Water Pressure, Sprinkler Backflow and noise

[Rimini]

Hi all, I recently moved into a brand new house which has a residential fire sprinkler system installed, with the equipment located in the garage. A few weeks ago we started experiencing some water hammer noise and a plumber came and saw our mains water was surging up to ~140PSI at its highest, and replaced the Pressure Reducing Valve on the main line which solved all the water hammer issues on the inside of the house.

Questions 1): The fire sprinkler system, as I understand is fed off a separate supply line (i.e the water which fills it is at mains water pressure, and this is refilled when we flush our toilet). The sprinkler system has a pressure gauge and the pressure in the system varies from ~40-130PSI throughout the day. Is this normal?

Question 2): I have started to notice that when our lawn irrigation system comes on there is a "thump" of "bang" noise which sounds like it is coming from the fire sprinkler system area. I assume this is a backflow device, or check valve working? Should this lawn sprinkler system coming on have such a large impact to cause this noise? This noise is particularly concerning to me; and I do not recall it happening last year when we moved in. When this happens the pressure gauge on the fire system flutters for a few seconds before settling again.

There is a backflow/anti syphon on the lawn irrigation too, there does not appear to be any leaks.

The annual fire sprinkler backflow test is scheduled for later in the month.

 

[Fitter30]

Backflow works when the supply water pressure drops lower than the system side. Didn't tell us what backflow you have. Pressure reducing valve should feed all your house plumbing tha main should have enough capacity that flushing a toilet wouldn't have any effect on the sprink line. 110 lb varying pressure is alot if it is the backflow a check valve before the backflow will take care of the problem.

 

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[Rimini]

Backflow works when the supply water pressure drops lower than the system side. Didn't tell us what backflow you have. Pressure reducing valve should feed all your house plumbing tha main should have enough capacity that flushing a toilet wouldn't have any effect on the sprink line. 110 lb varying pressure is alot if it is the backflow a check valve before the backflow will take care of the problem.

I'm sorry - I'm not massively knowledgeable about the different components but I will do my best here: The main line (from the street) comes into the house/the slab; there is a T-shaped split with one half going to the fire sprinkler system and the other side supplying the house; with a Pressure Reducing Valve in line. The fire sprinkler system therefore is at the same pressure as the mains water - which seems to fluctuate anywhere between thee 40-160PSI, and the sprinkler water is purged when we flush the toilet and there is another Pressure Reducing Valve on that line. There is a double check valve backflow device on the fire sprinkler system. I have attached a photo below from a neighboring house, but it is the same set up, although the PRV is on the other side of the wall of this photo.


Is the irrigation/lawn sprinkler system downstream of the house water/PRV mentioned above? I assume so or is the lawn sprinkler fed from a different supply line from the street and therefore a street water pressure? There is a PVB (Wilkins) on the irrigation supply and it doesn't appear to be leaking.

Basically, what I see happening is that when the lawn irrigation system comes the water feeding the lawn irrigation is at a differnt pressure than the fire sprinkler system, the fire sprinkler system water is either backflowing (probably not the correct term) due to the difference of pressure on the mains supply and it is hitting a check valve or the backflow valve which is snapping shut creating a water hammer? Does that make sense? I don't know I am just trying to figure out what the noise is and attempting to fix it. Could the lawn sprinkler PVB be faulty in some way?

My muni. water company is doing a pressure test tomorrow at the street.

 

[Fitter30]

What is the orange pipe just to the left of the backflow and is it teed into the sprink line? Just valve off the backflow turn on lawn sprinkler see if the noise stops.

 

[Rimini]

What is the orange pipe just to the left of the backflow and is it teed into the sprink line? Just valve off the backflow turn on lawn sprinkler see if the noise stops.

The thinner orange pipe is the fire sprinkler drain line. The blue pipe connected is the master bathroom toilet line (with a PRV further upstream). My understanding is upon flushing the sprinkler water flushes the toilet and fire sprinkler system refilled.

By valve off the backflow you mean turn both blue handles 90 degrees? To effectively lose that system - as shown in the attached photo earlier?

The water company just came out to do their pressure test and they are showing (right now) the pressure is ~80PSI at both our spigot AND our lawn sprinkler backflow (which is also fed directly from the mains/street). We have a PRV on the house supply already. I confirmed the fire sprinkler system was also ~80PSI at this time showing that hopefully the gauge is somewhat accurate. They also said that the pressure can surge when their pumps are on but not to the levels i'm showing so they will escalate it further. They recommend I put a PRV on the feed line to the lawn irrigation.

 

[Fitter30]

Just have to turn off either valve off on backflow. When a toilet flushes the backflow for fire protection water can't flow backwards out of the line one or both checks should close.

 

[Qiyansun]

Just have to turn off either valve off on backflow. When a toilet flushes the backflow for fire protection water can't flow backwards out of the line one or both checks should close.

How did you fix the problem? I am having a similar problem right now. Thanks!