Sizing electric tankless Domestic Hot Water system

[katwalatapan]

Hello,

I wanted to inquire if an electric tankless domestic hot water system could be sized per apartment square footage?

I'm looking to size a tankless DHW system for a 900 sqft apartment with one bedroom and den. The plumbing fixtures which require hot water are the kitchen faucet, washroom faucet, bathtub (which also has a shower head), clothes washer, and dishwasher.

Is there a rule of thumb which I could use to size a tankless system or do I require information such as fixture GPM, minimum inlet water temperature (42 degree fehrenheit), temperature rise (140 minus 42 = 98 degree fehrenheit)? Also what would information on a spec sheet such as Temp rise @ 1.5GPM - 98 F, @ 4GPM - 49 F, @ 6GPM - 33 F mean? Do I still have to meet a temperature rise of 98 F for any flow rate?

I'd appreciate your comments (preferably with a basic example if possible) to help me size tankless domestic hot water systems.

Thank you.

 

[IRstuff]

for a single apartment?

Perhaps you need to do an audit or survey as suggested by

http://www.rheem.com/products/tank_water_heaters/how_to_size_a_water_heater

see also:

http://learn.compactappliance.com/tankless-water-heater-sizing/

http://www.homedepot.com/c/water_heater_buying_guide_HT_BG_PL

http://energy.gov/energysaver/sizing-new-water-heater

TTFN
I can do absolutely anything. I'm an expert!
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[PEDARRIN2]

You will need to know how many (if multiple) fixtures could operate at the same time and add up the flow rate (gpm) you would require. Then you have to determine the minimum incoming water temperature (assuming it is 42F). Then you need to know what the maximum water temperature you will need. For a residential, I would use 120F, not 140F.

The instantaneous water heater manufacturers will have tables which indicate the temperature rise at different flow rates for their equipment.

Your washing faucet is likely to be the culprit. Typical rule of thumb is 5 gpm (although they can go as high as 10 gpm). This has nothing to do with water efficient washers, which reduce the total volume of water used, but not the flow rate into the equipment. However, you will not find an instantaneous water heater that will give you the temperature rise at this flow rate. A lot of the heaters have a flow restrictor which helps this. More than likely, your maximum flow will be 2.0-2.5 gpm.

You are likely looking at a 20 KW version with 96 Amps to get hot enough water.

 

[LittleInch]

I think your biggest issue is the bath tub and how long anyone will wait for it to be filled up.

My instant electric shower runs at about 8-10litres/min and is rated at 9kW. I personally would not want it to fill a bath at that rate as it would take a seriously long time.

Hence my guess is you're looking at a 15 to 20kW heater (!!)

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[HVACMech]

You want to do as Pedarrin suggested. You want your gpm's, decide how many fixtures could run at once, and your inlet water temperature. There is no better way to do it really. Newer, more expensive models are better at flow diversity than older models, but you can easily ruin/shorten the life span on these if you continually run at their minimum flow values, so select wisely.

 

[katwalatapan]

Thank you very much for your responses.

It does help me figure out the required size by doing the calculations and selecting the correct size by reviewing the manufacturer's spec sheet.

 

[chicopee]

Another point that you have to consider is the voltage and amperage per heater. These devices require a serious amount of electrical power which would require you to upgrade your service electrical panel. Your best bet would be to have a hot water storage tank which will sustain the required hot water supply.