Cold-Water Sandwich Behavior
The cold-water sandwich effect is one of the most frustrating quirks of any tankless water heater, and understanding how each brand handles it makes a real difference in Tampa Bay homes. A cold-water sandwich happens when you turn the hot water off briefly (say, to soap up in the shower or rinse a dish), then turn it back on. The residual hot water in the pipe comes out first, followed by a slug of cold water that was sitting in the heat exchanger while the unit was off.
Navien attacks this problem head-on with their NPE-A2 series, which includes the built-in ComfortFlow buffer tank and internal recirculation pump as standard equipment. The small internal reservoir holds a few cups of pre-heated water that gets pushed out first when flow resumes, eliminating the cold sandwich entirely.
Rinnai takes a different approach. The standard RU and RUR series do not include a buffer tank, so to get the same sandwich-free experience you need to add either the Circ-Logic feature with a dedicated return line or an aftermarket under-sink recirculation pump with a crossover valve. Retrofitting a return line in a slab-on-grade Tampa home is often impractical without opening walls, so the under-sink pump becomes the default workaround at an extra $400 to $700 installed.
Minimum Flow Rate Activation
Every tankless water heater needs a minimum flow rate across its flow sensor before the burner fires. Fall below that threshold and the unit thinks no hot water is being called for, so it shuts down.
Navien NPE units activate at 0.5 GPM. Rinnai RU and RUR series activate at 0.4 GPM. On paper that is a tiny 0.1 GPM gap, but in practice Tampa homes are now full of WaterSense-certified 1.5 GPM bathroom faucets, 1.8 GPM rain showerheads, and low-flow kitchen aerators that put you right on the edge of that activation threshold.
If your home is loaded with WaterSense low-flow fixtures, Rinnai’s 0.4 GPM activation point gives you meaningful headroom.
Vent Material Requirements
Vent material is one of the biggest hidden cost drivers in a tankless install, and the two brands diverge sharply here.
Navien NPE-A2 and NPE-S2 units are certified for Schedule 40 PVC, CPVC, and polypropylene. In Tampa Bay, that means we can run 2-inch or 3-inch PVC straight from the nearest Home Depot or Ferguson. Material cost for a typical 15-foot run sits around $60 to $90.
Rinnai requires UL-1738 listed polypropylene (PPs) vent systems such as Centrotherm InnoFlue or M&G DuraVent PolyPro. PPs is a high-temperature engineered plastic with gasket-sealed joints. Material alone runs $25 to $45 per foot plus specialty fittings, adding $300 to $500 to the install compared to PVC.
Rheem RTGH Series: The Third Option
We install plenty of Navien and Rinnai units when a customer specifically requests them, but the brand we actively recommend to most Tampa Bay homeowners is Rheem. The Rheem Prestige Condensing Tankless, specifically the RTGH-95DVLN (natural gas) and RTGH-95DVLP (liquid propane), is our preferred install.
On spec, the RTGH-95DVLN delivers 9.5 GPM at a 35-degree temperature rise, which is plenty for a 3-bath Tampa home running two showers and a dishwasher simultaneously. Uniform Energy Factor (UEF) sits at 0.94, matching Navien and beating most Rinnai condensing models by a hair. The heat exchanger is commercial-grade stainless steel with a 12-year warranty, tying Rinnai’s top-tier warranty and beating Navien’s 10-year coverage by two full years.
The real difference is parts availability and local service infrastructure. Rheem is the dominant water heater brand in Florida. Ferguson on Hillsborough Avenue stocks Rheem flow sensors, igniters, gas valves, and heat exchangers as counter items. When a Navien flow sensor fails on a Sunday morning, we are looking at a 2 to 5 business day wait for overnight shipping from the distributor in Irvine. When a Rheem sensor fails, we pick up the part before lunch and have you showering by dinner.
Installed cost for the RTGH-95DVLN runs $2,800 to $3,800 depending on vent routing, gas line condition, and condensate drain access. FREE estimates and FREE diagnosis on every quote.
Install Costs and Tampa Gas Line Realities
A typical Tampa Bay tankless install from any of the three brands lands in the $2,800 to $3,800 range.
Most Tampa homes built between 1980 and 2005 were plumbed with 1/2-inch black iron gas line running to a 40,000 BTU tank water heater. A condensing tankless needs 180,000 to 199,000 BTU, which demands 3/4-inch line from the meter to within a few feet of the unit. Upgrading that line through a Tampa slab home typically runs $600 to $1,200.
Vent routing is the other variable. Exterior sidewall termination through the garage wall is the fastest and cheapest path. Roof termination through the attic adds roughly $400 for flashing, storm collar, and the longer vent run.
Permits run through Hillsborough County, Pinellas County, or the City of Tampa building department, typically $85 to $165.
