HW preheater and on demand HW heater

[mikeyny]

Hello everyone,

        I am about to install a home brew hot water solar panel to preheat domestic water before it enters the cold side of an on demand gas water heater. If I have 110 degree water flowing to the unit will it cause a problem with the unit firing at the correct time. Has anyone had any experiance with this situation. How sensitive are the controls on those on demand heaters?  

                                               Mike

[kurt]

you have to get a on demand heater with a thermostat on the intake side some work great with solar preheating some don't you have to do your research before buying one....

[GaryGary]

Hi,

You need a tankless water heater that will modulate the burner to add only as much heat as is needed (or none).  

My Takagi JR does this.

I know some of the Bosch units do this.

If your solar system produces enough heat in the summer to provide all your hot water, you may just want to disconnect the tankless for the summer -- a lot of people do this.

Gary

[TheCasualTraveler]

     I was at Lowes looking at one on demand HWH and I don't remember the brand name but I called the company (thinking about preheating the water with solar) and was told the limit for water temp on the intake was 80 degrees. PowerStar and Bosch are the brands they carry, I think it was PowerStar I was looking at.

[richhagen]

There are specially made versions of on demand hot water heaters for this purpose which will adjust the amount of heat provided to the water based upon its input temperature.  I suspect that in most locations these will be more of a specialty item and will not be carried in the general stock of a diy center, but maybe in some of the more sunny areas.  If you use one that does not have this feature, you may put out extremely hot water capable of scalding and burning users, or trip the over temperature sensor on the unit, shutting it off, or open up the over tempurature/pressure relief valve (or if both of those were to fail blow the thing up)  Most basic units, like the Bosch 125NG units I have been using, provide the same amount of heat regardless of the input water temperature.  I had one where the valve that turns on the heater based upon the flow rate got stuck.  The unit turned off based upon the over temperature, and cleaning the unit was pretty straight forward.  Rich

[chainsaw]

I just installed(5 months ago) a 17 gal tank that is solar heated going into a Rinnai on demand heater.

Bottom line- it works great. If the preheated water is higher than Rinnai selected temperature, the heater does not kick on. If the incoming water is say 100 degrees and 110 is selected for a shower the heater will modulate its btu output to match what is needed to raise the temp 10 degrees. Great gas savings.

I'm not familiar with any other brands of on demand heaters other than the Rinnai.

The only downside is when the sun doesn't shine you burn more gas but even Mother Nature needs a break now and then.

[TheCasualTraveler]

     This is a good post. Combining two great energy saving features, solar and on demand hot water.

     To review, you need to get a unit that can work with solar input, having a thermostat control on the intake side. Two brands mentioned so far are,

Takagi Jr (gas)

http://www.takagi.com/web2003/c03.htm

Rinnai  (gas)  model?

http://www.foreverhotwater.com/#\

     Any other gas models? Has anyone any first hand experience with an electric model? As efficient as gas is getting set up for it in my case would be a pain.

[freeEnergy4me]

On a side note, the on-demand HW heaters:

Why are they affordable in the stores but outrageously expensive to install?  You can find one for $1000 or less at Home Depot....but when you call someone to install it, it's like $4000-5000??

Is it just me or does it seem like installation would be fairly easy? A) connect gas line B) connect water line. Viola!

Regular water tanks are not that expensive to install, isn't it the same connections, with maybe a mount on your wall?

Anyone ever install one on their own?

[jimjjnn]

I did install a Takagi on demand water heater myself except for the gas connection as it had to be inspected by the County.

The most difficult part was was installing the burner exhaust pipe through a a frame and brick wall.

Only mistake was in using wrong fibre glass insulation. I used as I recall the pink colored insulation. The inspector had me install the yellow insulation. He was great and gave me advice when he was there.

The gas connection was $460 . It had to be done by furnace installation company tho I could have done it myself.

[spinningmagnets]

I think most people are afraid of natural gas stuff, and are just willing to pay extra for a professional.

I recieved good advice when I moved a water heater, or I would have been lost and screwed it up. It would've worked, but would not pass inspection later when the house was sold, needing to be expensively re-done according to code very fast.

Use "black-pipe" that is made specifically for gas, and put a gas-rated brass on-off valve within reach of the unit in case a leak is discovered later. Use thread sealant tape on the joints that is specifically made for gas.

When the installation is done, check all the joints with soapy water. If there's a leak, it will produce bubbles. No matter how small the bubbles might be, you must have no bubbles at all. A small leak will make a high frequency noise humans can't hear, and will be read by the inspector with an acoustic meter.

[thirteen]

Most ondemand water heaters I know will raise the tempature roughly 50/60 degrees and all have heating sencers to regulate the water tempature. I installed an ondemand heater in one of my daughters rentals. An LP Bosh ondemand water heater. It works geat and saves the people $$ per month. The renter asked what I was going to do with the small gas water heater that was up stairs. He suggested putting it in line and exhuasting the heat from the ondemand water heater thru the tank thus warming the water before entering the ondemand. The gas company inspected it and said ok after they had me change some of the venting and I had to install a readable meter for the exhaust tempature and one in the water line before the ondemand heater. I also had to extent my roof vent to 30 inches. The gas company is monitoring the exhaust temp. I plan on doing something like this to my retirement home only using the wood heat for preheating my water. If you have a system that has excess power say from a solar panel or hydro system it can be used to preheat your water before going to your ondemand water heater. The electric water heaters normally used make a good pre heater and make a good dump load for your systems extra power. Just some ideas.

[freeEnergy4me]

But if you already have a gas water tank then you have the vent. I do.

I just can't figure out why they charge so much for the installation? I assumed like you, that I could probably do it myself.

I'm a single person and water just sitting in a tank being heated over and over without use is ridiculous.  But I got 2 quotes for over $4000 for installation. WTH?

Thanks for your comments, I knew it could be that hard.