Tankless water heater with a low minimum flow rate?

Stiebel-Eltron Tempra 12 is listed at >=.37GPM. We installed a Tempra 29 and love it!

Brett: At this time, standard gas (or propane) tankless water heaters do not have really low minimum flow rates. The type I am refering to do not have any tank built into the water heater. The reason for the relatively high minimum flow rates is due to the size of the low end of the burner's modulating range. In rough terms, it takes 40,000 Btu/hr to keep up with 1gpm at a temperature rise of 70F (50-120). If the flow rate is 0.5 gpm, then you only need 20,000 Btu/hr, but you must also have a 70F temperature rise. If the temperature rise is only 35F (think Tucson in mid-summer, where the incoming cold water temperature can be almost 80F), then you only need 10,000 Btu/hr in the burner. Rinnai has a burner with a low end of modulation at 12,000 Btu, which is close.

If you want even lower flow rates, you need even smaller burners. You alos need smaller burners if you want to provide supplemental heating to a preheated soruce of hot water, such as solar thermal. Here there can be as little as a 1F temperature rise (the water is almost, but not quite, hot enough). It the flow rate is 0.25 gpm, then we only need 125 Btu/hr (36 watts)! This is more than 100 times smaller than the low end capacity of most tankless gas water heaters.

Right now, Navien's water heater that has a built in 0.5 gallon tank can provide hot water at lower flow rates. US law defines a tankless water heater as one with no more than 2 gallons of water and large burner capacitities relative to the volume, so this model is legally a tankless water heater. Also, although not tankless, the new Eternal Water Heater (by Grand Hall) will provide hot water at low flow rates. This sealed combustion unit has a tank of about 4 gallons.

If you are are willing to look at electric tankless, then there are several companies that have models that can provide hot water at low flow rates, generally around 0.2 gpm and at least one can go lower. Look for whole house models from Seisco, Skye and Hubbell. Also a wide range of smaller models from these two companies and from Steibel-Eltron, Bosch, eEmax and American Hometec. There are more than 12 manufacturers of electric tankless water heaters suppling the US market.

The key to providing hot water at low flow rates with tankless water heaters is to match the combination of flow rate and temperature rise to fall within the range of the water heater's capacity. The other way is to have some amount of stored hot water (generally called a tank!) that can be used to accomodate low flow rates as well as provide for some amount of peaking capacity (which depends on the stored volume.)

By the way, having a water heater with a tank is not bad. What we should be asking about all water heaters is what is the thermal efficiency, what are the standby losses (for both heat loss and for the power supplies) and what are the cycling losses (as the water heater ramps up to full temperature)? We want high thermal efficiences and low standby and cycling losses, regardless of technology or fuel source, right?

Hope this helps.

Thank you Gary.
You are right. Ultimately we should focus on the efficiency of the unit regardless of type. I am not really hung up on tankless water heaters. If extremely high efficiency can be achieved with a hot water storage tank I'm all for it.
In your opinion, what are the better storage tanks out there? Preferably closed combustion units.

First question is what do you have for a heating plant? Can you create a combined heat and hot water system with a single burner?