A check valve or pressure reducing valve at the meter turns your plumbing into a closed system. Heated water has to expand somewhere, and without a tank it expands against your fixtures.
Quick Answer
It depends on whether the plumbing is a closed system. If a check valve, backflow preventer or pressure reducing valve sits at the meter, expanded water cannot push back into the municipal main and pressure spikes instead. Frisco Tankless Water Heater Pros verifies system type, sizes the tank and sets the precharge to your static pressure. Call (972) 430-9024.
Overview
Thermal expansion is straightforward physics with an inconvenient consequence. Water expands as it heats, roughly one and a half to two percent going from a 50F winter inlet to a 140F setpoint. In an open system that extra volume simply pushes back into the municipal main and nobody notices. In a closed system it has nowhere to go, and because water is very nearly incompressible, a small volume increase produces a large pressure increase. Systems that sit at a comfortable 65 psi at rest can spike well past 100 psi during a heating cycle.
What makes a system closed is a one-way device at the meter: a check valve, a backflow preventer, or a pressure reducing valve with an integral check. These are increasingly standard on municipal connections, and Texas code requires a PRV wherever static supply pressure exceeds 80 psi, which is not unusual in parts of North Texas. Most Frisco homeowners have no idea which category their house falls into. It is the first thing we determine, because it decides whether an expansion tank is a code requirement or an unnecessary purchase, and we will tell you if it is the latter.
Tankless changes the magnitude of the problem without eliminating it, and this is where a lot of bad advice circulates. A 50 gallon storage tank heating from 50F to 140F generates close to a gallon of expansion, which is a lot. A tankless heat exchanger holds well under a gallon of water total, so the expansion attributable to the heater itself is small. But the calculation is not about the heater. It is about the volume of heated water in the system, and a house with a recirculation loop holding several gallons of continuously heated water, or a unit with an internal buffer tank, has meaningfully more expanding volume than the exchanger alone suggests. Local code requirements also apply to the system, not to the appliance's internal volume.
The failures that follow are quiet and cumulative rather than dramatic. Pressure spikes shorten the life of faucet cartridges, toilet fill valves, supply line connections, washing machine and dishwasher inlet valves, and the appliance's own pressure relief. Homeowners replace a dripping fixture, then another, then a fill valve, without connecting them. A gauge on a hose bib during a heating cycle usually ends that mystery in ten minutes. And when there is an existing expansion tank, it is very often the original one from the tank heater it replaced, waterlogged, precharged to a pressure nobody ever verified, and providing no cushion at all.
Diagnosis
Symptoms rarely point at a single cause. These are the ones we find most often on North Texas systems.
Cause: The internal bladder eventually fails and the tank fills completely with water. It then provides zero cushion while still looking perfectly normal on the wall. Nothing about its appearance changes.
Fix: We test by depressing the Schrader valve at the bottom. Air means the bladder is intact; water means it has failed and the tank is replaced.
Cause: Tanks ship with a generic factory precharge, commonly around 40 psi. If your static pressure is 70 psi, the bladder is already fully compressed at rest and the tank absorbs nothing when expansion actually occurs.
Fix: We measure static pressure at a hose bib, then set the precharge to match with the tank isolated and no water pressure on the system side.
Cause: Someone reasons that a tankless exchanger holds less than a gallon so a minimal tank will do. That ignores a recirculation loop holding several gallons of continuously heated water, or an internal buffer tank.
Fix: We size from total heated system volume, measured static pressure and the relief setting rather than from the appliance nameplate.
Cause: Static supply pressure above 80 psi is a code violation on its own and stresses everything downstream. An expansion tank does not fix high static pressure; it only absorbs thermal expansion.
Fix: We measure static pressure first. Where it exceeds 80 psi we install or repair a PRV, which then creates a closed system and requires the expansion tank as well.
Cause: A two gallon tank weighs little empty and around twenty pounds full. Hung off a copper stub with no bracket, it fatigues the joint and eventually leaks at the fitting.
Fix: Tanks are supported independently with a bracket or strap so the piping carries no weight, per the manufacturer's instructions.
Cause: Placement on the wrong side of a check valve, or an orientation the manufacturer does not permit, can leave the tank isolated from the pressure it is supposed to absorb or shorten bladder life.
Fix: We install on the cold supply between the check device and the heater, in a permitted orientation, with a service valve so it can be tested and replaced without draining the house.
Process
We locate and identify any check valve, backflow preventer or pressure reducing valve at the service entrance. If the system is genuinely open, we say so and you do not buy a tank.
15-20 minutes
A gauge on a hose bib gives static pressure at rest, then we watch it through a full heating cycle to document the actual spike rather than assuming one.
20-30 minutes
If static supply pressure exceeds 80 psi, a pressure reducing valve is required regardless of expansion control, and we address that first.
10 minutes
Sizing comes from total heated system volume including any recirculation loop or buffer tank, the measured static pressure, and the relief valve setting. Not from the appliance nameplate.
15 minutes
With the tank isolated and no water pressure on the system side, we adjust the air charge at the Schrader valve to match measured static pressure. Setting it in place under pressure gives a false reading.
15 minutes
Mounted on the cold supply between the check device and the heater, in a permitted orientation, supported by a bracket or strap so the piping carries no weight, with a service valve for future testing.
45-90 minutes
We run a full heating cycle with the gauge still connected and confirm the pressure now stays within range instead of spiking.
20 minutes
Benefits
Cost
| Factor | Effect on price | Range |
|---|---|---|
| Tank size | A small tank suits a demand-only tankless system. A recirculation loop or an internal buffer tank raises heated system volume and the tank size with it. | $60-$260 equipment |
| Existing piping configuration | A clean accessible cold line with room for a tee is quick. A congested closet requiring repiping to create a mounting point is not. | $100-$450 |
| Support and bracketing | A larger tank needs a proper bracket or strap so the piping does not carry the weight. Cheap material, but it is not optional. | $25-$120 |
| Service valve | An isolation valve at the tank lets it be tested and replaced later without draining the house. Worth the small addition. | $40-$110 |
| Pressure reducing valve | Required where static supply pressure exceeds 80 psi. Installing one also creates a closed system, which is why the two jobs so often go together. | $350-$850 |
| Replacing a failed existing tank | Removal of a waterlogged tank plus replacement. Often cheaper than a first-time install because the tee and support already exist. | $180-$400 |
| Combined with a water heater installation | Adding the tank while the water connections are already open costs a fraction of a standalone visit. | $150-$300 incremental |
| Permit | Generally covered under the water heater permit when installed as part of that job. | $0-$150 |
Typical total: $225-$650 installed for most Frisco homes. Ranges, not quotes — a firm number needs eyes on the existing installation.
Comparison
| Factor | With an expansion tank | Without expansion control |
|---|---|---|
| Pressure during a heating cycle | Stays close to static pressure | Can spike well past 100 psi from a normal 65 psi rest |
| Faucet cartridge life | Normal | Shortened by repeated pressure cycling |
| Toilet fill valve life | Normal | Frequently the first component to fail repeatedly |
| Pressure relief valve behavior | Stays closed | Intermittent weeping or discharge that looks like a bad valve |
| Noise after hot water use | Quiet | Banging and knocking as pressure cycles |
| Stress on the heat exchanger and joints | Cushioned on every cycle | Repeated fatigue loading |
| Code status on a closed system | Compliant | Cited on inspection during any water heater permit |
| Cost | $225-$650 once | Recurring fixture and valve replacements |
Start with the physical constants, because the whole argument sits on two of them. Water at 50F has a density of about 999.7 kilograms per cubic metre. At 140F it is about 983.2. Divide one by the other and you get 1.0168, so heating water across that range expands it by roughly 1.68 percent. That is where the commonly quoted figure of one and a half to two percent comes from, and it is why the North Texas winter inlet of 50 to 55F matters here as well as in capacity sizing. A summer inlet closer to 80F expands less.
Now put a number on the volume. Type L copper holds about 0.0251 gallons per foot at 3/4 inch and about 0.0121 gallons per foot at 1/2 inch. A typical Frisco house with 150 feet of mixed hot distribution is carrying roughly 3 gallons in the pipe. Add a tankless heat exchanger, which holds well under half a gallon. Call the heated system volume 4 gallons. Expansion is 1.68 percent of that, or about 0.067 gallons - roughly 8.6 fluid ounces. It does not sound like a problem.
For scale, run the same calculation on the appliance this house probably used to have. A 50 gallon storage tank heating from 50F to 140F expands by 0.84 gallons. That is more than twelve times the expansion of the tankless case, which is exactly why storage water heaters made expansion tanks a familiar sight and why the assumption that tankless removes the need is so intuitive and so wrong. The expansion did not go away. It got smaller, and the pressure consequence of a smaller volume in an incompressible system is still a pressure consequence.
The reason this comes up so often here is pressure. Both IRC section P2903.3.1 and IPC section 604.8 require an approved pressure reducing valve where static supply pressure at the building exceeds 80 psi, and the UPC sets the same threshold in section 608.2. Texas recognises more than one code family, but every one of them draws the line at 80 psi. Parts of the North Texas distribution network run above that, so PRVs are common, and every PRV with an integral check converts the house to a closed system as a side effect. Homeowners buy pressure control and get expansion control as a new obligation, usually without being told.
Three practical consequences follow. The first is that an expansion tank is required by the closed system, not by the water heater, which is why the question of whether a tankless unit needs one is the wrong question. The second is that a house can become closed years after it was built, when the utility fits a check at the meter or a plumber installs a PRV for unrelated reasons. The third is that an expansion tank does not reduce static pressure and never has. If your house sits at 95 psi at rest, a tank will absorb thermal spikes on top of 95 psi and you will still be replacing fixture cartridges. That is a PRV problem and it has to be fixed first.
Bladder tank sizing uses one equation. Required tank volume equals the expansion volume divided by the acceptance factor, where the acceptance factor is one minus the ratio of the initial absolute pressure to the final absolute pressure. Initial pressure is your static system pressure and final pressure is the ceiling you are willing to reach. Absolute pressure means gauge pressure plus about 14.7 psi, and forgetting to convert is the single most common arithmetic mistake in the whole exercise.
The table below works it at a ceiling of 80 psi, which keeps the system inside the code limit rather than merely below the 150 psi relief setting. Expansion volume is 1.68 percent of heated system volume, per the 50F to 140F calculation above.
| Heated system volume | Expansion at 50F to 140F | Static 50 psi | Static 60 psi | Static 70 psi |
|---|---|---|---|---|
| 2 gal - exchanger and short runs | 0.034 gal | 0.11 | 0.16 | 0.32 |
| 4 gal - typical tankless house | 0.067 gal | 0.21 | 0.32 | 0.64 |
| 8 gal - long distribution or a modest loop | 0.134 gal | 0.42 | 0.64 | 1.27 |
| 20 gal - large recirculating system | 0.336 gal | 1.06 | 1.59 | 3.18 |
| 40 gal - large loop plus buffer volume | 0.672 gal | 2.12 | 3.18 | 6.36 |
Two things fall out of that table. The first is that the smallest commonly stocked potable expansion tank, at around 2 gallons, is comfortably more than a demand-only tankless house needs. Nobody is going to sell you something smaller, so on most jobs the sizing exercise confirms the default rather than changing it, and a contractor who tells you a tankless house needs a large tank should be asked to show the calculation. The second is how violently the required size grows as static pressure approaches the ceiling. At 70 psi static against an 80 psi ceiling, the acceptance factor collapses to about 0.11 and the tank has to be three times the size it would be at 50 psi static. High static pressure does not just stress fixtures. It makes expansion control expensive.
The most consequential step in an expansion tank installation takes about ten minutes, costs nothing, requires a tire gauge, and is skipped constantly. Tanks ship with a generic factory air charge, commonly around 40 psi. If your house sits at 65 psi static, that bladder is fully compressed the moment water pressure is applied and the tank has zero acceptance volume. It is a decorative object bolted to your cold line, and it will look and sound exactly like a working one.
Measure static pressure at a hose bib, at rest
No fixtures running, no irrigation cycling, ideally early morning when municipal pressure is at its daily peak. Record the number. Pressure varies through the day and you want to set the pre-charge against the high end of what the house actually sees, not the afternoon reading.
Stop if it reads above 80 psi
Above 80 psi static, code requires a pressure reducing valve and there is no point pre-charging a tank to 85 psi to accommodate a condition that is not allowed to exist. Fit or repair the PRV first, set it, then measure again and size the tank against the new number.
Set the air charge on the bench, with nothing on the water side
This is the step. The pre-charge is adjusted at the Schrader valve before the tank is connected to any pressurised water. A tank already plumbed and pressurised will read whatever the system pressure is at the Schrader regardless of how much air is actually behind the bladder, which is exactly why setting it in place feels like it worked and does not.
Use a gauge that resolves single psi
A bicycle pump with a low range gauge or a digital tire gauge reading to 1 psi is fine. A truck tire gauge marked in 5 psi increments is not accurate enough to match a 62 psi static reading, and the whole point of the exercise is matching.
Leave it ten minutes and re-read before installing
A tank that will not hold air on the bench has a failed Schrader core or a defective bladder. Finding that out on a workbench costs nothing. Finding it out after the tank is mounted, teed in and supported costs a second visit.
Install, then prove it through a heating cycle
Tank on the cold supply between the check device and the heater, supported independently so the piping carries none of its weight. Then leave a gauge on a hose bib and run a full heating cycle. Pressure should stay near static. If it climbs, something in the previous five steps did not happen.
Bladders fail. It is a rubber diaphragm flexing through a pressure cycle several times a day for years, and a realistic service life is somewhere in the five to ten year range, shorter if the tank is mounted somewhere hot. That is a genuine argument for the cold side placement beyond the code reasoning: a bladder living at 70F lasts longer than one living at 130F in a Frisco attic.
Finally, the thing that ought to be said plainly on a tankless page. A tankless heat exchanger holds well under half a gallon of water. On its own it produces a trivial amount of thermal expansion. The tank on your wall is not protecting the water heater and it is not there because the heater is tankless. It is there because a one-way device at the meter closed your entire potable system, and every foot of hot distribution pipe in the house, plus any recirculation loop and any buffer volume, expands into a system with no outlet. The appliance is a small part of the volume and none of the reason.
That has a practical implication worth acting on. If we determine your system is genuinely open - no check, no backflow preventer, no PRV - we will tell you so and you will not buy a tank. And if you are on the fence, the diagnostic costs almost nothing: put a gauge on a hose bib, note the resting pressure, run a long hot draw, and watch it while the system reheats. A needle that stays put means an open system. A needle that climbs 30 psi and stays there until somebody opens a tap means a closed one, and that reading is the whole answer.
Want this priced for your specific Frisco home? Talk to a tankless specialist at (972) 430-9024.
It depends on whether your plumbing is a closed system, not on the fact that it is tankless. If a check valve, backflow preventer or pressure reducing valve sits at the meter, expanded water cannot push back into the municipal main and pressure rises instead, so expansion control is required. If the system is genuinely open, it is not. Many Frisco homes have a PRV because static supply pressure exceeds 80 psi, which Texas code does not allow. We verify which case applies before recommending anything.
The volume of heated water is much smaller, so the expansion is smaller. A 50 gallon tank going from 50F to 140F produces close to a gallon of expansion. A tankless heat exchanger holds well under a gallon in total. But the calculation covers the whole heated system, not the appliance. A house with a recirculation loop holding several gallons of continuously hot water, or a unit with an internal buffer tank, has substantially more expanding volume than the exchanger alone would suggest.
Two quick checks. Tap the tank: an intact one sounds hollow near the top and solid near the bottom, while a waterlogged one sounds solid throughout and feels noticeably heavy. Then briefly depress the Schrader valve on the air side. Air escaping means the bladder is intact. Water coming out means the bladder has ruptured and the tank is doing nothing at all. A failed tank looks completely normal from the outside, which is why they sit unnoticed for years.
To your measured static water pressure, not to whatever it shipped with. Tanks commonly arrive precharged around 40 psi. If your house sits at 70 psi, the bladder is already fully compressed at rest and the tank absorbs nothing when expansion occurs. We measure static pressure at a hose bib, then set the air charge to match with the tank isolated and no water pressure on the system side. Setting it while the tank is pressurized from the plumbing gives a false reading.
It shortens the life of everything on the hot side, including the heat exchanger. Repeated pressure spikes are a fatigue load on joints, fittings and the exchanger itself, and they will eventually find the weakest point. Sustained high static pressure is a separate issue from thermal expansion and needs a pressure reducing valve, which Texas code requires above 80 psi. An expansion tank absorbs thermal spikes. It does not reduce static pressure. Some houses need both, and we measure to determine which.
The cold supply, between the check device at the meter and the water heater. That location keeps the tank on the pressurized side of the one-way device so it can absorb the expansion, and it keeps the bladder at a lower operating temperature, which extends its life. We also fit a service valve at the tank so it can be tested and replaced later without draining the house, and support it with a bracket or strap so the piping carries none of the weight.
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Background reading
Longer explanations of the tradeoffs behind this work, written for homeowners rather than trade readers.
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Coverage
We take tankless water heater expansion tank installation calls throughout Collin County and Denton County. These are a few of the areas covered.
See every Frisco neighborhood and surrounding city we cover.
Call (972) 430-9024 and speak to someone who works on tankless systems every day. Open 24 hours a day, 7 days a week.
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Frisco, TX Tankless Pros