New Domestic Hot Water Zone Pics

[Piker]

Alright, here's my new DHW zone that I recently just piped in. It's a little different than the norm... but so far testing has shown very positive results. Here were the design parameters:

1) I wanted to be able to heat my DHW off of the two 500 gallon pressurized thermal storage tanks, without putting coils into the tanks
2) I did not want to use a plate heat exchanger due to terribly hard water
3) I did not want to use a thermostatic mixing valve on the DHW, again due to terribly hard water.

Those parameters put me in a situation where I needed to have a fair amount of thermal transfer through a heat exchanger that could be easily cleaned... for this I build a double sidearm heat exchanger approximately 6' in length... connected at all joints with unions so that the heat exchanger could be removed for cleaning. I figured I would also need to circulate (as opposed to thermal siphon) the DHW through the heat exchanger in order to get the transfer I needed, though at the cost of screwing up stratification in the tank. I would also need to keep the temperature in the tank moderated with some sort of thermostat inside the tank itself, and the dhw loop off of the boiler would have to be part of the zone manifold that draws either from the storage or the boiler.

So here's what I came up with. I rearranged the prv to incorporate a 3/8" soft copper tubing well that reached into about the middle of the tank. I used a ranco digital thermostat to measure the temp in the tank, and to tell the argo 6-zone controller when to send heat to the DHW loop. The circ on the domestic side of the heat transfer has two tests involved before it can start churning the water in the tank through the sidearm. 1) the dhw loop from the zone manifold has to be at least 140*... this is done with a simple close on rise mechanical aquastat. and 2) the zone valve to that loop must be open... this was done with a simple 24 volt ice cube relay connected to the dhw zone on the argo controller. These two checks make sure that once the storage has cooled down past a usable temperature there will be no reverse btu transfer from the electric coils in the dhw tank to the storage (during backup use)... and also that you don't continue to circulate the dhw after the temp in the dhw tank has been met but the pipes leading to the sidearm from the manifold are still hot.

The dhw circulation is counter intuitive on this setup as well. Normally, you would pull cold water off the bottom of the tank, send it through the heat exchanger, and return it at the top of the tank in order to maintain some stratification. The only problem with this is that with the fittings available on the tank, you have to use the 'hot out' of the tank as part of that recirculation loop... if you circulated the water in the direction you would think it should go, you would then chance sending water that is way to hot out to the faucets directly from the heat exchanger. Sooo... I ran it backwards... I pull off the top, pass through the sidearm, and return at the bottom of the tank. This really screws up stratification... and thus would screw up the amount of usable hot water... thus the double sidearm. I was hoping to get enough heat exchange to almost use the tank like an on demand unit. It's not quite that good, but it's not too shabby. I keep the temp in the dhw tank at 130, with the control set to a 1* differential. As soon as you start using water somewhere in the house, the circ turns on shortly thereafter and begins to transfer more heat into the tank. We can do 2 adult showers, and 1 giant garden tub bath for the little ones and only reduce the temp in the tank to around 115 or 120... we can then recover back to 130 in 15 minutes or so. This is far better than I had anticipated. I think I am getting about 35,000 Btu's/hr transfer with 180* water in the boiler... might be more though. I am going to cool the tank down to around 75 degrees and do a full bore charge and see how long it takes... this should give me a more accurate measure of thermal tranfer. Not that it matters a whole lot... the fact that it works well for our needs is enough.

I have not yet had the chance to run the loop off of the storage tanks, since I do not yet have them insulated. I am sure it will be a totally different ball game altogether once those are online, and will likely have to slow the flow through the boiler side of the sidearm in order to increase the delta t and help support stratification. As soon as I can get the spray foam guys out, I'll post results for those tests as well.

At any rate... this was a fun project. Enjoy the pics, and feel free to offer suggestions.

cheers

 

[Piker]

by the way... there are plans to put some conduit around that orange 10-2 wire carrying the 220 to tank... just in case you were wondering.

cheers

EDIT: I MOVED TWO PICS FROM THE UPPER POST DOWN TO THIS ONE SO YOU DON'T HAVE TO SCROLL SIDE TO SIDE TO READ THE TEXT.

CHEERS

 

[Gooserider]

Sounds good, but it's a little hard to follow with just your pictures... Any chance you could put up a plumbing diagram as well?

Gooserider

 

[Piker]

I am unsure when I will have the time to post piping diagrams, but I do have some data for all of you on heat transfer of this setup.

I took all the water in my DWH tank down to 85*. I did this by circulating the water constantly as I drew hot water from a faucet. I allowed the tank to circulate for around 15 minutes to make sure that I was pegged at 85 degrees.

I then opened up the shut-offs on the boiler side of the heat exchanger (175*) and began to warm the tank. In 36 minutes the temp in the tank went from 85 to 130, at which point the circulator stopped. I gained an additional 2 degrees after all circulation had ceased (circulation stops totally because check valves prevent gravity flow) for a total delta T of 47* in 36 minutes of heat transfer.

(51 gallons of water) x (8.3 Btu's per degree/gallon) x (47* delta) = 19,895.1 in 36 minutes.

[(19,895.1 Btu's) x (60 minutes)] / 36 minutes = 33,158.5 Btu's/hr


So around 33,000 Btu's per hour heat exchange @ 175* water in the boiler... not to shabby. Not great, but definetely usable for our family of 4. Would I have better luck with a braised plate? Yes... would I be replacing the braised plate often? Yes. I might try putting some reflective insulation on the exchanger... not sure if that will make much of a difference or not, but worth a shot.

So, all in all... this is a good setup so far. I haven't tested the exchange with lower water temps yet... IE thermal storage. It should still work fairly well, though I will likely have to maintain higher temps in the storage than I would like if i want it to supply ALL of my domestic hot water needs. We'll see.

cheers

 

[Piker]

By the way... the current electric heater is about 9600 watts with both elements running... or just shy of 33,000 Btu's.

nifty.

cheers

EDIT: Not 9600 watts... 8200... looking at the wrong voltage rating. 28,000 Btu's per our

cheers

 

[Jim K in PA]

Piker - I have been researching before I build and incorporate my DHW side arm into my revamped distribution system and re-visited your post. I have a couple of questions (If you are amenable to answering them):

1-has this setup been working well for you all summer?

2-I see your hot-out (demand) line from the tank Tees, and the pump draws off one side of this Tee to circulate drinking water through the side arm. Do you have any supply issues when there is hot water demand (shower) while the pump is running?

3-I presume you are using a bronze Grundfos pump?

4-Where did you set your electric coil stat with respect to the pump control stat? Do the electric coils kick in often to compensate for a high demand?


I plan on using a similar aquastat setup with an open on rise scheme to activate a zone valve. That will start the primary circ pump on my manifold to send hot water to the side arm. I intended to use thermo-syphoning to transfer heat to the tank, but now am wondering if I will need to boost that with a small pump. How big is your tank? Looks like an 80 (mine is a 50). I do not have the water hardness issues you do, and will be incorporating a mixing valve for the DHW demand side.

I have another port on the top of my tank to either return the hot water to or transfer the PRV to, so I wont have the access issue you had.

My forward-planning includes a flat plate solar panel for summer DHW heating, but I have to figure out how to incorporate that with one side arm if I can. I want to be able to just throw couple pairs of valves to convert from wood heated water to solar heated water. Wish me luck!

 

[Piker]

Piker - I have been researching before I build and incorporate my DHW side arm into my revamped distribution system and re-visited your post. I have a couple of questions (If you are amenable to answering them):

1-has this setup been working well for you all summer?
Yes, it seems to work well for a family of 4... but circulating the domestic water really screws up stratification in the DHW tank. I may move the ranco controller's thermocouple that's in the tank closer to the bottom so that it's more sensitive to temperature change. As it works now, by the time the ranco controller detects a drop in temperature, there might be 15 gallons of cold water in the bottom of the tank... this causes a rapid temperature drop as soon as the domestic hot water circ turns on. It would be better to be closer to the bottom of the tank with the thermocouple so that as soon as cold water enters the tank the dhw circ turns on, thereby slowing down the rate of temperature drop... right now, part way through a shower you have to bump the hot water up on the faucet just a hair to maintain a good hot shower... but it's a small price to pay for nearly free domestic hot water yes?

2-I see your hot-out (demand) line from the tank Tees, and the pump draws off one side of this Tee to circulate drinking water through the side arm. Do you have any supply issues when there is hot water demand (shower) while the pump is running? As stated above, the temperature of the supply water to the faucets will drop a bit when you remove the thermocline between new water entering the tank from the well. Aside from that, there are zero issues. I purposefullly drew water from the hot side of the dhw tank. If I had circulated the other direction, you would have to about excessively hot water coming directly from the heat exchanger going to the faucets.

3-I presume you are using a bronze Grundfos pump? Stainless.

4-Where did you set your electric coil stat with respect to the pump control stat? Do the electric coils kick in often to compensate for a high demand? Most of the time the tank is shut completely off. Until I move the thermocouple closer to the bottom of the tank, the lower element will kick on an awful lot otherwise. ONce I make this change, I will run the pump control at 130 or so, and the electric coils fairly low... say around 105. This should keep the electric coils from coming on most of the time.


I plan on using a similar aquastat setup with an open on rise scheme to activate a zone valve. That will start the primary circ pump on my manifold to send hot water to the side arm. I intended to use thermo-syphoning to transfer heat to the tank, but now am wondering if I will need to boost that with a small pump. How big is your tank? Looks like an 80 (mine is a 50). I do not have the water hardness issues you do, and will be incorporating a mixing valve for the DHW demand side.
The tank is 50 gallons. I am actually suprised how many Btu's we go through here just for DHW... it's alot more than the 60k per day that I thought... more like 120k. Our electric bills have been cut by about half.

I have another port on the top of my tank to either return the hot water to or transfer the PRV to, so I wont have the access issue you had.

My forward-planning includes a flat plate solar panel for summer DHW heating, but I have to figure out how to incorporate that with one side arm if I can. I want to be able to just throw couple pairs of valves to convert from wood heated water to solar heated water. Wish me luck!

GOOD LUCK!!


cheers

 

[Duetech]

Piker,
I use a tempering valve on my dhw out to prevent scalding water from going to the faucets. It's adjustable so you can tweak to your desired thermal inclinations. I heard of a product called Easy Water (easywater.com) that might help with the hard water problem. Never used it but am looking into it to eliminate our salt usage. It's not good for iron but will work with an existing iron filter.

 

[Nofossil]

If I understand your approach correctly, adding a tempering valve on the DHW outlet would allow you to reverse the flow direction and regain your stratification. I would certainly go that way. The Honeywell AM101 valves seem to be able to give you the lowest difference between the hot in and the mix out. This is a good thing - trust me.

 

[steviep]

By the way... the current electric heater is about 9600 watts with both elements running... or just shy of 33,000 Btu's.

nifty.

cheers

EDIT: Not 9600 watts... 8200... looking at the wrong voltage rating. 28,000 Btu's per our

cheers

I need to correct you here your water heater is most likely a 4200 watt heater. The way a water heater works is the top element heats up the top half first then when top thermostat is satified it transfers power to the lower thermostat and element but never both at the same time. On most heaters the bottom element works more then the top.
This also how you can tell which element is bad, no hot water its the top element, hot for 2 to 3 minutes the bottom element is bad.

 

[Piker]

If I understand your approach correctly, adding a tempering valve on the DHW outlet would allow you to reverse the flow direction and regain your stratification. I would certainly go that way. The Honeywell AM101 valves seem to be able to give you the lowest difference between the hot in and the mix out. This is a good thing - trust me.

I certainly do trust you... and using a tempering valve would certainly have been my first choice. But... recall #'s 2 & 3 from the original post...

I want to keep as much "stuff" off of the domestic hot water side as possible to keep maintenance down given the crud that's in our water here. While better stratification would increase the useable btu's stored in the dhw tank, we appear to have more than enough supply to meet demand as it currently piped. Once I move the thermocouple closer to the bottom of the tank, the issue of dropping temperature suddenly when the thermocline is "erased" should go away.

cheers

 

[Piker]

By the way... the current electric heater is about 9600 watts with both elements running... or just shy of 33,000 Btu's.

nifty.

cheers

EDIT: Not 9600 watts... 8200... looking at the wrong voltage rating. 28,000 Btu's per our

cheers

I need to correct you here your water heater is most likely a 4200 watt heater. The way a water heater works is the top element heats up the top half first then when top thermostat is satified it transfers power to the lower thermostat and element but never both at the same time. On most heaters the bottom element works more then the top.
This also how you can tell which element is bad, no hot water its the top element, hot for 2 to 3 minutes the bottom element is bad.

yes, you are correct. now that I think about the fact that there is only one 220 volt line to the hot water tank on a 20 amp circuit, it makes sense that only one element can be running at a time... something i have never really thought about.


You learn something new every day!

cheers

 

[Piker]

Just a quick note... I was originally concerned that the sidearm would not work well with thermal storage. As an update to that concern, it appears that the storage will deliver adequate btu's to supply our dhw demand all the way down to around 140*. Below that we still have hot water, but it rarely reaches the 130 mark, and the circs run constantly until the storage drops below 120 when everything shuts down.

cheers

 

[Jim K in PA]

Thanks for the additional feedback. I'll post pics of my revamped system in a few weeks when it is all done.