Tankless water heater question

[steph746]

Hello Everyone,

I have a question about the possibility of installing a tankless water heater. I'm in the process of researching various brands and models. One general question I have is about the gallon per minute rating. Should the size of a house be a factor into what level of GPM I select or is it solely based on how many appliances, showers, etc I might be running simultaneously? I have about an 1800 sq. ft. home and have been looking at tankless heaters in the 7.5-9.8 range.

Thank you in advance.


-steph746

 

[CT-18]

I have not had good luck with tankless and running multiple fixtures. I had 2 customers have us take out the tankless because of the drop in flow when multiple appliances were running. In both cases it was washer and shower. It kicked the showers down to a trickle when the washer was running. i cant stand those things

 

[KULTULZ]

Sizing Tankless or Demand-Type Water Heaters-

Tankless or demand-type water heaters are rated by the maximum temperature rise possible at a given flow rate. Therefore, to size a demand water heater, you need to determine the flow rate and the temperature rise you'll need for its application (whole house or a remote application, such as just a bathroom) in your home.

First, list the number of hot water devices you expect to use at any one time. Then, add up their flow rates (gallons per minute). This is the desired flow rate you'll want for the demand water heater. For example, let's say you expect to simultaneously run a hot water faucet with a flow rate of 0.75 gallons (2.84 liters) per minute and a shower head with a flow rate of 2.5 gallons (9.46 liters) per minute. The flow rate through the demand water heater would need to be at least 3.25 gallons (12.3 liters) per minute. To reduce flow rates, install low-flow water fixtures.

To determine temperature rise, subtract the incoming water temperature from the desired output temperature. Unless you know otherwise, assume that the incoming water temperature is 50ºF (10ºC). For most uses, you'll want your water heated to 120ºF (49ºC). In this example, you'd need a demand water heater that produces a temperature rise of 70ºF (39ºC) for most uses. For dishwashers without internal heaters and other such applications, you might want your water heated at 140ºF (60ºC). In that case, you'll need a temperature rise of 90ºF (50ºC).

Most demand water heaters are rated for a variety of inlet temperatures. Typically, a 70ºF (39ºC) water temperature rise is possible at a flow rate of 5 gallons per minute through gas-fired demand water heaters and 2 gallons per minute through electric ones. Faster flow rates or cooler inlet temperatures can sometimes reduce the water temperature at the most distant faucet. Some types of tankless water heaters are thermostatically controlled; they can vary their output temperature according to the water flow rate and inlet temperature.

SOURCE- -Sizing a New Water Heater - energy.com-

From what I have read and studied on the tankless WH (I am not a professional), it is usually their not being spec'd properly and/or installed correctly.

 

[CT-18]

Yes that is true. You also have to check gas meter for volume and the correct rise at what flow rate on the water side. We did several back in 2008 when the rebates were the draw until the homeowner's had a 700 upgrade charge on the gas meter.

 

[steph746]

I just contacted our gas company and got some information. They told me that our "service line" is 1/2 inch and can handle 742,000btu and our water meter can handle 325,000btu.

I'm not sure how these numbers compare with averages, but do you think based on them that I could install a tankless water heater and that it would work well in our home?

I contacted our water company and they had no idea of what the incoming water temperature is. I assume in the winter it would be fairly cold because we live in Maryland.


Thanks in advance for your continued assistance.

-steph746

 

[KULTULZ]

There is (was) an HVAC specialist that posted here that was very knowledgeable in tank-less. I think he favored Rinnai.

You would have to talk with such a person or a knowledgeable dealer.

-Pros and Cons of Tank-less Water Heaters-

 

[jakester]

May I ask why you want to go with a tankless wh? I had a navien gas tankless wh put in about a year ago when we moved into our current house and the only reason i did that is because my water heater was in the garage (1 car garage) and i needed to save as much space as possible so we went with a tankless wh and hang it on the wall. If i had the space I don't think i would have gone with a tankless, I don't think they are better than a regular water heater.

just my 2 cents.