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how to determine size for tankless water heater?


I live in a suburb about 30 miles from Chicago. We get our water through a pipeline from Lake Michigan. In the dead of winter, my current conventional gas 40 gallon water heater can only give me about a 10 minute shower before starting to get cold.

I need to replace that heater and am considering tankless. How can I determine the capacity I need? I live alone and very rarely use any two hot water consumers (i.e. shower and dishwasher) at the same time. I don't have radiant floor heating, so that is not a concern.

Also, if I need a high capacity to overcome that cold incoming water in January, will I need a bigger gas pipe? I *think* I have 3/4", maybe 1", coming into the house, but after it gets split and reduced I'm pretty sure it's 1/2" at the water heater.

Finally, the current one sits right next to a brick chimney. Can a tankless be mounted on the chimney to minimize rerouting the plumbing all over the place? (This question is NOT asking about using the chimney for venting the new tankless.)

Thanks for any advice,

Re: how to determine size for tankless water heater?

You need to know a few things to be able to determine the right size for you.

Briefly, you need to know the water demand of each "user" and anticipated demand at any given moment. Then you read the label on the unit you select, so it's capable to supply the water to meet the demand.

Re: how to determine size for tankless water heater?

First off the gas line must be 3/4" minimum to any tank less heater. As far as the flue pipe most are 4". There are some other disadvantages with tank less heaters. They are also not cheap they run 2.5 to 3 times as much as a tank type. The new clothes washers have a very low flow rate of water that will not allow the heater to fire. Some of the newer dish washers are also in the same category. Why not just replace the 40 gal. heater with a 50?


Re: how to determine size for tankless water heater?


I would tend to agree with dj1 and John

You would have to determine your "peak usage" of domestic hot water (DHW) in making a decision on which type of unit to buy.

"Peak usage" is usually the amount of HW you use during the "morning rush" when, for example, getting ready for school or work, which usually involves a shower, shaving, washing, etc.

There are charts that one can fill in to determine dhw peak usage if you Google such phrases as "DHW peak usage", etc.

Before deciding on a heater replacement it would be wise to contact your heating system service person & have them over the house to see if they can do anything with your current gas heater---a repair may be possible that will solve the problem with little expenditure of $$$ on your part.

If no repair is possible, consider what type of heating system you have---if you happen to have forced hot water, a 30 or 40 gal indirect HWH is a good option; if you have forced hot air heat, perhaps a new gas HWH may be best.

I must admit I am not a fan of tankless heaters; it's my understanding that they require a large gas supply line into the house; are costly to install & the user often runs out of hot water during a "peak usage" even such as the morning rush.

I'm not familiar with local codes in your part of the country as far as strapping a unit onto the chimney; it seems to be more common practice to strap them to the studs of exterior walls for support.

If you have no regular service tech, consult the Yellow Pages under "Heating Contractors" and have several over the house to give equipment & price quote estimates.


Re: how to determine size for tankless water heater?

Truthfully, I won't consider a tankless water heater, because of installation costs, unit costs and poor hot water supply at times.

No wonder tankless water heaters are not that popular. The conventional tank heaters have proven their effeciency and simplicity over the years. With life expectancy of 6 to 12 years, they're a better buy than tankless heaters. True, they have problems, but they are some of the most dependable appliances in most homes.

Another fact: in case of emergency, when there is no water supply, due to a natural disaster, you know that your tank can give you 30, 40, 50 or more gallons of water. There are ways to purify tank water for drinking. Remember that.

Re: how to determine size for tankless water heater?

I can speak from experience since I have a tankless water heater as do my parents and a friend of mine in his vacation home.

I have never run out of hot water while taking a shower, washing dishes, etc. and have often run the dishwasher while taking a shower without a problem. My parents have a brand new front loading washer and have no problem with low water flow not firing up the heater. Yes the initial cost of the heater is more and luckily my father is a plumber so I had mine installed for free but the life expectancy of a tankless is 25+ years as opposed to 6 to 10 years for a regular water heater. You are also not heating water when no one is using the water. Depending on your water usage will depend on whether it will pay for itself or not.

Some things you will have to consider will be whether you need a larger gas line or not. If you have to have a new gas line installed from the street to the house it is probably not worth it. Also you need to be able to vent it so you have to have wall space that is several feet from windows, doors, roof vents, etc. Your best bet is to check with a plumber who is knowledgeable in installing tankless heaters or a good plumbing supply house.


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