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
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
