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Boiler Was Plumbed Wrong...

I just purchased a 10 year old home and after we moved in I realized that the original boiler had been plumbed completely wrong. I've attached a diagram of how it is currently laid out. There are only three zones, one is a slab (1st floor) the other two are stapled under the 2nd story subfloor (bedrooms and living space). As you can see the mix valve is useless installed the way it is and also the boiler setpoint is 140F with the return water temp well under the 130F recommended for a cast iron Gold CGs. Also, the circulation pump installed on the secondary loop is ALWAYS on; if the boiler has power to it the pump is running, even if there is no demand for heat and all the zone valves are closed! I can't believe that this thing has been setup this way for 10 years and I'm just not sure the best way to go from here. I'm in Durango, CO where it's DRY so hopefully condensation will not be as big of an issue but I'm not holding my breath for the boiler lasting me for another 10 years. Any pointers on how to make this thing right other than completely tearing it all out and starting over? I'm perfectly comfortable with doing the required mods and wiring and would love your input.


  • ZmanZman Member Posts: 3,516
    Clean Work

    That is a clean install.

    You are correct that it leaves much to be desired in the design department.

    The boiler pump is pumping directly into the expansion tank. It should be pumping away from the tank. This may or may not give you problems. the good news is that there is very little resistance in that loop, so the pressure differences are slight.

    The closely spaced tee's going to the secondary are not so close. They should have 4x pipe diameter (max of 8x)  between them That would be 5" to 10" with 1 1/4" pipe. What you have might work, but again it is not quite right.

    You are correct that the mixed loop will never work as drawn. It can't.

    You are also correct that the boiler will not last long at those return temps.

    I am guessing that your staple up install does not use aluminum plates. Is there a way to confirm this?

    If your system is plateless, you could run the boiler and the staple up zones at 150 then mix down the slab zone with a device that will provide outdoor reset and boiler protection. The outdoor reset will help prevent indoor temp overshoots do to overheating your high mass slabs. You could use something like this and another circulator. You would also want to change how it ties into the system. you don't want the other heating circ pushing on it.

    Normally, the circ that will not turn off would be wired into the green controller. I am guessing it is presently wired always on.

    Do you know what the boiler aquastat is set to now?

    What size is the boiler?

    How big is the house?

    What type of construction/insulation?

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • heatpro02920heatpro02920 Member Posts: 991
    Does it work?

    If its been there that long it must heat the house, if it works and you are comfortable you can leave ignore it...

    I wouldn't, I think I would repipe and wire the entire system..

    I install many WM CG- boilers...

    I would wire it with a zvc4--exp control and maybe throw on a PC700... I would repipe it primary secondary with a couple grundfos Alpha's, one for the primary one for the zones, let the exp control handle the primary pump, and I wouldnt mix if its not needed, if you have no high temp zones figure out your temps and go from there, I would need a lot more info to really figure the best way to do it all, like your system temps, heat loss, heat loss and radiant per zone, ect..

    Its too bad its piped like that because the piping itself doesn't look terrible..
  • WanabeePlumberWanabeePlumber Member Posts: 7
    edited March 2014
    Thanks! How about this?

    Thanks for the responses! I was looking at it again today and was wondering if just moving the secondary loop pump to pull through the loops and mix valve would fix things or if that's a bad idea. I would also wire it up through the zone controller to shut down when not needed. I've attached a diagram of my proposed changes.

    ps - I'm a certified electronics tech and work on industrial control systems for a living, that's why have have a small clue as to what's going on... :). Also I plan to answer the above questions but I don't have time. I'll post those later.
  • heatpro02920heatpro02920 Member Posts: 991
    What are your temps

    what is your boiler temp and your floor supply temps, are your floors plated or just stapled up?

    What is your aquastat set at?

    You may see the most savings with the controls I listed above, and leaving the piping as it is, if its working... Are you going to repipe the system yourself or have a contractor do it? If you are doing it your self, I would just start over, use more efficient pumps go full ps system and if you do actually need to mix I would do it differently {not sure if you do or not, but since that mixer isnt working, I would say no}...

    What do your manifolds look like?
  • ZmanZman Member Posts: 3,516

    If you answer the earlier questions you will get better advise.

    I think the only reason the system worked before is that the boiler temp is turned down and the mixing valve is turned up.

    Your drawing will not work very well.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • IronmanIronman Member Posts: 3,526
    edited March 2014
    Not like that

    The circulator MUST draw through the mixing valve and pump out toward the manifold.

    Also, the primary circ should pump away from the expansion tank, not toward it. As Zman said, this may not be critical in your installation, but the position of the mixing circ is.

    I don't think that what he said about controlling slab temp completely register with you. It doesn't with most folks initially.

    Let me explain: the slab is a high mass heat emitter; it's a huge rock that takes a long time to heat up and will continue to give off heat for hours after the heat to it is shut off. We call this the flywheel effect: it takes a lot to get it going and a lot to slow it down or stop it. It requires much lower water temps than staple up which is low mass. If supplied with water that is too warm, the slab will drastically over-heat it's zone because it will give off heat for hours after it's thermostat is satisfied.

    The question then is: what temp is right for the slab? That answer depends on what the heat load is at any given outdoor temp. What's right at 45* won't be right when it's 25*, etc. The colder it gets, the warmer the water needed and vise versa. And I purposely use the term "warm water", not hot.

    By using outdoor reset (ODR), the water temp is varied according to OD temp to give the precise water temp to the slab that is needed and prevent the flywheel effect. In fact, when the heating curve is properly set on the ODR control, the thermostat actually becomes unnecessary though I would leave it to act as a high limit. I have a radiant slab next door that we installed and commissioned this fall and it's been running all winter without a thermostat. My neighbor has confirmed that the temp inside stays a constant 68*.

    The slab presents another concern: the issue of flue gas condensation that you mentioned. If the radiant system in your slab was done correctly, then it should not require more than 110* supply water temp, and that's stretching it. That means you would be returning 100* or cooler to the boiler. That will definitely cause flue gas condensation and possibly thermal shock to the boiler block.

    The solution to both issues is the smart valve that Zman gave a link to. It will modulate the water temp based upon ODR and has provision to protect the boiler also. Variable speed injection mixing with ODR is the other option, but that would require some re-piping and an additional circ. Look up Tekmar 356 for more info on this. The IValve may be easier for you.

    The issue of heat transfer plates that Z mentioned is also very important since the btu output of the staple up floor would be reduced by at least half without them. Hotter water would then be required to compensate if you don't have them. All This effects your control strategy and piping arrangement.

    What I'm trying to say is that you need more than just some re-piping: you need some design work to determine the proper approach to correct whatever issues your system has. That all starts with a heat loss calculation. That's the foundation for everything.

    I would recommend that you google "SlantFin Heat Loss Calculator" and download that. It's free and its self explanatory and simple to use. It will require taking measurements of your windows, doors, walls, floor space, etc. and entering the data in the program.

    Once you have the heat loss calc and know approximately how much tubing is in your floors, a determination can be made upon how to proceed.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • WanabeePlumberWanabeePlumber Member Posts: 7

    Thanks everyone for chiming in so far. I had some time to investigate things a little more and have answers for most of the questions. (Square footage is rough estimates)

    Boiler Aquastat set @ 140°F (turns on at approximately 110°F off at 140°F)

    Boiler is 100,000btu

    House is 2,500sq ft

    2x6 Construction, blown-in attic, not sure of insulation on walls.

    Because of poor design all zones have same feed temp which = boiler outlet, no mixing

    Zone1 - (Rec Room, laundry, bath, entry 900sq ft) Carpeted concrete slab with three poured concrete walls in the hillside and the fourth wall is interior. This room stays comfortable because of being buried in the hillside. No ODR needed in my opinion.

    Zone 2 - (Bedrooms 800sq ft upstairs) Stapled up pex with fiberglass insulation, carpeted floors. Southern exposure with a fair amount of solar heating midday.

    Zone 3 - (Living room, kitchen, dining 700sq ft upstairs) Stapled up pex with fiberglass insulation. Flooring is 3/8" floating wood on 1/4" cork... Very slow response time. I currently have an Ecobee Smart thermostat (which has built in ODR) on this zone and it seems to be learning quite well, overshoot has been improving the last couple days.

    I do plan to do all modifications myself. Ironically my last house had an eerily similar setup with radiant, a Gold CGs and low return temps; I had to plumb in a primary loop which it did not have. I'm comfortable sweating copper and wiring.

    As far as a solution moving forward, I do want to make it right but also only to the degree that there is a return on my investment. ie. mixing for the slab so that I'm not sending 140°F water to it is important for the slab and comfort but relocating the pressure tank probably won't make a discernible difference in the end.

    Could anyone comment on the idea I presented in my second post, the diagram that relocates the circ pump to pull water through the mix valve and other zones? If that will work it seems like a relatively simple fix that would allow me to turn the aquastat up fixing the low temp issue with the boiler while not cooking my slab.


  • ZmanZman Member Posts: 3,516

    The problem with your present setup and the one you are proposing is that the mixing circulator is not within the heating loop. As soon as the valve is too hot, it will close the hot port and circulation in the loop stops. The only reason it "works" now is that the boiler is turned down so low the valve never reaches that position.

    Your boiler must not receive sustained return water temps below 130 degrees or it will be damaged by the condensate. Your aquastat temp needs to be raised.

    I think the easiest solution is the one I have sketched. It may be possible to utilize your existing valve. My concern with doing that is that the cold slab may pull the boiler temp into the condensing range.

    The I valve will give you outdoor reset which is nice, more importantly it will measure your boilers return temp and automatically keep it in the non-condensing range.

    You need to  be sure you have check valve on the cold side of the mixing valve, some have internal ones.

    You will need a simple relay to control the new circ, zone valve and I valve. That is another subject.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • WanabeePlumberWanabeePlumber Member Posts: 7

    I appreciate the detailed response! I follow what you're saying and can definitely implement that solution. It's considerably more expensive than I was hoping to spend but I know that's all relative to the cost of replacing the whole system when the boiler fails...

    Please humor me here... :) I'm still not sure why my proposed drawing wouldn't work. Temporarily imagine that the 2nd and 3rd zones don't exist, now why would it matter whether the zone 1 circ pump was upstream of the slab, between the slab and zone valve or just on the downstream side of the zone valve? It seems to me that where I'm proposing would still "draw" through the mix valve but also function to circ the other two zones when they call for heat. FYI zone 3 is only calling for heat about 5 hours each morning then it stays warm for the entire day. (currently setting back zone 3 ten degrees at night.) I've attached a modified photo to show in real life what I was thinking.
  • ZmanZman Member Posts: 3,516

    I see what you mean.I don't think you will have a problem with the mixing in that drawing.

    I have not tried it that way. I don't see why it would not work.

    Anyone else?

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • ZmanZman Member Posts: 3,516

    The condensate comes from the combustion process.

    A dry climate will not help.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • WanabeePlumberWanabeePlumber Member Posts: 7
    You're right I should have remembered that

    I forgot about that, thanks. CH4 + 2*O2 = CO2 + H2O
  • WanabeePlumberWanabeePlumber Member Posts: 7
    Any More Input?

    Does anyone else have any input as to my proposed modification? Wanting to get to this withing the next week or two. Thanks!
  • WanabeePlumberWanabeePlumber Member Posts: 7
    Any Further Thoughts?

    Does anyone else have any input as to my proposed modification? Wanting to get to this withing the next week or two. Thanks!
  • GordanGordan Member Posts: 891
    edited April 2014
    It'll be fine.

    That will work, but the piping layout as is would not create a good spot for a circulator - it's sandwiched between elbows and it really needs some straight piping, especially on the discharge. While you're cutting in the circulator at that spot you could also correct the too-wide spacing of the tees on the boiler loop. You also do not want to be pumping into the "bull" (middle part) of the tee between the system return and the mixing valve intake.
  • Mark EathertonMark Eatherton Member Posts: 5,584
    What you've depicted will not work...

    When there is a simultaneous call for low and high temp, the return water from the high temp will cause the mixing valve to lock up.

    There is supposed to be an integral circulator on that boiler, and THAT pump could serve the high temp calls, if the pressure drop is not to great.

    Then set the low temp pump up as a secondary with its own mixing valve with the pump pumping away from the 3 way and you should be fine. Do not allow the mixer to see the higher temperature return water. The integral pump is for boiler protection against long term condensation production.

    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.
  • GordanGordan Member Posts: 891
    Lock up

    The typical wax capsule thermostatic design one sees would swing to "full cold", which in this case is, of course, not cold. Are there valves that feature internals that would actually shut off flow completely? There might be an issue with longevity if the valve is consistently operating at full tilt to "cold" and the capsule wants to expand more but it has nowhere to go.

    Regardless, you're absolutely right that the mixing valve in the proposed setup (whether thermostatic or electric) would not have "full authority" over the supply temperature to that circuit, as it receives mixed "higher temp" and "lower temp" return. This may not be an issue unless we're talking WILDLY divergent supply temperatures, and I don't think we are, in this case.
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