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[american-pacemaker]

Why does everyone use 5 x 12 and not 10 x 20 flat plate heat exchangers ? Is it purely a cost reason or is ?
Also can a heat exchanger be to big for outside wood boiler to inside oil boiler installation ?

[LittleJohn]

I would say $$$ & availability, are two big facters. 

...short web search later, do you see the MINIMUM flow rates and/or the related BTU outputs  for the 10x20 -> I don't know anyone running pumps at that kind of speed (>45gpm on the boiler side, thru 1.5" orifices)  ***Betcha those pumps dim the lights***

Now back to oversizing heat exchangers (question of FPHX too big for OWB); its only an issue if you are pulling more than 20F across the HX, and sending water back that is too cold, causing condensation in the firebox of the OWB.  That is why most boiler like the water to come back at a certain temperature.

This loss of heat can be controlled in several ways:

• Increase speed of pump on OWB or boiler and reduce pump speed on other side of HX
• Install a delta T pump, like a Taco Bumble bee on the HX, opposite the boiler/heat source
• Install a thermostatic mixing valve on the opposite side of the HX from the the boiler/heat source
• Install a Mechanical mixing valve on the opposite side of the HX from the the boiler/heat source

[american-pacemaker]

Littlejohn, I have been told to use a 50 and an 80 plate exchanger for my system. I have a 50 with 1" connections but I have a
1 1/2 primary tube on my oil boiler. If I get a 5 x 12 with 1 1/4 connections and 80 plates I want to make sure it is right.
Thanks

[LittleJohn]

Pacemaker, if you are planning to run 2 HX, I would plumb them in parellel

In gerenal the larger you go with a HX (number of plates), the less head pressure you have, but the more $$$ you have to spend to buy the HX.  Now that beign said, you may be able to contact any HX company in your area, and have them size your a heat exchanger based on your system. 

I want to say that I am pretty sure I am running a 5x12 - 50 plate and it was rated somewhere between 250-500k BTU/hr, depending upon allowable temperature drops; which gets back to delta T across HX it ifs too high it can cause condensation in firebox, which is not good.  The larger the allowable delta T, the higher the BTU.

...quick story, had an issue last fall gets new in-slab radiant working in house and it took a couple of weeks to find issue  .  Once I got everything working, and restarted system the HX were cold to the touch, I thought I plugged one, nope I was just pulling so many BTUs the HX felt cold even though I was pumping 180f into the heat exchanger on the OWB side.  This only lasted about an hour and then things started coming up to temperature and then there was no noticeable temperature drop across the HX

[RSI]

Why does everyone use 5 x 12 and not 10 x 20 flat plate heat exchangers ? Is it purely a cost reason or is ?
Also can a heat exchanger be to big for outside wood boiler to inside oil boiler installation ?

Since they cost about 5 times as much as a 5x12 I doubt many even considered them.

I use 5x13" heat exchangers.