Going to replace hot water tank. Need some advice

[RNeumann]

I think I linked Takagi above- check that out.

How many people in the house? How many bathrooms etc? I've never had to put two in a structure- unless it was for geographic reasons.

 

[jdom84]

Im not a pro plumber, but I have installed and used many different brands and types of tankless hot water heaters and combi boilers in my houses and family members houses, both condensing and non condensing. Rinnai and Takagi would be at the top of the list. I currently have a takagi t-h3-dv-n which runs about $1,000, plus hookup/flush valves kit, I swapped out a BOSCH tankless unit and instantly noticed a difference. These units are built for light commercial use and I use this single unit to heat my 2700 sqft house via in floor hydronic heating and provide DHW using separate loops and water to water heat exchangers. I have a thermostatic mixing valve to keep people for scalding them selves. The good thing about the condensing units, they uses 3" pvc for flue and intake, are 95% efficient, can be used for space heating up to 180 degrees f. They are modulating and only the set temperature exits the heater. We can run every hot water fixture in the house and the thing keeps cranking out the hot water.

As far as flushing them... I first ran a non condensing tankless for 7 years with hard water and never had an issue. I decided to flush it because I was told I should and after the flush I couldn't tell a difference in performance and the flushing fluid (vinegar) took on a shade of light green, but wasn't like sediment came out or anything. I do pre-filter all water in the house to keep the faucets from clogging up though.


Although, there are things to keep in mind, like "cold starts" and waiting longer for water at the tap. I wont buy another hot water tank, to me some things are just worth spending the extra dollar and this is one of them.

 

[LittleBill21]

Im not a pro plumber, but I have installed and used many different brands and types of tankless hot water heaters and combi boilers in my houses and family members houses, both condensing and non condensing. Rinnai and Takagi would be at the top of the list. I currently have a takagi t-h3-dv-n which runs about $1,000, plus hookup/flush valves kit, I swapped out a BOSCH tankless unit and instantly noticed a difference. These units are built for light commercial use and I use this single unit to heat my 2700 sqft house via in floor hydronic heating and provide DHW using separate loops and water to water heat exchangers. I have a thermostatic mixing valve to keep people for scalding them selves. The good thing about the condensing units, they uses 3" pvc for flue and intake, are 95% efficient, can be used for space heating up to 180 degrees f. They are modulating and only the set temperature exits the heater. We can run every hot water fixture in the house and the thing keeps cranking out the hot water.

As far as flushing them... I first ran a non condensing tankless for 7 years with hard water and never had an issue. I decided to flush it because I was told I should and after the flush I couldn't tell a difference in performance and the flushing fluid (vinegar) took on a shade of light green, but wasn't like sediment came out or anything. I do pre-filter all water in the house to keep the faucets from clogging up though.


Although, there are things to keep in mind, like "cold starts" and waiting longer for water at the tap. I wont buy another hot water tank, to me some things are just worth spending the extra dollar and this is one of them.

any pictures of your setup?

 

[jdom84]

I have a couple pictures from some text. These were from first install/testing, there was no primary loop yet and I had it hooked up to a turbonics mid velocity blower to test heat output and cycling rate compared to the old boiler. Each heat exchanger is capable of 140k btu, I put in 2 because I wasn't sure if 1 was going to transfer enough energy but it surely was... I still only use 1 and the other is hanging out there in case someday I want to do some sort of alternative heating source. This system seems to works better then the 70yo 160k btu cast iron boiler that it replaced. I use less than half the fuel I did before and the cost of the complete setup was a fraction of the cost of a new boiler...



If I remember ill get some pictures of the completed setup.

 

[LittleBill21]

are your heat ex-changers working in the first photo? im interested in how the DHW loop works? there is no tank it simply goes through 1 the exchanger and it transfer enough heat? what does the main loop run at? what temp? and what is the DMW temp at the loop before the thermo valve?

 

[jdom84]

First photo. just the DHW is hooked up and very crudely I might add... it was getting late in the day and had to hook the system up to the existing piping so my wife could shower. the next day, second picture. the two pipes going straight up are cold water in left and DHW out right. the two test pipes that are kinda just hanging there... are the testing to the turbonics unit.

Right now I run the system at 150 degrees f which seems to be a pretty good number. The temp up to the mixing valve is 150f.
I wanted DHW to be priority, there is no loop for the DHW. the cold water comes in goes past the "out port" of the pump/check valve into the ex-changers and into the heater - gets heated - comes out the heater and past the "in port" of the pump, to the mixing valve reducing the temp to 120 degrees f to the fixtures if they are on.

If the pump is on, the "ex-changer loop" is running, heating the ex-changers for the hydronic loop. There is another pump running (running a pump is cheap, cost ~ $7 a year) on the primary hydronic loop. If someone turns on a fixture its almost instantly hot at the fixture (or more like a tanked hot water system) because the hottest water (150f) is already being produced and 120f is delivered to them (no cold start). If the pump is on and a fixture is on, the cold water coming in mixes with the hot water the pumped into the ex-changer loop. making a kind of lukewarm heat ex-changer but the DHW is still getting the hottest water because its pulling from the same line as the pump.. I manage this system with a raspberry pi and some temp sensors/relays and some python code, shutting off the primary hydronic loop (which would be called a boiler loop on a boiler system) until the water in the ex-changer loop is hotter then the water in the primary hydronic loop. Combi boilers have a similar priority system where they shut down the boiler side when DHW is running.

Having the 2 heat ex-changers, if I had alternative heating, such as a wood boiler/solar evacuated tubes. It would be able to "pre-heat" DHW going into the Takagi unit because the ex-changers are before unit which would reduce heating cost. Also would be able to just run the ex-changer loop, transferring energy from the wood/solar to the hydronics loops.

I designed this system so I could complete turn off the hydronics heating in the spring/summer and still have DHW. It all sounds complicated but its very simple, aside from some magic pixies in the raspberry pi.

There are several ways this could have been achieved. using a heat exchanger and a storage tank for DHW is one of them.

Ill draw a picture. :thumbsup:

 

[LittleBill21]

can u draw a picture of your current system? i think i understand it.

i also use a pi for water heater control. like minds think alike

how are u tying in your temp sensors? and a link to them?

if i understand your heat exchanger loop. your sending 150f water back into the cold side of the tankless? i thought there was a limit on the cold side input?

 

[jdom84]

sorry OP for all this off topic stuff

here is a quick go at the drawing. there is also the 5 in slab loops (ran as one) that run at 100 degrees f and a 10 port manifold that feeds 7 individual in floor loops on the first level that run at different temps depending on the outside temp. I did not draw those.

I use ds18b20+ sensors which are digital and all run on the same single wire interface (i2c I believe). I got them in bulk on amazon (20 for $20 I think). I have found that they are not the most responsive sensors but they get the job done. If you want instant responsive sensors you need an ADC and some sort of thermistor or something like that, but that is a far more time consuming to setup. I just use some high grade electrical tape to attach the sensor to the pipes, shielding the pins of the sensor of coarse. I also use them in each of the rooms as a thermostat or way of reading the room temperature and turning on or off the heat. All managed via web app... which I haven't found time to actually finish.

this guy has a pretty good guide for the ds18b20, which is where i learned about them a few years ago.
DS18B2 Temperature Sensor with Raspberry Pi | REUK.co.uk

 

[jdom84]

if i understand your heat exchanger loop. your sending 150f water back into the cold side of the tankless? i thought there was a limit on the cold side input?

not on the Takagi. its designed to run just like a boiler. it will ramp up to 150, if the intake temp reaches 150 it will stop heating.. cool down to 140 and ramp back up to 150. It can reliably handle intake temp up to 170 degrees f. but you limit the warranty by half running it like this

 

[LittleBill21]

yea i understand it now.

i use the ds18b2 on my arudino projects, u can get them in water proof versions, but yea they have about 2second read response time for arduino.

does your heat exchanger pump cycle alot, i would think it would not need much time to satisfy the heat exchangers, but i guess if the primary loop is on, maybe not, i guess the primary loop pump would cycle more.

your floor setup sounds kewl, how are u adjusting the temp based on outside temp?