Pressure, PRVs and expansion tanks
Water Pressure, Pressure Reducing Valves and Expansion Tanks in Bothell, WA
High pressure, low pressure, water hammer, and the dripping relief valve that gets misdiagnosed as a dead water heater more often than anything else in this market.
Code requires a pressure reducing valve when static pressure exceeds 80 psi. Once you have a PRV, a check valve or a backflow assembly, your house is a closed system and thermal expansion has nowhere to go, so it needs an expansion tank. A weeping temperature and pressure relief valve on your water heater is usually that, not a failing heater. A 12 dollar gauge on a hose bib answers it in 30 seconds, and we check it free on any visit.
The most expensive misdiagnosis in Bothell
Your water heater is probably fine. Check the PRV first.
Symptoms: the temperature and pressure relief valve on your water heater drips or weeps. Toilets run intermittently. Faucets hammer when you shut them. Fixtures and supply hoses wear out faster than they should.
The sales answer is a new water heater, sometimes a very expensive one. Public BBB complaint records for large companies working this market include a 429 dollar diagnostic fee followed by a single recommendation to replace a system for over 22,700 dollars, and a separate boiler replacement quote of 22,744.85 dollars. Pressure relief valve replacement recurs across years of complaints as a suspected upsell.
What is actually happening
Code requires a pressure reducing valve when static pressure exceeds 80 psi. The moment you have a PRV, a check valve or a backflow assembly on the incoming line, your house becomes a closed system. Water expands when it is heated, and in a closed system that expansion has nowhere to go, so pressure spikes on every heating cycle. The relief valve then does exactly the job it was designed for. It relieves.
That is why a closed system requires a thermal expansion tank. If that tank was never fitted, or it is waterlogged and has lost its air charge, the water heater absorbs the abuse and takes the blame. Replacing the heater without fixing the pressure means the new one does the same thing.
Why Bothell specifically
Seven hills, 460 feet of relief, and what that does to your plumbing
Bothell sits on seven hills formed by glacial channels, with Nike Hill topping out at 510 feet. The Sammamish River valley floor is essentially at lake level, the river dropping only about 12 feet across its entire 13.7 mile run from Lake Sammamish to Lake Washington. That is over 460 feet of relief between the valley floor and the ridge tops at Norway Hill, Beckstrom Hill, Bloomberg Hill, Westhill and Shelton View.
Water systems are pressurised by elevation. If you live low, at the bottom of a pressure zone, you get the accumulated head of everything above you. That is why downtown, Bothell Landing, North Creek and the lower Waynita streets run genuinely high static pressure, and why the PRV conversation matters far more down there than it does on the ridge.
If your irrigation coverage suddenly dropped, or fixtures feel weak on the Snohomish County side of Bothell, that project may be the reason rather than a fault in your house.
Diagnosis
High, low, or neither. What your symptoms actually mean.
| Symptom | Usual cause | Right fix |
|---|---|---|
| Relief valve on water heater weeping | Thermal expansion in a closed system | PRV check, then expansion tank. Not a new water heater. |
| Banging pipes when a tap shuts | Water hammer, usually high pressure, sometimes loose strapping | Pressure reduction, arrestors, strapping. |
| Toilets running intermittently | High pressure forcing past the fill valve seal | Fix pressure first, then the fill valve. |
| Weak upstairs shower, worsening over years | Galvanized supply corroding shut, common in 1950s to 1970s Maywood and Queensborough | Repipe that section. A booster will not fix a restriction inside your own pipe. |
| Pressure collapses when a second fixture opens | Undersized or restricted supply | Diagnosis at the meter, then at the house. |
| Whole house went weak recently, Snohomish side | Possibly the Alderwood pressure reduction project | Gauge test, then PRV adjustment or replacement. |
| Supply hoses and fixtures failing often | Sustained pressure over 80 psi | PRV. This one quietly costs people thousands over a decade. |
Notice that only two rows on that table end in replacing an appliance. Pressure is a system problem, and diagnosing it at the source is almost always cheaper than replacing whatever failed downstream of it.
What it costs
PRV and expansion tank prices in Bothell
| Job | Typical range |
|---|---|
| Static pressure gauge testFree as part of any visit | No charge |
| Diagnostic visitCredited in full to any repair you approve | 75 to 200 dollars |
| Expansion tank fittedRequired on a closed system | A fraction of a water heater replacement |
| Pressure reducing valve replacement | Typically inside a 284 to 760 dollar service call |
| Water hammer arrestors | Quoted per fixture |
| Water heater replacementThe thing you may not actually need | 800 to 2,000 dollars |
Compare the middle rows against the last one. That gap is the entire reason this page exists, and it is why we check pressure with a gauge before anyone in our van says the word replacement.
Backflow
The other reason your house may be a closed system
Backflow prevention assemblies also close a system, and in Bothell more homes have one than people realise. Northshore Utility District is specific: most multi family units and all commercial and industrial properties are required to have a backflow prevention assembly, and single family homes need one too if they have fire protection systems, irrigation, boilers, or plumbing three or more floors above the water meter.
Testing is not optional either. Under WAC 246-290-490, assemblies must be inspected and tested by a Washington Department of Health certified backflow assembly tester at installation and annually thereafter. NUD puts it bluntly: if you do not test your backflow assembly on an annual basis, your water service may be terminated.
If you have irrigation, a boiler or a fire sprinkler system, and your relief valve started weeping, those facts are related. The backflow assembly closed your system, and nobody fitted an expansion tank.
Straight answers
Water pressure questions from Bothell homeowners
Why is my water heater relief valve dripping?
Usually thermal expansion rather than a failing heater. Code requires a pressure reducing valve above 80 psi, and once a PRV, check valve or backflow assembly is present your house is a closed system that needs a thermal expansion tank. If that tank is waterlogged or was never fitted, the relief valve does exactly its job. Replacing the heater without fixing the pressure means the new one does the same thing.
What water pressure should a Bothell house have?
Code requires a pressure reducing valve above 80 psi. Anything sustained above that shortens the life of every fitting, supply hose, fill valve and appliance in the house. A hose bib gauge costs about 12 dollars and gives you the answer in 30 seconds, and we check it free on any visit.
Why did my water pressure suddenly drop on the Snohomish side of Bothell?
Alderwood Water and Wastewater District is running a Water Pressure Reduction Project, lowering system pressure by approximately 30 to 80 psi across its 380 West and 370 zones. AWWD notes irrigation and fire sprinkler systems may be affected, and that any recommended testing or modifications are the property owner responsibility. Get a gauge reading before assuming a fault in your house.
Do I need an expansion tank in Bothell?
If your house is a closed system, yes, and code requires it. A house is closed the moment there is a pressure reducing valve, a check valve or a backflow assembly on the incoming line. Since Northshore Utility District requires backflow assemblies on homes with irrigation, boilers, fire protection or plumbing three or more floors above the meter, more Bothell houses are closed systems than owners realize.
Why is my upstairs shower pressure so weak?
In 1950s to 1970s Maywood, Beckstrom Hill and Queensborough homes it is usually galvanized supply corroding from the inside and narrowing the bore. It worsens over years, first draw water is discolored, and pressure collapses when a second fixture opens. That is a repipe question rather than a pressure question, and a booster pump will not fix a restriction inside your own pipe.
What causes banging pipes when I turn off a tap?
Water hammer, most often driven by high static pressure, sometimes by loose pipe strapping. Fix the pressure first with a PRV, then add arrestors and secure strapping if it persists. Chasing the noise without checking pressure treats the symptom.
Does my backflow assembly need annual testing?
Yes. WAC 246-290-490 requires inspection and testing by a Washington Department of Health certified backflow assembly tester at installation and annually thereafter. Northshore Utility District states that if you do not test annually, your water service may be terminated.
Is a pressure reducing valve worth it if I have not had problems?
If your static pressure reads over 80 psi, yes, and code requires it anyway. Sustained high pressure quietly shortens the life of supply hoses, toilet fill valves, faucet cartridges and appliances across the whole house. The valve costs a fraction of what a decade of premature failures costs, and a burst braided supply hose is one of the most common causes of major water damage.
Related
Other things we fix in Bothell
Water Heater Repair & Replacement
Tank, tankless and heat pump. Plus the PSE and Snohomish PUD rebate deadlines nobody explains.
Leak Detection & Pipe Repair
Pinhole leaks, crawlspace leaks, and the free meter test you can run yourself first.
Repiping, Polybutylene & Galvanized
Canyon Park poly-B and Maywood galvanized, diagnosed honestly before anyone sells you a whole house.
24 Hour Emergency Plumber
Burst pipes, sewer backups and no-water calls, answered around the clock across 98011, 98012 and 98021.
Drain Cleaning & Clogged Drains
Cleared through a cleanout, then cameraed so you see the line is actually clear.
Gas Line Repair & Installation
PSE owns the pipe to your meter. Past that it is yours, and it needs a City of Bothell inspection.
Service area
- 98011
- 98012
- 98021
- Kenmore
- Woodinville
- Mill Creek
- Brier
- Lynnwood
- Bothell East
- Bothell West
Relief valve dripping? Pipes hammering?
Get the pressure checked before anyone sells you a water heater. Bothell, Kenmore, Woodinville, Mill Creek, Brier and Lynnwood.