Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Mod con and plate exchanger

heatpro02920 Member Posts: 991
I am trying to perfect this as much as possible, so I can make any mod con any size a combi unit... Ill install all the components in a sheet metal enclosure when I get it perfect...

I have been playing with it here and there since last year, I have come to the conclusion simpler is betterer.... I was jumbling it up with flow restriction and dhw circulators, I stripped all of that away and just a simple 3 way zone valve, aquastat, circ and mixer work well...

Then I ran into an issue where the aqaustat was taking too long to react, in other words, I start the faucet and would get hot water but then by the time the a-stat sensed the drop, the boiler started and circulated hot water through the plate exchanger the temp would drop on a large consumption and start to feed warm vs hot...

So I installed a flow sensor and it starts the zone as soon as water flows vs minutes longer with just the a-stat...

My goal is to make a simple unit I can assemble and hook into any boiler for a mild DHW system, nothing crazy just enough for the basic 3-bed 2 bath home..

I have had good luck running a small plate exchanger on the return side of mod cons and feeding the indirect water feeders with that water, it lowers the return temps and helps the indirect... I have only done it on 2 systems {1 was my neighbors house and the other an office building} both worked very very well... Lowered return temps and put the energy in the DHW... Is it worth the expense {$280 plate exchanger, that may not last a very long time?} time will tell, I got a great deal on some plate exchangers from a gentleman who used to install outdoor woodstoves {got a great price on 3 wood stoves, a ton of 0011-0015 pumps, plate exchangers, insulated pex, ect..}

But anyway, I noticed veissman is selling a unit similar to what I want to accomplish, so maybe all my playing is useless...


  • icesailor
    icesailor Member Posts: 7,265
    Linear thinking:

    Sounds like inquiring minds think alike.

    Yours & Veissmann's.
  • Gordan
    Gordan Member Posts: 891
    Why circulate through the fphx with no DHW call?

    Seems like it complicates things (3-way valve needed) and adds head loss for no benefit. If you have no flow on the DHW side of the FPHX, you're not going to reduce return temperatures.
  • icesailor
    icesailor Member Posts: 7,265

    Sounds like R & D at work.

  • SWEI
    SWEI Member Posts: 7,356
    TT Prestige Solo and plate exchanger

    Not sure if this works on the TriMax control or not, but it did on the MCBA:  If you install a 3-way that has a 120V motor, you can tell the control it's installed in an Excellence and it will manage your external diverter valve automagically.
  • heatpro02920
    heatpro02920 Member Posts: 991
    This was my original idea

    I thought the same thing, water moves and then you circulate through the plates... Problem is, it takes too long, I was getting very long run times to catch up, so what was happening was the water would move, the boiler would kick on and circulate through the fphx and now the domestic was flowing cold water cooling the fphx so fast it took a long time to get hot water...

    By keeping the fphx 165 degrees on the domestic side the water comes hot instantly... no wait.. If the box I put the unit in is insulated well, I don't think there will be much for stand by losses...
  • heatpro02920
    heatpro02920 Member Posts: 991
    but I want the return to feed through my

    plate exchanger when there is not call for heat, is that still possible?
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,840
    It is, but why...

    As Gordan pointed out it will add resistance to the CH circuit, and really not reduce the return unless there's a call for DHW. And if that's the case you should probably prioritize all output to the DHW call anyway.

    If you REALLY want to do something unique, place a 120 gallon tank so that you have a dedicated "Cold Source" tank that you can then circulate between the FPHX and the tank, thereby diverting return water temperature into the pre heat storage tank thereby lowering the return water temp going into the boiler. The incoming cold water to the DHW system would be piped with the incoming water hitting the FPHX first, then into the PHS tank then to auxiliary. Then place a small (or large depending upon the largest single dump load) reverse indirect for the storage and production of DHW, hook that up to a prioritized pump and let 'er rip. I guarantee that under normal considerations (not more than 2 showers or 1 shower, 1 machine running simultaneously) you will not run out of hot water with 100 k BtuH output boiler, and with a proper recirculation system you shouldn't have to wait more than 30 seconds for hot water.

    I have set a plate heat changer for DHW production without storage, had about 40K input, and we just adjusted flow for when we needed hotter water. It was prioritized, and when doing dishes, it was best to leave the water running at a slow rate (we wash by hand up there) and it wasn't the best conservation theme, but we had a virtual endless supply of hot water at about 1.5 to 2 GPM. It's amazing what you can get out of one of those little buggers. Use caution in areas of hard water. They WILL scale up. I use an electromagnetic water conditioner, and I haven't had any issues.

    I now have a GFX vertical flow drain waste heat recovery preheater piped in series with a Solar DHW preheat tank (120 gallons) piped in series with an 80 gallon tank (HTP). During the summer, when I am not there, if the solar PHS tank over heats, I have a 15' heat pipe drilled into the Earth, grouted in well, that is a tube in a tube heat exchanger (2" OD, 1/2" ID as my heat dump. During the Winter, when I am not there, I put all solar output into my home to keep the propane modcon boiler off line during the day, and let the SPH tank drift. It never gets much below 40 in my basement mechanical room.. When I know I am going to be going up there during the winter, I disable the solar input into the house and start preheating the DHW. I also have a loop and a small pump between the SPH tank and the Aux tank, and if I want, again when I am not there, I can turn the pump on between the two tanks and raise the aux. tank and cool down the SPH tank.

    All of this is controlled with an internet based PC logic that I can manipulate from any where that I can get an internet connection.

    I don't use much propane in the summer :-)

    But if I die, my wife is going to have to marry my friend, Der Heatmeister to figure out WTH I did :-) He'll just bypass the PC, set a EcoBee thermostat and going four wheeling in Lil Pepe. :-)

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • clammy
    clammy Member Posts: 2,912

    I did something simalar to your thoughts about 15 years ago becausethe HO needed more Hot water at her jacuzzi .The plumbers who did the bath re do installed a 80 gal amtrol but it would not give her hot enough water and the boiler could not keep with the indirect rated output or atleast the 140 the HO wanted .What i did was parrell feed the flat plate and used the domestic  output of the amtrol into the flat plate and installed a taco 5000 mixer on a thermal trap worked great ,this amtrol and the flat just feed the master bath they had 2 40 or 50 gal tanks for the rest of the home ,The women wanted as insane as you think like 150 water at the tub with this set up she had it .I believe i used swep plates rated at about 45mbtu or something very long time ago and this was on a ci boiler using seperate pump and priority for domestic boiler was about 150 mtu output fair sized home .worked pretty well and was one of my former bosses sisters never heard a peep or complaint.Really seemes to let her let the tub spout rip  peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • heatpro02920
    heatpro02920 Member Posts: 991
    120 gallon tank

    I am trying to make something small that will be like a combi unit that can mount on the wall and cost around $500, lol.. 120 gallon tank isnt going to fit the project..

    Your system sounds fun, I have done my share of systems that would have other techs scratching their heads, but I try to keep it as simple as possible...

    As far as adding resistance, I havent noticed anything that would hurt performance, and believe it or not the return temps are lower even if the domestic isnt running {it cant run while the heat is circulating since it is priority}, but the px still gives up some energy... I have been thinking about this since I started, This will not work in a low temp radiant system, and with a odr with low return temps, it may work backwards with a px full of hot water and return temps lower than the dhw side... I realized that... I have been successful using a fphx in the return loop to preheat dhw on tanks though, that works well...
  • Gordan
    Gordan Member Posts: 891
    What happens outside of the heating season?

    If you need the FPHX to be "warm start" then it seems like you're outta luck when there is no heated return water.

    What you're describing is the "cold sandwich" that's endemic to instant-type DHW sources with little to no storage capacity. The way around that is a buffer. I think that the important thing to realize is that DHW production and space heating are sufficiently dissimilar in the nature of their cycles, that the only way to synchronize them and reap significant benefit is to have a LOT of buffer capacity, have the boiler maintain the buffer temperature, and have both space heating and DHW draw from the buffer. Most things in between will not add sufficient benefit to justify the cost and complexity, and some may actually increase the operating expense. A small (or not so small) buffer tank charged by the FPHX that's hydraulically separated from the space heating circuit is the typical alternative to a coil-type indirect.