Sizing exchanger block for DHW to hydronic heat?

Jells

I'm building a system for a 450 ft apartment. I've heated other identical units with just a 40 gal atmospheric DHW and bronze pump successfully before, but now I'm upping my game and doing it correctly with an exchanger block and 2 pumps. I understand most of the proper design, basically same as a boiler hydronic loop but with an exchanger block replacing the boiler.

My biggest question is sizing the block. Since I'm already running relatively low 140 temp from the tank I want the most efficiency I can get from the exchanger for the most BTU throughput to the radiators. Does upsizing the block past the DHW 40k BTU rating do anything for this? The extra cost for a higher BTU rating than the cheapest 60k Bell & Gossett BP400 is nominal.

EBEBRATT-Ed

It's all about water temp and flow on both sides of the loop. A bigger HX will allow lower temp from the water heater ....to a point of diminishing return

hot_rod

https://www.caleffi.com/sites/default/files/file/idronics_29_na.pdf

Jells

EBEBRATT-Ed said:

It's all about water temp and flow on both sides of the loop. A bigger HX will allow lower temp from the water heater ....to a point of diminishing return

Thanks, but sorry, I'm not really understanding how to apply that. I'd be running the BW atmospheric DWH at max no matter what. If I have 40k BTU input to the DWH, and total potential radiator emittance of 20k@140 deg, will a bigger block than 60k BTU actually make a noticeable difference in the water temp in the loop? Or is it already so far above my emittance that it won't make a difference? Since losing 20 deg in the block means losing 10% of BTUs emitted, it seems important.

I'm also trying to decide whether to do my 2 kickspace radiators in a simple loop or a use Monoflo loops off a main loop. The units are 1/2 copper connections and I'm plumbing it with 3/4 pex and clamps.

@hot_rod That's a big doc, I'm not sure what there answers my question. When you're not trying to optimize for cost those kind of design manuals become less useful.

EBEBRATT-Ed

@Jells

I think what your asking no one here can do. You have to pick a manufacturer and go to them.

Give the gpm for both sides of the exchanger (water or glycol)
Btu output of water heater
Btu output of the radiation (load)
Some idea of the SWT you need

They can quote you a smaller HX and then an oversized one as well

Jells

EBEBRATT-Ed said:

@Jells

I think what your asking no one here can do. You have to pick a manufacturer and go to them.

Give the gpm for both sides of the exchanger (water or glycol)
Btu output of water heater
Btu output of the radiation (load)
Some idea of the SWT you need

They can quote you a smaller HX and then an oversized one as well

I think I'm just going to go for it. My gut says the input and the HX rating is so far above the output that the issue is nonexistent. The return water will still be very hot after the initial flow of a heat call, the rads simply don't have the output to create a problem there.

Zman

http://flatplateselect.com/site/pub/login.aspx?ReturnUrl=/site/default.aspx

GroundUp

There is no such thing as a "60k BTU block". A given 20 plate 3x8 heat exchanger might transfer 15,000 BTU with 150* boiler water and 140* load SWT. That same 20 plate HX might transfer 150,000 BTU with 190* boiler water and 40* load water. What the guys are trying to tell you, is that it depends on what you need the HX to do. Do you need the load SWT to match the water heater temp? What type of heat emitters will be used and what is the flow rate on both sides? The closer together those temps are, the larger the HX needs to be. A 20k high temp baseboard system will require a larger HX than a 20k low temp radiant system.