-to-maintain-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

1) If possible, it is Best to solve pressure problem before installing new shower.
http://waterheatertimer.org/Low-hot-water-pressure.html

2) If the cold water has good pressure, then Electric shower could fix problem ... but shower would have to be connected to the cold water line and not the hot.

3) Seems the expense of adding a electric shower would be greater than having plumber fix low hot water pressure.
Assuming you have access to plumbing pipes, and this is not apartment building.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Open following link for troubleshooting resources including Rheem contact information.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

High water pressure might be causing problem.
Use Brady BTG-100 pressure gauge to check pressure on nearby spigot.
Add a pressure reducing valve on incoming water line.
http://waterheatertimer.org/images/Check-water-pressure-250.jpg
Water heaters, tankless and tank-type are damaged by high water pressure 80 psi.
House plumbing fixtures also have shortened lifespan.

However if water pressure or water flow are too low, then burner will not activate.
Tankless are like bowling on a ship sometimes.

Repair most tankless water heater problems yourself, same day, any brand, for $320
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Before calling service technician, open following link for Rheem troubleshooting resources.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

List of causes MIGHT include one of following:
Bad household circuit breaker that powers electronics, or loose wire inside outlet where tankless plugs in.
Tankless fuses and control board.
Water flow rate too low. Minimum .8 gallon required (depending on model)
Tankless inlet filter clogged.
Household aerators clogged at faucets.
Crossover at single-handled faucet
If installed, there can be crossover at mixing valve.
Faulty household check valve, including check valve on re-circulation system if that system is installed.
Restriction in household water line, such as clog at elbow.
Unequal pressure in household hot and cold lines, caused by restriction in hot line. When single-handled faucet turns on, the cold water pressure is higher so hot does not flow and tankless will not turn on.
Loose wire connection inside tankless.
Unit temperature set too high.
Unit temperature set too low.
Water temperature is very hot coming out of tap, so more cold is added, but that reduces amount of hot water flow, so tankless turns off because flow rate is too low.
On some tankless, if water is preheated by solar or other indirect source, the burner will not activate.
High mineral content in water > delime tankless yearly
Misaligned temperature sensor
Low gas pressure. Wrong type of gas. Undersized gas line.
Too many faucets on at same time.
Faulty tankless water valve. Water valve stuck.
Fluctuating household water pressure, for example MUD and PUD district water pressure problems or water well pressure.
Faulty household pressure reducing/regulating valve if installed.
Faulty household pressure tank, if installed -or- pressure tank needs to be installed.

See image larger

-Tankless water heaters are susceptible to lime build-up if not de-limed each year.
-Hard water will clog pressure-sensor and cause failure within 2 years if filter and softener are not working.
-Water flow and water pressure sensors inside tankless are succeptible to hard water deposits.
-Low water pressure from clogged pipes to municipal water problems to water well pressure valve can affect tankless burner
-Low-flow shower heads may not engage burner.
-With tankless-gas water heaters, sensors inside combustion area detect when incoming air flow is not adequate ... inspect inflow
-Your manual probably has a troubleshoot section.
-The manufacturer will refer you to installer.
-If you self-installed unit, that will void warranty to my understanding.
-There are water heater forums 1 2 3 4
-There are plumbers who will each have a different opinion
-There are Fixya experts who get paid to tell you the above.
-And there are local plumbers who are authorized dealers of the brand of tankless who get paid to fix them

there shouldn't be a proble it actually would work less if you are getting low pressure,, but how did you come to the conclusion that it is low pressure,, do yo have a guage, you should not loose that much pressure on any fixture