# How do I size a flat plat exchanger to heat DHW



## MrEd (Nov 4, 2009)

How do I pick which size flat-plate exchanger I need to add to my 50 Gallon electric hot water tank to heat it from my tarm. I am going to run it as a seperate zone, with a potable pump to circulate the DHW. I assume that too big is better than too small, but I have no idea how to pick other than just oversizing it, which gets expensive.

I don't necessarily need a formula with the exact calculation, I'd be happy with a "I used a xxx and it worked for me"....


----------



## MNBobcat (Nov 4, 2009)

I have the same question but in my case I need to heat two 80 gallon electric hot water heaters that are currently set up for off-peak electric.


----------



## emesine (Nov 4, 2009)

Can you explain your system a bit more?  

Plate HXs need a pump on either side to transfer heat.  What is the temperature of the hot side, what pump is going to get the water there, and for how long will it turn on?  Ditto for the cold side- it sounds like you will have drinking water circulated through the cold side via a potable circulator, which will work great.  How fast do you want your heater to heat up?  I have been looking into HXs for the lasts few weeks intensively, so I think I can answer your question if you give me a bit more info.  

Andrew


----------



## MrEd (Nov 5, 2009)

emesine said:
			
		

> Can you explain your system a bit more?
> 
> Plate HXs need a pump on either side to transfer heat.  What is the temperature of the hot side, what pump is going to get the water there, and for how long will it turn on?  Ditto for the cold side- it sounds like you will have drinking water circulated through the cold side via a potable circulator, which will work great.  How fast do you want your heater to heat up?  I have been looking into HXs for the lasts few weeks intensively, so I think I can answer your question if you give me a bit more info.
> 
> Andrew



Guess I want it to heat up at least as fast, and hopefully faster than, the existing 50G 4500W electric tank does it. All the regular temps, well water coming in at whatever well water is, boiler water is going to anywhere from 160->200 degree depending on where I am in the burn cycle. I plan on superheating the 50G tank and using a tempering valve to stretch out the hot water; plenty of extra BTU's coming out of my Tarm. 

The 50G electric marginally gives enough hot water for the family, so rough calcs I'd want it to go from cold to hot 50% faster than the electric does, if it could produce twice as much hot water, all the better. Not sure how to be more specific than that...is that enough info to make a reasonable estimate?

Is there a simple calc to get me from 4500Ws to the equivalent BTUs?


----------



## emesine (Nov 5, 2009)

4500 watts X 3.4 = 15,300 BTU.  

That will be very easy to obtain with a flat plate HX.  You will need a pump or some way of getting hot water out of your boiler loop and into your HX....  Let's say you use a taco 003 which will give you about 3 gpm at 3 feet of head...

According to www.flatplateselect.com you will need a 5 X 12 inch by 10 plate HX.   You will get 60,000 BTU transfer with the following parameters

Hot side
3 gpm
in  180
out 140

Cold side
2 gpm
in 100
out 150

Hope this helps!

Andrew


----------



## maplewood (Nov 5, 2009)

Thanks - I'm just about to install one, too.  Perfect timing for this thread!


----------



## DaveBP (Nov 5, 2009)

> Is there a simple calc to get me from 4500Ws to the equivalent BTUs?



The unit you convert to BTUs is watt-hours.

1000 watts for one hour = 1000 watt-hours, or more commonly 1 kilowatt-hour. 

1kilowatt-hour = 3413 BTUs

4500 watt-hours = 4.5 KWH  X 3413 =15358 BTUs

Don't sweat the exact number. Like emesine says, easier to think 15,300


----------



## rowerwet (Nov 5, 2009)

MNBobcat said:
			
		

> I have the same question but in my case I need to heat two 80 gallon electric hot water heaters that are currently set up for off-peak electric.


for that kind of install I would go with a sidearm HX, a FP would require a pump to circulate the water through the water heaters, a sidearm set up would use normal convection to do the work for no extra electric. I pre-heat the supply to my tankless hot water heater using a shell HX. I made an 8' long shell HX out of copper pipe 3/4" for the water supply and 1 1/4" for the shell that holds my boiler loop water. The heated water goes to a mixing valve and then to the water heater. 
 for your set up you should put a side arm HX on each water heater and a mixing valve on the outlet so you don't scald anyone, save the electric for summer.


----------



## MNBobcat (Nov 10, 2009)

rowerwet said:
			
		

> MNBobcat said:
> 
> 
> 
> ...



I'm sorry, I just now found your post.  For some reason I'm not getting all of my emails alerting me to replies.


With my OWB I can plumb it two ways.  I can either run it in series where the water is pumped from the boiler to the DHW HX and then to the furnace HX and back to the boiler, or I can go from the OWB to a manifold and then take whatever runs I need off of the manifold.  I'm thinking of going with the latter.  In which case I would have a pump for the DHW.  I wonder if there isn't some way to put a tem sensor on the hot water heaters which would cause the pump to turn on as needed?


----------



## Mid Michigan (Nov 10, 2009)

My set up uses a plate heat exchanger. The OWB circ. pump is always running so that side of exchanger is always hot. I put a small circ. pump on the domestic side to recirculate water from farthest point of DHW system back to my propane HW heater. It goes through the heat exchanger on its way back to tank. I even have the cold water make up going through heat exchanger and than into tank. I put a aquastat with a capillary tube down into my DHW tank. I took the anode out and used that opening for a well I made out of copper. I screwed in down into tank and inserted aquastat. I never run out of hot water. I could leave it running all day and it would be hot. It is nice though to know the aquastat is shutting down circ. pump when up to temp. Also it keeps my insulated pipes hot so I don’t have to wait for hot water at the farthest bathroom. To be fool proof I should add another aquastat to the return line from the farthest point as the lines in crawl space will cool faster than the tank. Will have to add check valves to your system to keep everything going in the right direction.

My plate exchanger is 50,000 btu/hr. and here are specs for it. HW in 180 out 160 at 5gpm. Domestic in at 40 out at 140 1 gpm.


----------



## rowerwet (Nov 11, 2009)

MNBobcat said:
			
		

> rowerwet said:
> 
> 
> 
> ...


if you want to go that route, you need an aquastat in the hot water tank, set it to the top temp you want in the tank, If you want to go to the trouble and expense. Either way you will save electric as a  motor is cheaper to run than a heating element.
I always go for simplest, a side arm downstream from your boiler using convection, it doesn't get simpler than that. (I work on airplanes for a living and deal with unneeded complexity breaking down all the time)


----------



## dogwood (Nov 11, 2009)

MrEd, try this link, https://www.hearth.com/econtent/index.php/forums/viewthread/40632/ for answers to a similar question I asked a few months ago here. I ordered a Bell and Gossett 125,000 BTU 20 plate low pressure flat plate BPX HX from PexSupply.com a few days ago for this same purpose (cost $183). I also acquired a low flow Laing recirculation pump, model SM-303-BTW, to go with it on the DHW side from BuyPlumbing.net (cost $145). There are links on the bottom of the PexSupply ad for the for the B&G flat plate hx that will take you to sizing, (btu, pressure drop and gpm) tables. I hope the components do the trick with my new Tarm, install starting soon. Good luck. 

Mike


----------



## Gooserider (Nov 11, 2009)

MNBobcat said:
			
		

> rowerwet said:
> 
> 
> 
> ...



If your only loads are the DHW and furnace HX's what I would consider doing is using a sidearm for the DHW and putting the two in series, with the DHW HX first, and put a ZV controlled bypass loop around the DHW HX...  If the house calls for heat, water goes through the sidearm and warms the DHW tank, unless the tank is hot, in which case it opens the ZV and the house gets the heat direct...  This gets rid of a pump, which reduces operating costs, and lets you use a cheap sidearm instead of an expensive flat plate...  It also gets rid of the manifold.  One could also do the manifold w/ two zones on it, one for the house and one for the DHW, each with a zone valve, but you'd probably find the DHW loop had so much less flow resistance that when it opened the house HX wouldn't get any flow....

IMHO while there are things that FP HX's are good for, and there aren't good substitutes, I don't really like them - they are expensive, have high head resistance (leading to higher operating costs) and are prone to clogging and other failures (especially on an open system) I think it is best to use them as a only last resort if there aren't any other more efficient alternatives.

Gooserider


----------



## dogwood (Nov 11, 2009)

Gooserider, my understanding is there is a slow turnaround time for reheating your DHW with a sidearm which is not acceptable for all applications. There are a couple of threads on how to keep them clean in a fairly painless fashion. If I remember, you flush them with vinegar? every so often, and HR recommended some fittings recently so you can do this without removing the flat plate from the piping. I think when you want to take a shower or run the dishwasher the hot water needs to be there in a timely fashion. On the other hand I don't know how long it would take a sidearm to reheat the DHW. I went with the added expense of a flat plate and we'll see what happens once our install is done. I thought of it as an investment, like the rest of the system. 

Mike


----------



## rowerwet (Nov 11, 2009)

even simpler connection for a flat plate HX, shark bites and pex. the whole thing comes off in 30 seconds.


----------



## Gooserider (Nov 11, 2009)

dogwood said:
			
		

> Gooserider, my understanding is there is a slow turnaround time for reheating your DHW with a sidearm which is not acceptable for all applications. There are a couple of threads on how to keep them clean in a fairly painless fashion. If I remember, you flush them with vinegar? every so often, and HR recommended some fittings recently so you can do this without removing the flat plate from the piping. I think when you want to take a shower or run the dishwasher the hot water needs to be there in a timely fashion. On the other hand I don't know how long it would take a sidearm to reheat the DHW. I went with the added expense of a flat plate and we'll see what happens once our install is done. I thought of it as an investment, like the rest of the system.
> 
> Mike



You are right Mike, a convection driven sidearm is not going to be as fast as a pumped HX at recovery, or initial heating of a cold tank.  But I feel that if one does appropriate sizing of the DHW tank, and plumbs the sidearm so that it sees a relatively steady flow of heat, then the sidearm should be able to keep up with recovery as the hot water as used...  It probably does take a bit more attention to planning, and sizing to fit the loads, but should be doable in most cases.  I just don't like the added expense of purchasing and running another pump if it can be avoided...

There are also situations where a flat-plate IS the right answer, and if it is, I would use one without hesitation.  I just want to be sure about the answer, and possibly see if I can change it by asking a different question.

Gooserider


----------



## dogwood (Nov 11, 2009)

Gooserider, If you take a shower, run the dishwasher, and then are out of hot water when your wife decides to bathe, it is a not a trivial problem. The extra expense to avoid this is a trivial expense in comparison. My wife has shown remarkable forbearance in light of the massive expenses so far acquiring the boiler and parts. I would like to have the end result of our install to be a fully functional system with both on-demand heat and hot water. Waiting around a few hours for the water to heat up is not going to make the grade hereabouts. Other's situations may differ.

On another note, I also went for a twenty rather than ten plate to allow for lower than optimum supply input temps and to quicken the heat exchange time. Somebody recommended the twenty plate in a previous thread, if I recall, but thought a thirty plate would be overkill.

Mike


----------



## MrEd (Nov 11, 2009)

> Waiting around a few hours for the water to heat up is not going to make the grade hereabouts. Other’s situations may differ.



That was my thinking...a passive sidearm might work for some, but with a family of 6, and soon to be 4 teenagers (and already owning a potable pump and electric DHW tank), the pump powered, flate plate heat exchanger ($129) seemed like the safer, cheaper bet for me.


----------



## Gooserider (Nov 11, 2009)

Not telling anyone how they must do stuff...  I agree with the idea that running out of hot water isn't acceptable, but I don't agree that a properly configured sidearm or equivalent non-plate / non added pump setup would be unable to handle the makeup load...  However each person needs to make their own choices about that sort of thing.

Gooserider


----------



## dogwood (Nov 11, 2009)

Mr. Ed, maybe someone with a bigger family than mine (two only) could chime in a let you know how many plates and what dimensions your flat plate should have to service that big a family effeciently. I'm thinking a ten plate may not be enough as previously suggested, but I am a rank amateur at this and have relied on the advice of others in previous threads in planning and making my calculations prior to purchase and pending installation. And I haven't a clue how fast a properly sized sidearm would replenish your tank. 

Mike


----------



## MNBobcat (Dec 29, 2009)

I'm ready to plumb my plate heat exchanger for my DHW

I have two 80 gallon electric hot water heaters.  On HWH1 my cold supply comes into the cold inlet on HWH1.  Then the hot outlet on HWH1 goes into the cold inlet on HWH2.  The hot outlet on HWH2 goes to the faucets, showers, etc.  I guess the concept is that the hot water sitting on the top of HWH1 will thermosiphon into HWH2.

I will be running a pump to push hot water from the plate exchanger into the hot water heaters.  Where should I tie into the hot water heater(s)?  Should I pump hot water from the plate HX into the bottom drain on HWH1 and then hook the return into the bottom drain on HWH2?  Where would I mount the aquastat to control the pump?


----------



## in hot water (Dec 29, 2009)

The biggest mistake when sizing HXers for DHW is not accounting for incoming water temperatures.  In cold climates incoming water, in the winter months, can be in the mid 30's.  So you are looking at a 110 Δ T to provide 140 °F  DHW.

In warm climates the incoming water will be much warmer, 50 °F is typical where I live from wells or city supplied.  Phoenix has an average water temperature of 78 °F.

Many of the new mod con boilers has flat plate DHW options.  They can produce endless,  DHW at a certain temperature rise and gpm flow rate.  It's limited by the BTU size of the boiler.  The plate HX is sized to that output to provide the DHW.   If that is the customers desire, endless DHW then a HX with a pumped flow on the boiler side would more than likely win the race over a typical thermo siphon exchanger.

DHW can be a tricky load to design to.  Unless a customer can tell you EXACTLY how much and how fast then it's a guess based on "averages"  Like the industry suggested 20 gallons per per person per day at a 90 degree rise.

Here is a simple formula to determine the BTU needed  dhw=  8.33 (gallons) (Th-Tc)      8.33 X 60 gallons from 50- 140 °F would require 44,982 BTUs and enough heat exchanger to transfer that amount in the time period you desire.

In my years in the trade I have never had a customer tell me I provided too much DHW, but I have under estimated a handfull and had to go back and upsize storage and horsepower.

hr


----------



## dogwood (Dec 30, 2009)

MNBobcat, I've been trying to find a definitive answer on where to locate the aquastat to control the circ pump for my DHW too. If I rememeber correctly the closest answer I could find was in a previous thread where someone mentioned that manufacturers usually place their controls near the bottom of the DHW tank. That's where they are on mine now, prior to re-plumbing. So I am going to place my aquastat near the drain port on the bottom of the tank. I am also going to put my circ pump next to it and pump the heated water upwards through the flat plate into the top of the tank, mimicking the direction of flow a sidearm convection system would have. The boiler water will flow in the opposite direction from top to bottom through the flat plate to mimic the counterflow setup on my w/a hx. That's the best I could figure. Hopefully someone with experience will chime in and give you a more definitive answer. 

Another aspect of aquastat usage I have yet to figure out is what is the appropriate temperature differential setting to have on the aquastat for the DHW. Maybe someone can answer you question and mine so neither of us mess this up. I'll try to find the previous thread for you on the aquastat placement. I looked at again just last week while trying to figure this out. 

Here's that thread: https://www.hearth.com/econtent/index.php/forums/viewthread/40993/. It was HR who said to locate it toward the bottom third of the tank. 

Mike


----------



## MNBobcat (Dec 30, 2009)

Mike,

I've read every post on here that I could find that pertained to hot water heaters.  Some said that its important to have stratified water with the hot water on top and the cold on bottom and that the heater is more efficient that way.  

The problem I'm having is I can't find anyone here or anywhere on the Internet who has connected a flat plate HX to a pair of formerly off-peak electric hot water heaters.

In my case, I'll be pumping the hot water from the flat plate HX into one of my hot water heaters.  Presumably, the hot water in the first hot water heater tank will be forced into the second hot water heater tank by the pump.  I'll then be pulling water out of the second hot water heater (return line) back into the HX and pushing the heated water back into the first tank.    Instead of having stratification, both of my tanks should be at full temp from top to bottom.  I'll have 160 gallons of hot water which I'll heat to at least 140 degrees. I'll put a mixing valve on the outlet of the second tank.

What I'm trying to figure out is where to connect the supply line, from the pump, into the first tank and where to connect the return line to the second tank?  Should both connections be made to the drain valves on each tank?





			
				dogwood said:
			
		

> So I am going to place my aquastat near the drain port on the bottom of the tank.  Mike



How will you place the aquastat?  Is there a hole in the tank in that location?


----------



## dogwood (Dec 30, 2009)

MNBobcat, I am going to tee off a place for the aquastat as close to the drain pipe port as possible, hoping that is close enough to get a decent temp read of the bottom of the tank, and insulate that fitting to prevent getting a read on temps outside the tank. Don't have another port into the tank to utilize. The other thing I considered was as someone else suggested, to cut a hole in the outside covering of the tank and place a temp sensor directly on the tank inside and then reinsulate and re-cover that hole. I think that person called it a snap disk sensor in the thread I referenced, and provided the link to above. 

I don't know the answer for your other question but you might try posting it on Dan Holohan's site in "The Wall" section. I managed to get an answer there once to a question I couldn't get or find an answer to elsewhere. It's address is http://www.heatinghelp.com/Forum

Mike


----------



## dirttracker (Dec 30, 2009)

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 (Dec 30, 2009)

dirttracker said:
			
		

> 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.
> 
> ...



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 (Dec 31, 2009)

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 (Dec 31, 2009)

dirttracker said:
			
		

> 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 (Jan 2, 2010)

MNBobcat said:
			
		

> 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 (Jan 3, 2010)

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


----------

