Bosch Aquastar Instantaneous Water Heaters

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Are you tired of wasting energy and money by constantly heating
 and re-heating a tank of water?


Do you have a large family and frequently run out of hot water just when itís your turn to take a shower? If you answered "yes" to either of these questions then an Aquastar instantaneous water heater by Bosch may be the solution for you. Why you ask? Simple. As their name implies an instantaneous water heater only heats the water when you need it and as long as you supply it with natural gas or propane and cold water youíll never run out of hot water again.

Bosch does this with their simple flow-through design which only turns the gas burners on when a hot water faucet is opened and water is flowing through it. They have several units available to meet your needs based on the gallon per minute flow rate and temperature rise that you desire. There are limitations to an instantaneous water heater in that you canít open every hot water faucet in the house at the same time and expect them all to deliver piping hot water. The higher the flow rate through these units, the lower the temperature rise.


The most popular Aquastar line for residential use is the 125 unit, and there are several models within this line which are all listed in the chart below. Basically all of the 125 models can provide about 3.3 gpm of hot water with a 55į F increase in water temperature from that of the incoming cold water supply. This should be enough to supply one major outlet (bath tub, shower, washing machine, etc.) or two minor outlets (sinks) at a time. The 125 line offers three different options to ignite the gas burners. The 125FX uses a 120 VAC piezo electronic ignition, the 125B uses a standing pilot light and the new 125HX uses the water flowing through it when you open a hot water faucet to power a piezo electronic ignitor. Kind of like having your own miniature hydro electric dam built inside!
Their 125BS model uses a pilot light for ignition and it can be used in conjunction with a solar water heating system where it will automatically adjust its burners (or not even turn them on at all if it isnít needed) based on the temperature of the incoming water. The Aquastar 38B model is well suited for small homes or cabins where the flow rate requirement is small. Bosch recently expanded their Aquastar line with the addition of the 240FX model which is actually manufactured for them by Takagi industries. The 240FX needs a 120 VAC source to ignite the burners and it will provide 4.75 gpm at a 55 degree increase in temperature which should supply two showers at the same time (unless you arenít using low flow shower heads).


In February í02 I replaced my old small tank-type electric water heater with an Aquastar 125B propane unit and overall have been very pleased with its performance. My electric bills are less than half of what they were with the old electric water heater (honest) and even when I have guests, nobody ever gets a cold shower. There are some things that are a little different about owning and using an instantaneous versus a tank type water heater, but they just take getting used to. First is that the Aquastar 125 needs a minimum of 0.5 gpm to light the burners (0.75 gpm on the 240FX). All this means is that you canít get a small trickle of hot water out of your faucet. The other difference is that the Aquastarís outlet water temperature varies slightly with water pressure. I have a well pump and small 30 gallon pressure tank with a 30-50 psi pressure switch. If you start your shower when the pressure tank is full, the water temperature decreases slightly along with the pressure requiring you to adjust the faucet a couple times before your shower is done. If you are on city water or have a larger pressure tank than I do, then the water pressure wonít fluctuate as much during your shower and this wonít be a problem. Call us.
Model 240FX 125FX 125HX 125B 125BS 38B
Gallons per minute
@ 50 degree rise 5.2 3.6 3.6 3.6 3.6 1.2
@ 55 degree rise 4.8 3.3 3.3 3.3 3.3 1.0
@ 65 degree rise 4.0 2.8 2.8 2.8 2.8 0.9
@ 75 degree rise 3.5 2.4 2.4 2.4 2.4 0.8
@ 90 degree rise 2.9 2.0 2.0 2.0 2.0 0.6
Energy Factor 0.84 0.78 0.78 0.69 0.69 N/A
Efficiency (percent) 83 80 82 82 82 80
BTU/Input (thousands) 165 125 117 117 117 40
Minimum flow rate (gpm) 0.75 0.5 0.5 0.5 0.5 0.6
Ignition source 120V 120V Hydro Pilot w/
Pilot Pilot
Flue size (inchs) 4 5 5 5 5 4
Water inlet (NPT) 3/4" 1/2" 1/2" 1/2" 1/2" 1/2"
Gas inlet (NPT) 3/4" 1/2" 1/2" 1/2" 1/2" 1/2"
Height (inches) 24.5 29.75 29.75 29.75 29.75 25.38
Width (inches) 16.5 18.25 18.25 18.25 18.25 10.62
Depth (inches) 8.3 8.75 8.75 8.75 8.75 9.12
Shipping weight (lbs.) 65 52 44 44 44 25
Price $1,299 $949 $799 $699 $862 $441