Cold water pressure in primary loop rises over time

[Bunny Laroche]

Hello, we noticed that the cold water pressure in the primary loop of our newly plumbed hot water heater system tends to rise over time to 30psi at which point the safety valve begins to discharge. The rise from about 15-20 psi to 30 takes about a week to occur.

We have an appointment with the gas company to turn on our gas next week so we can begin using the boiler to heat our home. So although we expect the water pressure to rise after the boiler is operation, we are stymied why the cold water pressure in the primary loop tend to rise over time. We were hoping there might be a reasonable explanation to explain this phenomenon perhaps that is not readily apparent to us?

 

[Dana]

There are a few possibilties among which are:

The seat of the auto-fill valve is seeping, letting water in slowly over time.

If the domestic hot water is served from a tankless coil in the boiler, those often develop pinhole leaks. It's rare but not unheard of for a brand new tankless coil to leak from the potable side to the boiler side.

If the domestic hot water is served from a indirect-fired tank, the internal heat exchanger may have developed a pinhole leak.

The auto fill valve usually has a full shutoff valve between it and the potable supply. It's usually a ball valve (a lever handle, not a knob, 1/4 turn from "on" to "off") on newly installed equipment. When the system is at the correct pressure, turn that valve to the off position. If the pressure is still stable in a week, it's the auto-fill. If it continues to creep up, it's somewhere else.

Also note "primary loop" has a specific meaning in hydronic heating systems, and it's probably different from what you intended.

 

[Sponsor]

 

[NY_Rob]

One of a few things... hopefully... and it has to be corrected before commissioning.

Makeup water pressure regulator valve set too high.

Makeup water pressure regulator valve defective.

If you have an indirect domestic hot water tank- there could be a leak in the internal coil that's letting street pressure water into the boiler loop.
Hope it's not that... because the first two possibilities are easier and cheaper to fix.

You can shut off the supply to the makeup water regulator valve and see if the pressure continues to rise, if it does and you have an indirect tank- you now know the problem, if it stops rising- it's the makeup valve.

.... EDIT... looks like I was composing while Dana was posting.

 

[Bunny Laroche]

The near-mirror replies is very helpful to confidently suss out the the likely culprit for the problem. Since we are not using the boiler to supply hot water for the house yet, that's a project for next year (one step at a time for us), the problem must then likely be with the "make-up water" valve/unit.

This should be easy enough to test I think since it can be shut-off at the Viega water manifold my girlfriend installed prior to the boiler plumbing, but she also installed a shut off valve after the make-up water valve before entering the boiler's primary loop.

Also note "primary loop" has a specific meaning in hydronic heating systems, and it's probably different from what you intended.

I think I described the primary loop correctly. It is basically the copper pipe loop my girlfriend installed with all the components to control the hot water system that is finally connected to two secondary loops of plastic oxygen barrier PEX pipe separated from the copper primary loop by the Zone Valves. Although the pressure rise is surely across both the primary and secondary pipe systems, I guess my girlfriend described it across the primary loop because that is where she installed the system's two temperature/pressure meters, where the rise in pressure is readily apparent over the course of a week.

I am sure we will try what you have both kindly suggested and see if the problem is indeed with the water-make-up valve unit.

 

[Dana]

For the pressure to rise the water has to come from somewhere, and if the potable is only connected at one point, that is the point of failure.

Improperly machined valve seats or grit/sediment in the water supply can cause it to not seal completely. Most of the auto-fill valves have levers where you can open it wide-open, so you may be able to flush the grit out if it's grit problem by opening up a system drain and running 10+ gallons of water into the system before closing off the drain. Either way, a complete shut-off ahead of the fill valve would verify the location of the problem.

In most systems you would have:

potable water pipe --> shut off valve --> check valve --> automatic fill valve (adjustable pressure) --> heating system/boiler

The auto-fill is often a "solution-problem", in that it prevents the heating system from running dry, but (as is likely in your case) it can also seep, resulting in overpressurizing the heating system plumbing.