[BLDG-SIM] Tankless Water Heaters

[Chris Balbach]

Tim:


As usual, the answer is 'it depends"... The (annual) efficiency of 
instantaneous hot water heaters is a controversial topic and is a 
function driven primarily by whether or not the a standing pilot is 
used, the frequency and duration of hot water loads, and the estimated 
amount of cold water usage 'wasted' if/when users are waiting for hot 
water to be delivered or when the minimum flow requirements are exceeded 
and the hot water draw becomes cold.

At any rate, the TREAT (Targeted Residential Energy Analysis Tools) 
software suite allows one to model the annual effects of changing from a 
tank storage type of system to a tankless type of system.

More importantly, TREAT employs a very unique savings determination 
algorithm which allows users to combine multiple individual improvements 
into 'packages' to see the interactive effects - which more accurately 
capture and quantify the 'interactive' savings that occur when multiple 
improvements are simulated. For example, TREAT could accurately quantify 
the interactive savings from changing out existing shower-heads and 
faucet aerators with 'low flow' devices, while switching hot water 
heaters from say a tank type 40 gallon electric to a tankless propane 
model, combined with improving the insulation of the DHW piping (piping 
surface areas are user specified) in unconditioned spaces from from R1 
to R4, and resetting the DHW temperature from 135 degrees F to 120 deg F. 

As always, users must provide accurate input (GIGO), if they expect 
accurate output, which is especially important when working with 
simulating an existing building.  To assist with this, TREAT integrates 
a billing analysis tool for use against the baseline building model 
which compares baseload (weather independent) and heating/cooling slope 
(weather dependent) energy consumption of the model against utility 
billing data. Good data from residential utility bills are required for 
this feature, and the data is weather normalized (corrected) using daily 
high/low temperatures correlating to the billing data period, while the 
model uses TMY2 30 year average weather data . If billing analysis is 
used to 'match' the baseline model , users can improve the confidence of 
their predictions when investigating baseload (hot water, appliance and 
lighting) energy improvements.

This is probably more than you wanted - but I wanted to make sure you 
were aware of the possibilities when you are considering annual 
simulations. You can download a 30 day fully functional trial version of 
TREAT (including program documentation) from http://www.treatsoftware.com.

Have fun - Chris


Chris Balbach, P.E., LEED AP
Manager of Energy Engineering Services
Performance Systems Development
http://www.psdconsulting.com
(607)-277-6240

tim wrote:
> Any thoughts on modeling tankless water heaters v. traditional storage 
> tank DHW heaters for a residential application?Obviously there are no 
> stand-by losses but how about efficiency(HIR)? How much more efficient 
> are these systems?
>  
> thanks




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