Tankless water heater Sizing

[PCustoms]

Making an offer on a house tomorrow, one of the immediate items is going to be getting rid of the current "rented" electric water heater.

Property already has 1000gal propane on-site for the forced hot air furnace, so it seems natural to go with a propane water heater.

House has 1 bath, and just myself right now. Would like capacity for at least 1 other person (i.e. 2 showers in the morning), as well as some overhead for washing dishes/laundry.

Is a 6.5GPM 150K BTU unit adequate?

Takagi or Rheem? Amazon has a Takagi at 50% off MSRP right now. Going to call the HVAC house tomorrow and see what they can get for me, we have an account through work I have used to get discounts on fixtures in the past.

 

[jt777]

Take it from me. I work and a store that sells appliances, water heaters, etc. Those tankless ones do not last if you have hard water. The plumbers around here refuse to install them cause they and Always breaking down with our hard water. Even worse for the guys with well water.

 

[nadogail]

Because we all have unique situations, i respectfully suggest you investigate your local water supply to determine the suitability of a tankless water heater.

The size of the heater you mentioned seems IMHO, adequate for the situation you described.

 

[Git]

I started looking at tankless water heaters earlier this year. Our current water heater is over 12 years old and I want to have a plan in place when it looks like it is starting to die

I was looking at Rheem

There are a number of sizing calculators out there - you need to do the math to see what will be adequate for you

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

www.rheem.com/products/tank_water_heaters/how_to_size_a_water_heater

 

[cwii]

I have a Rheem (8yrs)and we are on a well. I had previously invested in a water softener as well as a RO system for drinking water. The other appliances, ice makers, washer, dish washer etc benefit from the softener as well the tankless system.
Previous provided info is good guidance.

Chuck

 

[kd3pc]

the calculators do not address water chemistry, and that is one of the most damaging things to the heater..

best to speak to a local business, county agent, or get the water tested at a pool/spa place. At least then you will know what has to be done to the water, prior to the heater.

Most of these heaters work on temp rise of X degrees at max flow, it may pay to have a "local holding tank" in the basement or crawl space that will be at room temp, as opposed to the well/system temp.


bests,

 

[PCustoms]

Drilled well has a holding tank already in the basement. I will be getting a water test prior to closing, I am familiar with hard water issues (we had to install a system for my families camp in order to protect the tankless, among other things).

Glad to hear the sizing calculations are as simple as I thought. Thanks for the links!

 

[Jackfre]

Do get the water test. Maintain the unit. That requires that you flush the unit by circulating 4 gallons of white vinegar through it annually. I sold many thousands of tankless water heaters in VT when I was the Rinnai rep in that area (91-11). It is just like anything else. You take care of it and it will take care of you. A 150 will be sufficient for you. Be aware that your 6.5 gallon unit will probably make about 3.4-3.8 or so gpm. The 6.5 rating is a maximum possible flow through the unit, or possibly an output at a 35* temp rise. In my home with a 50-55* input water my .82EF, 180,000 btu water heater set for 120* would use 2.1 gallons of hot and .4 gpm of cold to make a 2.5 gpm shower. That Rinnai RL75 (max flow 7.5 gpm) at a 70* temp rise would produce 4.3 GPM. You can get higher outputs on the same inputs by buying the 95EF units, but I still prefer the .82 EF units for simplicity.

 

[Git]

I forgot to mention that one of the reasons why I was looking at Rheem was there "condensing tankless" really got my attention

It would be very easy to replace my existing natural gas water heater with a condensing tankless - I would just have to install a 90 degree vent and go straight out the back wall of my garage

Here is a brochure
http://www.tanklessking.com/media/u...eem-prestige-condensing-tankless-brochure.pdf

 

[jbwilkins]

Making an offer on a house tomorrow, one of the immediate items is going to be getting rid of the current "rented" electric water heater.



Property already has 1000gal propane on-site for the forced hot air furnace, so it seems natural to go with a propane water heater.



House has 1 bath, and just myself right now. Would like capacity for at least 1 other person (i.e. 2 showers in the morning), as well as some overhead for washing dishes/laundry.



Is a 6.5GPM 150K BTU unit adequate?



Takagi or Rheem? Amazon has a Takagi at 50% off MSRP right now. Going to call the HVAC house tomorrow and see what they can get for me, we have an account through work I have used to get discounts on fixtures in the past.

One thing to check is the size of the line from the propane tank to the house.....150k BTU is probably 2-3+ times the current demand if the house now (guessing based on 1 bath).....you'll need significantly more flow if it's running at the 'max'.......

 

[chrispyny]

I was in a customers basement last week doing some work for him and noticed a recently installed Rinnai tankless hot water heater. House was set up not far off from mine. I asked him if he noticed a long period of time between turning on the hot faucet, and waiting for hot water to actually show up. He said thats the only thing he hates about it. He claimed it occassionally took up to TWO MINUTES before hot water hit the faucet, at the CLOSEST faucet to the Rinnai which was beautifully installed with all the appropriate fittings, sized pipe, and sized Ng pipe/pressure/volume.

I told him that is not acceptable for me. I thought about it. I need instant hot water for my dish washer and HE clothes washer. My raised ranch hasboth bathrooms, kitchen, dishwasher, and clothes washer situated almost perfectly centered in the house. I bet the longest pipe length is to the kitchen faucet and it might be 15 feet at an absolute most. When i turn on the hot water, i get full hot water within exactly three seconds.
In summary, i will NOT be purchasing a tankless water heater for my home. The waste in water, time to hot water, and cost of the unit makes it completely out of the question and unappealing.
I would have LOVED to upgrade to tankless as i need the space for my welder and a plasma cutter in the works, but alas, it's a no go.
This is just my opinion. Just wanted to share my limited experience with them.

 

[lakeroadster]

We have a Noritz NR98DVC tankless unit, on natural gas using well water. Works great. Our first experience with one of these. Have had it for about 10 months.

Only water filters are (2) canister types, one is a pleated type, the other a carbon type.

Hot water is reaches the user as fast as the tank unit we previously had.

 

[Adk Mike]

Tank less water heaters are great. I sell them once in a while in my day job. The thing where they work the best is if you lack space like in a Condo. You can mount in a closet on an outside wall.
Me I have a basement and lots of room. For me it is a 40 Gallon Bradford White TTW. They are the unit the trade uses. They only seem to fire when the hot water is running. Old fashion Quailty

 

[Crash&Burn]

There are so many factors to consider before making this decision. As mentioned, what is the composition of water coming into the home...impe less than roughly 80 ppm or ~5 gpg should be gtg with normal cleaning routines. Where will the wh be located in relation to the pou? Is there currently a recirc pump/line or can one be easily installed?

Answer those 3 and then many might give you a better/different response.

I've installed hundreds and personally owned 3 tankless over 15 years and they aren't always ideal so that's why I pose these questions. I also now operate a water treatment plant for roughly ~1mil people so I understand more than most corrosion concerns as well as mineral composition of the water.

 

[PCustoms]

^^^didn't take any measurements yesterday but I would guess maybe 25 feet to the kitchen sink and 35 max 5o the bathroom. No ceiling in basement so could add recirculation.

Need to get te specs from my father on the condensing unit at camp. It's tied to the basement and 1st floor radiant, I know it's designed for that and IIRC has dual heat exchangers. Anyway a 30ft run to the kitchen and water is HOT within a minutes of turning the faucet on.

Tanked or tankless you've always got to drain the line. From what I remember of installing the last it needs to have a minimum flow to kick rhe burner on.

 

[jbwilkins]

"..... From what I remember of installing the last it needs to have a minimum flow to kick rhe burner on."

Min flow rate to activate have dropped considerably since they were
introduced in the US...I think a Rinnai is somewhere around .2 GPM.....

 

[Crash&Burn]

^^^didn't take any measurements yesterday but I would guess maybe 25 feet to the kitchen sink and 35 max 5o the bathroom. No ceiling in basement so could add recirculation.

Need to get te specs from my father on the condensing unit at camp. It's tied to the basement and 1st floor radiant, I know it's designed for that and IIRC has dual heat exchangers. Anyway a 30ft run to the kitchen and water is HOT within a minutes of turning the faucet on.

Tanked or tankless you've always got to drain the line. From what I remember of installing the last it needs to have a minimum flow to kick rhe burner on.

That's not a bad run if that's total pipe footage and not just a linear footage from point a to b. The typical complaint is wait times, and wasted water for that matter. Minimum flow is usually in the .25gpm ballpark as was mentioned above me. My wife had a bad habit of just cracking the hot valve and wondering why there was no hot water.

 

[Jackfre]

When I have done contractor training on tankless installs I always say, yes, quote the job where the current water heater is. It will likely be the least expensive option or position A. Now go for a walk around the house and look at the lay-out of the space. Is there a better location? Closer to the primary point of use? Easier to vent? If there is, quote it that way and let the customer know why that second or position B is really the best location for the water heater and the customers comfort. You have to ask why the water heater is in the garage to begin with. Convenient and cheap? Why perpetuate the mistakes of the lowest cost bidder? As well, analyze pipe sizes against fixture flow rates. To large a diameter of pipe will delay hot water delivery.

There are many recirc options for tankless. The best of them will be available this summer. Stay tuned! Oh, hell, it will be a wireless control with either wireless motion sensors or push buttons. PC, If you run a recirc line, do it in 3/4".

 

[Pwrgeek]

I had a Rinnai RUR98i installed last fall. Went 199kbtu because we're on a well and our ground water comes out at about 55 deg F. With three females in the house (and me) it's the best thing I've ever done. No more cold showers for me (I'm always the last through the shower at night). For what it does for us cheap at twice the price.


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[gbcanadian]

As an HVAC tech who works in urban and suburban communities, I would not recommend and tankless to anyone. They are such a flawed design with so many problems. Get a tank and a protection plan for something like 20$ a month, repairs are costly and even one service call cost will be more than what you'd pay on the annual plan.

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[gbcanadian]

Also a note that many are misconceived by! Tankless water heaters do not store water (duh?) So it has to initiate the combustion process each time someone requests hot water which is why you will sometimes expect to wait up to 2 minutes for hot water. The reason for this is the purge process. When combustion starts, the ventor motor will start and prove a pressure switch, checking if the vent is obstructed. After the purge then, it will restart the process and begin to ignite the burners and heat water as it goes through the heat exchanger. A traditional tank has a gas valve with temperature settings. Set it, tank will fill and heat when its full. It will remain at that temp until its empty and then refill and begin to heat again. The advantages are you have the readily available water and the choice to size a tank, up to 75 gal (which is huge!!) little savings they claim on tankless's are lost when ONE problem occurs $$

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[Jackfre]

The tankless in my home reaches set-point temperature, from a cold start in 5-7 seconds. The sequence of operation is unit sees flow, immediately starts the PRE=purge of the HX. PCB sees fan operation and releases spark. PCB sees spark and releases gas. PCB then continuously monitors microamps indicating that there is indeed flame in the unit and it can continue to supply gas. That entire process takes approx 2-3 seconds. Yes, there is a short delay in hot water delivery and that is what causes the "cold water sandwich". The 2 minute delay in delivery is caused by having to move the cooled hot water in the piping through the system so the hot water behind it can get there. Depending upon the flow rate of that particular fixture, if it is low, and the volume of water in the pipe you can wait a while. There are effective re-circ option available. I would note that you have to wait on a tank water heater as well. To get an idea of pipe volume look at the cross sectional diameter of 1/2, 3/4 & 1" tubing. It is .19, .44 and .77 sq in respectively.

I am not aware of any tankless units that still use pressure switches. That is old an obsolete technology and they were one of the primary points of failure in those units. The better tankless manuf offer free training on the equipment. Perhaps, gb you might enjoy that and not feel so poorly about these excellent products if you understood them better.

In the States, as of 4/15/15, water heaters over 55 gal must be condensing or heat pump to meet the new energy regs and they are substantially more expensive than previous models.