For a Maricopa County restaurant, add up the hourly hot water demand of every sink and machine from the county table, at a 60 degree F rise. Heater recovery must equal that total, and the county may allow a 75 percent factor. Tanks start at 50 gallons for open food service and 75 with a dish machine.
The rules that set the size
The guide gives four rules that apply to every food business.
- Use a 60 degree F rise. "To properly size a water heater in Maricopa County, the required temperature rise shall be 60ºF when calculating hot water recovery."
- Match recovery to demand. A storage heater "must have a recovery rate equal to or greater than 100% of the calculated hourly hot water demand, in gallons per hour (gph)."
- Use the manufacturer's numbers. The guide says to use the maker's listed recovery rates "in conjunction with a 60ºF water temperature rise" and to refer to "cut sheets" for the particular heater.
- Give each business its own heater. Each establishment "shall have its own properly sized dedicated hot water supply source." A shared facility with three or more permittees under different owners can be approved for a single heater if it is properly sized.
The plan review checklist adds minimum tank sizes. A facility that only sells prepackaged food needs a 30-gallon heater with a 100 percent recovery rate, and it can drop to 10 gallons if it has no more than one mop sink and two lavatories. An open food or beverage business needs 50 gallons. One with a mechanical dishwasher needs 75. The county guide lists the same three tiers. The checklist says heater sizes "may be adjusted based on the number of sinks and the type of facility," so these are floors, not targets.
Step 1: Add up the hourly demand
Table 1 in the guide lists average hot water use in gallons per hour (gph) for common equipment. Each row has a High column and a Low column. The guide says "High" is for "multi-use eating utensils," such as a full-service restaurant, and "Low" is for carry-out operations that use single-service utensils. Some of the rows:
| Equipment | High gph | Low gph |
|---|---|---|
| Hand sink | 5 | 5 |
| 1-compartment food prep or rinse sink | 15 | 15 |
| 3-compartment food prep sink | 45 | 45 |
| Pre-rinse hose sprayer | 45 | 45 |
| 3-compartment warewash sink | 60 | 45 |
| Service (mop) sink | 15 | 5 |
| Automatic warewasher, door-type single rack | 65 | 40 |
| Bar 3-compartment sink | 20 | 20 |
| Showerhead | 60 | 20 |
For equipment that is not listed, the guide says to "refer to the manufacturer's specifications." The guide also gives a shortcut for small sinks of "5-gallons per compartment." If you have a sink of an unusual size, the guide has a formula: multiply length, width and depth in inches, divide by 231 to get gallons, and multiply by 0.75 because sinks are filled about three-quarters full.
Step 2: Apply the 75 percent allowance if it is granted
Not every fixture runs at the same time. The county recognizes that. The guide says "a 75% allowance of the calculated hourly hot water demand may be granted if deemed appropriate (based upon fixture demand), and approved by the Department only when sizing the hot water storage capacity for conventional tank water heaters."
Two cautions come with it. The allowance is not automatic, since the county has to approve it. And it is not the same as the 70 percent rule used for first-hour rating, which the guide keeps separate.
Step 3: Check the tank and the first-hour rating
Recovery is how much water the heater can heat in an hour. Storage is how much hot water is waiting when the rush starts. The guide explains the link with the first-hour rating: "(Tank Capacity) x .70 (70%Rule) + (Recovery Rate) = First Hour Rating." Its example is a 60-gallon gas heater of 50,000 Btu with a 60 degree F rise, which recovers 77 gph. That gives 60 x 0.70 + 77, or 119 gph in the first hour.
Our own arithmetic on the guide's example shows where 77 comes from. Heating a gallon of water one degree F takes about 8.33 Btu. So 50,000 Btu an hour over a 60 degree F rise is about 100 gph at perfect efficiency. The guide's 77 gph means the heater is treated as about 77 percent efficient. Real heaters vary, which is why the county sends you to the manufacturer's cut sheet.
The guide's own sample calculations show how the pieces fit:
- 1A carry-out business at 70 gph with the 75 percent allowance needs 53 gph of recovery and a minimum 50-gallon tank.
- 2A full-service business with multi-use utensils and no machine, at 105 gph, needs 79 gph and a 50-gallon tank. The guide says "a 75-gallon storage capacity water heater is highly recommended."
- 3A full-service business with a dish machine, at 175 gph, needs 131 gph and a 75-gallon tank. The guide says "a 100-gallon storage capacity water heater is highly recommended."
A worked example
This is our own example with the county's table values, not a county sample. A small full-service restaurant has two hand sinks, a 3-compartment warewash sink, a pre-rinse sprayer, a 1-compartment prep sink and a mop sink. The High column gives:
- Two hand sinks: 2 x 5 = 10 gph
- 3-compartment warewash sink: 60 gph
- Pre-rinse hose sprayer: 45 gph
- 1-compartment prep sink: 15 gph
- Service sink: 15 gph
The total is 145 gph. If the county grants the 75 percent allowance, the target is 145 x 0.75, or about 109 gph of recovery at a 60 degree F rise. Without the allowance, it is 145 gph. The storage minimum for open food service is 50 gallons. The guide is not fully consistent here, though. Its allowance example on page 22 turns 105 gph into 78.75 gph and says "a minimum 75-gallon tank water heater is required," while its Table 2 sample with the same numbers gives 50 gallons. At 109 gph a reviewer may well ask for 75 gallons, so plan for that. Add a door-type dish machine at 65 gph and the total is 210 gph, or about 158 gph with the allowance, and the tank minimum moves to 75 gallons. A machine also raises the booster question, covered in our page on whether a restaurant needs a booster heater.
### Tankless heaters and other options
Tankless sizing uses flow, not recovery. The guide says a tankless heater "must be sized properly, in gallons per minute (gpm)" using a 60 degree F rise, and it says typical units "can provide hot water at a rate of 3 to 6 gallons per minute." It warns that a small unit may not cover a busy kitchen: "using sinks and running an automatic warewashing machine may exceed the hot water supply capability of a tankless water heater." The fix it gives is "multiple tankless water heaters connected in parallel," or a separate unit just for the machine. The plan review checklist says tankless heaters "may be approved but require prior review and approval." Small under-sink "insta-hot" heaters are limited to single point-of-use jobs like a hand sink.
Every properly sized heater, tank or tankless, still needs "final on-site evaluation and approval" by the county. Our page on types of commercial water heaters compares the options, and why a commercial water heater runs out of hot water covers what happens when the size is wrong.
### Before you buy
Do these five things before you order a heater.
- 1List every hot water fixture on the plans, including bar sinks, mop sinks and hose reels.
- 2Decide High or Low. Multi-use plates and glasses mean High.
- 3Ask if the 75 percent allowance applies. The reviewer decides.
- 4Pull the cut sheet for the heater's recovery rate at a 60 degree F rise. Our page on water heater recovery time explains the term.
- 5Check the gas or electric supply. A bigger heater draws more. Our page on upsizing the gas line for a commercial water heater covers that.
HQ Plumbing & Air is licensed and insured, and we can review your fixture list against the county's table and tell you what the heater has to do. We offer free estimates and upfront pricing.