What is your house water pressure? Do you have a pressure reduction valve? With a tankless, you don't normally need an expansion tank, but that could be an issue. A pressure relief valve is a safety device. Normal house pressure should not be high enough to trigger it. It could be defective. If your house water pressure is quite high, you should install a PRV. Note, water pressure can change quite a bit from daytime to night time, with late night/early morning hours possibly the worst. When is the valve opening?