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[RSI]

Is anyone here running a plate heat exchanger for dhw and no mixing valve? If so, what size and is the water too hot normally or usually good?

[WoodMOJoe]

We run a 20 plate with no mixing valve and it works great, notice no variation in temp while taking a shower.

[mlappin]

If the plate exchanger is in the cold line going into the water heater wouldn’t expect much difference in temp at all.

[RSI]

We run a 20 plate with no mixing valve and it works great, notice no variation in temp while taking a shower.

I am not talking about variation, I mean max temp out of the faucet.

When you run hot water, how cold does the boiler line out of the plate get?

[E Yoder]

We use 3X8" 30 plates. This will give about 140 - 145 F output of domestic water side, about 130-135 after going through the water heater. Will do those temps all day long if needed.  Furnace line will run a 40-ish degree Dt while you're drawing hot water but that's very intermittent. We rarely use mixing valves. Keeps it simple. Bigger FPHX would run hotter domestic water output but there's really no point.
And a 3X8" 30 plate is big enough  to not restrict flow on a typical 4-6 gpm house circuit.

We've used hundreds. I love them .

[RSI]

Are you sure that it isn't restricting the flow? Have you ever compared flow with and without the plate hx in the circuit? From those numbers, it looks like you are taking all the available heat from the water line.

My home system runs at 7gpm (according to the pump display) and I get about 7 degree delta t when running DHW.  I have a 5x13" 30 plate, I used this size instead of a 20 just for better flow.

When I put in the G200 last fall, I got the pipes mixed up and was pumping through the system backwards. I left it that way since it was working ok and the DHW was staying in a good temp range. Saturday I swapped it with a G100 and connected the correct way. Right away the DHW got much hotter.

I am thinking the way to go it flow same direction instead of cross flow for DHW,

[shepherd boy]

If you have a faucet flowing at 2gpm, and raising temp from 55 degree water to 145- 90 degree rise x2 =180. Divide that by 7gpm stove flow you get a 25.7 degree drop in stove temp water. Depends on DHW gpm draw as to DT. We always counter flow heat exchangers.

[RSI]

I doubt the faucet was drawing 2gpm from the water heater with the 7 degree delta t. Mostly because it is so hot in the tank that there is a lot of cold added at the faucet. I just looked at my logger graph to get the numbers.
What size heat exchangers do you use?

[mlappin]

20 plate 5x12’s with a mono flo T.

[shepherd boy]

I doubt you are running 2gpm, but when installing one we need to be prepared for anything someone wants to do, Like run three showers at once. We like a narrow heat exchanger, like 3" wide. Seems like we get better heat transfer, usually use a 3x8 30plate , have used 40, but couldn't tell much dif. 20 plate does seem a little less effective.Seems we get more restricted flow from long pipe and too many ells than we do out of flat plate heat exchangers.

[E Yoder]

I have not checked exact flow rates for a long time. Once I checked an NRF-22 pumping 1" Pex circuit going through 1 3x8" 30 plate, several tees and 90's, 3 hot air coils and had 5 gpm.  Made 140F domestic water all day long. That was an experiment at our shop, normally I'd do P/S piping on something like that to boost flow.
I did submit my flow to an online calculator and the restriction wasn't bad. The 40 degree Dt was a rough ball park estimate pulling several gpm on domestic side. Most house domestic load is less than 1 gpm. Then Dt drops accordingly.
BTU load is the familiar gpm X Dt X 500. I'm sure you're way ahead of me on calculating stuff like this.
From my experience long distance piping tends to be the problem a lot more often than 1 FPHX.

One difference in most of the installs we do is we are tying into heat pumps which tolerate big Dt's much much better than boilers.

[E Yoder]

Pushing heat into a low temp system like a heat pump or low temp radiant is so easy. You can run huge Dt's and small pumps and it heats wonderfully. Big houses heated on 3-4 gpm. The job I did today had a 2 1/2 ton heat pump and a 5 ton heat pump. I doubt I had more than 5 gpm. But it heated very nicely, with a 30-ish degree Dt.
Copper baseboard is a whole nother animal! Tight Dt, High temps.

[E Yoder]

One thing I've noticed with low flow rates through a hot air exchanger (coil) is if you counter flow the water so the hottest manifold is at the downstream side if the airflow it can run quite a bit hotter air temps. Same principle applies in any HX but I've been surprised how much that helps.
Usually I run into air problems tho if it goes too low. Can't pick up the air and slap it through a big coil.

Good idea RSI, I should try a loop with and without the flat plate. Would be interesting.

[aarmga]

How long does a typical water heater tank hold hot water with the plate exchanger feeding the cold side?  I was planning on installing a plate exchanger on the hot side of the water heater and bypassing the tank all together and turning off the water heater durning the winter months.  I don't like the side arm at all and heating the tank seems like a waste of energy to me.  When I wash my vehicles I have no problem running my 40 gal hwh out of water.  O boy does the wife get mad at me. 

The reason I ask how long the tank holds hot water is really the only question I have for which side of the  hwh I will be installing the plate exchanger.  If I install it on the hot side, turn the unit off and run cold water through the tank to keep it from getting stale basically my setup would be nothing more than a tankless hwh.

[E Yoder]

I've always cut the cold line entering the water heater and routed it through the flat plate. The reason for this is the hot water entering the water heater holds the electric off so you have auto backup. Also running it through the tank keeps you from noticing fluctuations in temp due to changes in flow rate through the flat plate (multiple hot water uses throughout the house).
I've found you need to take about two showers per day to replenish the tank temps so if you only have one person living there then you might have to put it on the hot side and turn off the electric. And when you turn on the electric in the summer be sure to turn off the stove pump if you're not burning to avoid cooling your domestic water.

So there's more than one right way, but I normally preheat the tank for simplicity.
And you'll love your unlimited hot water! We did a 3X8" 30 plate doing domestic hot water for a fraternity house once and they'd do 30-40 showers back to back . Would burn an MF10,000 firebox load of wood in 4 hours!