Heat exchanger or hydraulic separator?

Bernie_the_Brewer · May 6

I apologize for the fusillade of questions lately. I am getting very close to pulling the trigger on accepting a bid, and am trying to take care of a few details.

I have a converted gravity system, so ~170 gallons of water in the 100-year-old iron/steel pipes and rads. I specced a magnetic dirt trap, strainer filters, demineralized and treated water, etc.

However, the pro that won the job is offering two options for the primary/secondary interface: The expected/requested hydraulic separator, or an upgraded option of a flat-plate heat exchanger.

Naively, it seems like the flat-plate HX would be a big improvement for this application. I imagine that it would subject the boiler HX to a lot less gunk/scale, and presumably extend the service life. So I wonder why a HX is not used as the separator for the primary and secondary loops more often? The upgrade is not cheap, but not ridiculous, about the same as the sticker price on a middle-of-the-line modcon.

Any comments on desirability of the flat-plate HX option? Thanks!

(I couldn't help but notice the one in @hot_rod 's AWHP project: )

GGross · May 6

any time you have a heat exchanger there is an efficiency drop. in smaller systems a hydro sep, or buffer tank can provide some added volume allowing for less cycling. There is an argument to be made where a heat exchanger can be a better option, but in my mind the heat exchanger is typically a last resort to protect the boiler due to site conditions.

Gunk and sludge is not good for your boiler, and it will also impede heat transfer. typically I would rather put money to cleaning the existing system and using something like a caleffi sep4, however your contractor may have info I don't have. with a heat exchanger you also will need 2 expansion tanks, and pumping requirements for the system may change with the added pressure drop of the additional heat exchanger.

an example of when I opted to use a heat exchanger, there was a system that was used as a chilled water pump and dump loop in the summer months. they were killing boiler heat exchangers annually so I recommended separating the boilers from the system via heat exchangers.

hot_rod · May 6

For my application the HX is to separate glycol in the HP circuit from the buffer/ separator and the radiant loops.

If you select a separator with a 4 in 1 function you get the hydraulic separation with excellent air, dirt, and magnetic particles separation.

I don’t see a need for the plate hx and I would definitely add a Y strainer to it if you go that route

The Sep 4 is one of the high performance options.
But as @GGross mentioned, a good power flush of the rad and piping before you connect to it would be wise

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Caleffi-549597A-Brochure.pdf

mattmia2 · May 6

the hx will be a lot less tolerant of the muck in the gravity system than a fire tube boiler will be. you will still have a circulator on the system side and that is the thing that is most likely to have a problem with the iron much in the system. it can stick to the magnets in the wet rotor circulator and damage things which is why the magnetic dirt separator is important.

if it is a single zone and you do your math right you probably don't need to do primary secondary on a gravity conversion with a modcon. if it is also running an indirect or is several zones primary secondary is probably better.

jumper · May 6

»and you do your math right you probably don't need to do primary secondary «

one pump is better than two. For one thing two pumps double pump trouble

Bernie_the_Brewer · May 6

Just for completeness: The system will have 3 space heating zones and an indirect. And we are planning on a thorough flushing (and demineralization and water treatment, as said earlier).

hot_rod · May 6

what type of boiler? The manual should typically show the required minimum flow and operating delta

So look at the gpm of the smallest zone to see how it matches up with the boiler requirements

While some fire tube boilers can be direct piped, installation manuals generally steer you to PS or hydraulic seps

pedmec · 1:49AM

HX is good for one thing really. Exchange heat energy from one medium to another. As hot rod has stated you are much better off with a sep 4. You get four critical functions out of one device. In my mind its a no brainer.

leonz · 2:30AM

You are better off with a steel compression tank, Airtrol valve and a drain-O-Tank valve to maintain your point of no pressure change.

A 30 or 40 gallon Quick Tank brand compression tank or a pair of them with two airtrol valves and two Drain -O-Tank valves properly plumbed will provide you with greater thermal mass, keep your circulator flooded with low pressure water and allow your circulator to work with less effort and you will have a greater air cushion than you would ever have with a bladder expansion tank air scoop and automatic air vents that will leak AND IF IT IS PLUMBED CORRECTLY YOU WILL NEVER HAVE TO BLEED AIR from your system as the 1/3 to 1/2 air to water ratio in a steel compression tank will absorb any air bubbles rising from the boiler through the Internal Air Separator feeding the air bubbles and hot water to the steel compression tank or tanks through the airtrol valves in the case of 2 steel compression tanks.

mattmia2 · 4:09AM

if the airtrol is working properly the thermal mass of the compression tank is more or less isolated from the system.

Heat exchanger or hydraulic separator?

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