A plate HX for DHW

hot_rod

This is my second time using the SWEP software, no one to check my math at SWEP today :)

So to use 150° SWT, 25∆ return to boiler at 125°, 8 gpm flow rate

incoming domestic water at a fairly low 40°

98,000 btu/hr, a 110 mod con for example, gives me 2.8 gpm of 110° dhw

So a 22 plate HX in this case.

The plate HXers in combis are much smaller and need 180° SWT to get 'er done.

DHW HX st 150SWT.png

LRCCBJ

The beauty of this is that you can design your own HX for whatever output temperature you need. Here you are able to transfer 2.8 GPM @ 110°F. I'd be inclined not to design it so close to the edge and see if I could get 130°F from 150°F supply.

Furthermore, you can clean it with ease (if you pipe it correctly).

The question that begs is whether you can utilize it WITHOUT a tank and be pleased with the results? If the mod-con is not ready and waiting at 150°F, you're going to have to wait for it to start and climb. This isn't very long but it is a consideration in the summer and shoulder seasons.

To eliminate the indirect and all associated piping to and from: PRICELESS.

You cannot eliminate the tank if you're using a CI boiler on cold start. You could use the electric water heater as a tank (less costly).

I suppose you'd need to add a flow switch on the incoming supply…………..notably problematical.

hot_rod

The combis that I have owned could be either cold start or always hot, using a bit more energy.

From cold start on the Gianomi type boilers warm up was fairly quick.

I tried both Harwil and Sika .5 and .35 gpm flow switches and they both worked fine. Most tankless have a flow switch to trigger them on. Sika seems to be the most common.

With this 500 gallon buffer tank in my last shop, I could shower with 110 tank temperature across that large plate HX.With close approach HX sizing you can get within 3- 4°

The tank was connected to a wood boiler and 160 sq ft of solar thermal. Boiler back up would just heat the very top of the tank. A big air bubble at top was the expansion space and drain back siphon break. Plain water in the tank

Also a circulator test bed :)

It got neatened up before I sold the place. A rookie hydronic guy came out, filled the buffer tank, causing all the collectors to freeze after we moved away.

Their kids left the garage door open and froze the wood boiler! I suspect the entire system has been abandoned.

Screenshot 2025-01-19 at 9.31.15 AM.png

PC7060

https://forum.heatinghelp.com/discussion/comment/1836198#Comment_1836198

sad but not uncommon. I try to keep my systems understandable by moderately experienced techs but it’s to hard stop errors when they won’t read the tech data.

LRCCBJ

With this 500 gallon buffer tank in my last shop

OMEG, that is quite the tank. How did you even get it there?

LRCCBJ

From cold start on the Gianomi type boilers warm up was fairly quick.

Define "fairly quick" considering the fact that you are standing beneath the showerhead…………………waiting.🤔

I don't believe any of the FT heat exchangers can get up to 150°F (from 70°) in one minute. 2500 BTUH required to lift 4 gallons in the unit. A touch under two minutes……………..I would think this is unsatisfactory for most folks.

hot_rod

https://forum.heatinghelp.com/discussion/comment/1836283#Comment_1836283

me personally I wait outside the shower until it reaches the desired temperature. So 2 seconds or 2 minutes doesn’t much matter. Although the city treated water down the drain is more of a concern for me, in a water starved state.

My wife prefers the tub and the wait is a non issue.

Keeping a 3 gallon FT boiler hot isn’t that expensive, if you want instant hw output. Certainly not compared to a tankless coil in a cast iron block😳 what’s your time worth.

Any tank type hw tank is costing you standby loss $$ also.

My warm up coil type mod con example was in comparison to a cold block of cast iron warming up.

LRCCBJ

Your solution of keeping the FT reasonably warm (120°F) at all times appears to be the best solution for the off seasons. This would be a good compromise between waiting two minutes and instantaneous. The only thing I don't like is the cycling that the FT would suffer. I'd see if it could manage a DT of 30 and go from 140° down to 110°F.

Some of that wait time is inevitable anyway without full time recirc. I have recirc here and I still wait exactly 35 seconds……….the recirc is only on the first floor.

Compare the nominal cost of keeping the FT warm for six months versus the cost and maintenance of an indirect………….AND the additional space requirement for it…………..and the standby losses from it!

I love the fact you can feed it with 150°F supply vs. the 180°F for the indirect. A big savings right there.

It's a fine solution if you tweak it a bit!

Contractors should utilize it more often!

hot_rod

https://forum.heatinghelp.com/discussion/comment/1836370#Comment_1836370

the cycling time and noise grates on me also. My mother in law would hand wash every dish, water on, water off in a 3 second time frame. I could hear the combi fan start to rev and immediately ramp off. A small buffer in a tankless or combi seems like a good compromise.

LRCCBJ

I was thinking the exact same thing. How about this…………and the FT never needs to cycle to maintain a minimum temp:

https://www.supplyhouse.com/AO-Smith-EJC-6-6-Gallon-ProLine-Compact-Residential-Electric-Water-Heater?

mattmia2

Some combis have a small tank built in.

You can feed the indirect with far less than 180f if you size it to handle that.

I already don't believe the .5 degree f/hr standby loss of the indirect but i especially don't believe it after coming home from work on friday to the boiler firing to heat it when it had just recovered before i left for work about 9 hours earlier

LRCCBJ

I already don't believe the .5 degree f/hr standby loss of the indirect

Figure about 2X that value………..minimum………..maybe more if you store high and mix down.

hot_rod

if the test is done in a room held at 80 degrees with a 120 tank temperature, heat trapper nipples??