Regular Guest
I am building a new home and just had a system analysis completed for my house they spec'd a 4 ton unit for the house and it looks great but I had some questions on the desuperheater and the most efficient way for hot water. What I am considering is using a 50gal Electric HWH as a holding tank and use an LP Tankless HWH after the Electric HWH.
Or I could use a LPHWH and use a Electric Tankless that way in an event of a prolonged power outage I could fire up the LPHWH and still have Hot water
Opinions and suggestions are very appreciated.
Jason
Professional Member*
Except during prolonged operation, you won't really create enough hot water to satisfy your needs using the desuperheater alone. So your suggestion to use a holding tank and a gas fired tankless water heater make good sense. I would use a similar system if designing it myself. You'll need the gas unit during the shoulder seasons when you're not running the heat pump at all or very little.
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Regular Guest
good to know I am on the right track, anyone else have a different suggestion and why it would be better?
Professional Member
For your electric tank, look into the Marathon brand.
I'm using the 85 gallon model for the holding tank for my heat pump water heater and the combination is working very wel.
Professional Member
You will actually use desuperheater in cooling and heating. Water furnace also give you the option to shut it off with the flip of a switch. Personally I would use the electric water heater as the holding tank and L.P. as primary tank.
Definition of insanity is to do the same thing over and over expecting a different result.
Regular Guest
Does brand really matter for my holding tank since I will not be running any electric to it?
Originally Posted by paul42
Regular Guest
That's basically what I am thinking except except I will be using a tankless L.P. heater after the EHWH which will not be hooked up to electric.
That is unless I am misunderstanding what you are saying.
Professional Member
The Marathon is guaranteed to never leak.
Regular Guest
excellent good to know! thanks for the tip.
Professional Member
First, the Marathon tank is a great tank, and the configuration you are considering is the correct way.
I am not one to suggest cheap, but what you want to do is just connect the desuperheater water circuit to a hot water storage tank. It can be any insulated vessel (with pressure relief). I would recommend a regular electric tank for this, and don't bother wiring it. This holding tank will then feed your Tankless water heater.
You can get most of your summer water this way, and a good portion of the winter, but little to none in shoulder seasons.
Your tankless water heater has a built-in water temperature sensor, so it will modulate its firing rate depending on the entering water temperature from the holding tank.
I would caution you on using a regular 85% efficient model - get one of the new condensing models (92% AFUE or better) - yes about $1,000 more. The regular 85% efficient units can suffer from a lot of condensation rolling back down the flue because of short run-times in this configuration - so make sure the vent pipe is drained correctly. I've actually seen the gas valves flooded full of water because of this!
Good luck.
Regular Guest
Do you have a particular Tankless in mind that is one of the new condensing models? Only one I found so far is a Bosch Pro
Regular Guest
I have a Rinnai tankless and am getting ready to hook up the DSH. I am going with the Marathon 85 as storage to feed the tankless. The tankless is great but the DSH has never been used as I was uneducated when I built. The heat is just going to waste so it is time to use it. I just hope the water into the tankless is not to warm to cause problems...
Professional Member
Noritz, Takagi and Rannai all make excellent units. Noritz is the larget manufacturere and is releasing a new > 92% efficient unit soon (if not already). Takagi already has a condensing unit out (has for over a year). I have not received any announcements from Rannai, but I'm sure they have one coming out soon too.
I've got a Bosch at my house, it is OK, but not as easy to service. I've had some condensing issues with this unit, and had to service the thing several times and modify the installation to overcome some of its design shortcomings, but is still a decent unit. This unit is over 5 years old, so one of the first generation units - hopefully they've made a few improvements (the components are fine).
These systems have internal water temperature sensors. Yu pick the output water temperture during setup. The units modulate their firing rate to deliver this temperature based on water flow rate and entering watering temperature. If the entering water temeprature is alreeady 100 deg F, then it will only add 20 degrees. The problem is because this is such a low firing rate, you will get condensation in the exhaust pipe (at least in colder climates) that you need to make sure is drained properly. This is why I recommend a higher efficiency condensing unit. The high-efficiency units are designed to deal with condesnate much better than the typical 85% effient units.
If you use solar or a desuperheater tank and the water is already over 120 deg F, then the unit shouldn't fire at all! This makes it a very efficient and convenient system.
Please post back on your experiences. Thanks!
Regular Guest
Buck
My tankless is direct horizontal vent outside maybe 3 feet max. Do you know if there is a condensate problem on these the same as vertical or longer runs?? I tried to contact Rinnai about supply temp. limits to the heater but have not heard back....
Professional Member
3 feet is good and should help a lot. Make sure you dont exceed the number of elbows the unit is typically allotted (for equivalent lengths) - usually 3 - 45 degree elbows allowed, and the closer they are to each other, the greater the restriction.
Check the manufacturers websites. They usually have PDF copies of their equipment installation manuals and specification available and is the first thing everyone should look at when choosing and installing equipment for their applications.
Regular Guest
Since I am using the tankless after the Marathon holding tank how should I size the tankless for a 3 bathroom, 4 person house?
Professional Member
What is the raw (cold) entering water temperature into your house (from well or city) and how many showers do you typically operate coincidentally?
Most of the time you could probably get away with the smallest unit they make, and this may actually be the best choice, but you can look at the GPM versus temperature rise charts to see how many GPM the device can deliver at 106-108 degrees.
Most modern shower heads are rated at 2.5 GPM flow, so if you can tolerate 108 degree water (pretty hot), and you are using 2 showers at 100% hot water, and your entering water temperature is 50 degrees, then you would need 5 GPM at ~ 58 degree temperature rise.
Most of the charts and settings are for 120 degrees, so if the unit will deliver
3 GPM at a 70 degree rise (50 in, 120 out), then this should be close because you are mixing at the shower valve with cold water. If you have the shower valve at 75%, then you need .75 x 5 GPM or 3.75 GPM of 70 degree temperature rise.
I know a lot of plumbers who are afraid to install these units because they say they don't make enough hot water, yet these are rated at, and have higher BTU output (150 to 175k) for H20 than most big boilers (usually 60 to 100k output)!
You have an advantage of the Marathon tank, so your entering water will be a LOT hotter most of the year, so your units rating will be almost twice of what is published - in other words, you could probably double the GPM load with no noticable effects.
Regular Guest
Water is coming in from a Well and usually only one shower at a time will be run but I am sure there are times where there will be two running at the same time. So with that said I am assuming I can get away with a smaller .92 effic unit and prob be good to go?
Regular Guest
I have a Rinnai R85. I believe 188k btu rated at 8.5 gallons max depending on rise. Any way, in a 4 bedroom house. I intentionally have run dishwasher with two showers going and it handles all with no problem. I actually run at 110 most times and this is fine for us. Might save a little gas...All should be much better once Marathon is inline. Also J.Hartbarger my propane feeds the tankless and gas range and my use is about 150-160 gallons per year with 2 adults/2 infants.
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