How do I size a flat plat exchanger to heat DHW

[dirttracker]

FWIW, I use a 5x12 10 plate to heat DHW directly. No tank or other storage, DHW comes out of the plate and though a tempering valve. I've been running this setup for a year without issues, as long as I keep my storage above 130 I have water hot enough to bathe with no issues. Based solely on this, I believe you will be able to recover your tank temperature quickly depending on the size of the circ pump used for the DHW.

I suggest you should plumb the HX in series with the inlet to the 1st water heater. Plumb the HX in a counterflow configuration to obtain the best efficiency. I assume your OWB circ will be running all of the time, otherwise you will need controls to turn on the OWB circ when you have DHW demand. Set up the DHW circ to pull water out of the bottom of the 2nd water heater and pump back though the HX to the first tank. Depending on the OWB flow rate and temperature, the DHW circ may not have to run much to keep the tanks hot.

I ran my 5 x 12 flat plate for 5 years with an OWB. The plate was connected in series with the inlet to my 40 gal water heater, the water heater was shut off for the winter months. I never ran out of hot water with this setup.

I did not have a problem with corrosion on the OWB side, even after 5 years on an open system. I did have to pull the HX once to flush out the potable water side after someone unplugged the water softener.

 

[MNBobcat]

FWIW, I use a 5x12 10 plate to heat DHW directly. No tank or other storage, DHW comes out of the plate and though a tempering valve. I've been running this setup for a year without issues, as long as I keep my storage above 130 I have water hot enough to bathe with no issues. Based solely on this, I believe you will be able to recover your tank temperature quickly depending on the size of the circ pump used for the DHW.

I suggest you should plumb the HX in series with the inlet to the 1st water heater. Plumb the HX in a counterflow configuration to obtain the best efficiency. I assume your OWB circ will be running all of the time, otherwise you will need controls to turn on the OWB circ when you have DHW demand. Set up the DHW circ to pull water out of the bottom of the 2nd water heater and pump back though the HX to the first tank. Depending on the OWB flow rate and temperature, the DHW circ may not have to run much to keep the tanks hot.

I ran my 5 x 12 flat plate for 5 years with an OWB. The plate was connected in series with the inlet to my 40 gal water heater, the water heater was shut off for the winter months. I never ran out of hot water with this setup.

I did not have a problem with corrosion on the OWB side, even after 5 years on an open system. I did have to pull the HX once to flush out the potable water side after someone unplugged the water softener.

DT,

That's very interesting that you were able to heat your water with no tank. Those plate HX must work really well! Did you run a bronze or stainless pump on the DHW side? I know they recommend those for corrosion resistance. All I can find locally are cast iron pumps. Didn't know if it was worth ordering a bronze pump or not?

I have a taco switching relay that will turn the DHW circulating pump on when the aquastat (yet to be installed) calls for heat.

If I plumb in series to the water heater inlet, I assume that means your cold water supply goes into the circ pump then into the HX and out from the HX to the hot water heater inlet? If I do that, then I may want to also install a means of bypass so that in the summer months I can use electric to heat my DHW and not have to run the OWB.

 

[dirttracker]

I did not use any pump at all on the DHW side of my system. When I was using the OWB, the DHW was heated by the plate HX when there was demand for hot water. The circ on the OWB ran continuously (side note, the OWB was an open system and used a cast iron circ. After 5 years it was pretty well trashed by the corrosion). The water in the water heater tank was only kept warm by the tank insulation. I never had a problem with having the water go cold in the water heater, but we never went more than 12 hours without using hot water. I suspect if I had to go longer with this setup, the water in the tank would have eventually cooled down.

When you set up your circ for the DHW, I would pull water out of the last water heater and force it back through the HX and on to the inlet of the fist water heater. This will put the pump in parallel with the plate HX and water heaters. You will need a pump with a built in flow check to keep cold water from bypassing the plate HX and water heaters through the DHW circ. I'd draw a simle picture, but the computer I usually use is broken and won't be fixed until Monday. Hope my description will be sufficient.

 

[MNBobcat]

I did not use any pump at all on the DHW side of my system. When I was using the OWB, the DHW was heated by the plate HX when there was demand for hot water. The circ on the OWB ran continuously (side note, the OWB was an open system and used a cast iron circ. After 5 years it was pretty well trashed by the corrosion). The water in the water heater tank was only kept warm by the tank insulation. I never had a problem with having the water go cold in the water heater, but we never went more than 12 hours without using hot water. I suspect if I had to go longer with this setup, the water in the tank would have eventually cooled down.

When you set up your circ for the DHW, I would pull water out of the last water heater and force it back through the HX and on to the inlet of the fist water heater. This will put the pump in parallel with the plate HX and water heaters. You will need a pump with a built in flow check to keep cold water from bypassing the plate HX and water heaters through the DHW circ. I'd draw a simle picture, but the computer I usually use is broken and won't be fixed until Monday. Hope my description will be sufficient.

DT,

From what I've read, the inlet of the hot water heater is supposed to enter the tank and go down through a pipe and dump near the bottom of the hot water heater. So I'm thinking I could use the drain valve and have about the same effect but a lot easier way to tap in.


I'm also thinking I could use the bottom thermostat on each tank to control the pump. If either or both thermostats determine that water isn't up to temp, it would close the contacts (like flipping a switch) and would tell my taco switching relay to turn the pump on. That way both tanks need to be up to temp before the pump turns off.

Sound like a good plan?

 

[Gooserider]

From what I've read, the inlet of the hot water heater is supposed to enter the tank and go down through a pipe and dump near the bottom of the hot water heater. So I'm thinking I could use the drain valve and have about the same effect but a lot easier way to tap in.

I agree, but would suggest that you make the tap using a Tee, so that you still have a drain valve...

I'm also thinking I could use the bottom thermostat on each tank to control the pump. If either or both thermostats determine that water isn't up to temp, it would close the contacts (like flipping a switch) and would tell my taco switching relay to turn the pump on. That way both tanks need to be up to temp before the pump turns off.

Sound like a good plan?

Strikes me as a very reasonable approach - the only thing that might be worth considering is whether or not there would be a value in adding a differential function to look at the boiler system temps and NOT turn on the pump if the tanks are hotter - you don't want to do so in such a case, as this would result in pumping the heat OUT of your hot water tanks...

Gooserider

 

[dirttracker]

MN,

Your HX plumbing will work and it has the added benefit of being able to use the water heaters on electric if you have to remove the HX for cleaning. When you do the plumbing, put a shut off on the inlet to water heater #1 to be sure the inlet is forced through the HX. This will give the fastest regeneration as you will always have preheat on the inlet water as long as there is some heat in the OWB.

I like your idea of using the water heater thermostats to run the circ. I've never had an electric water heater and don't know how the thermostats work. I could see this getting a bit comlicates if the thermostats are just a set of contacts to feed current to the heating elements. You'll have to spend some time sorting that out. I would suggest you use both thermostats in parallel to command the circ. That way the circ will run when either tank needs to be warmed. If you are concerned about drawing down the water heaters, you could use an aquastat to not allow operation of the circ if the boiler temps are below say 140. This depends on how you plan to run the OWB