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Thread: Heat Exchanger issue with Viessmann boiler

  1. #1
    DIY Junior Member billdoors's Avatar
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    Default Heat Exchanger issue with Viessmann boiler

    I've been having a problem with the heat arrangement in my house not quite providing enough heat in the fall and winter ever since the new system has been installed. While I trust the craftmanship of my plumbing installer, I'm not so sure that his suggested remedy is the right one and was hoping for a second opinion from the experts on the forum, as it seems like any of the possible remedies involves roughly equal $$$$$.

    First, my house and equipment.

    I live in Vancouver, BC (Canada). The house is a 2700 sq ft, 2-level, slab-on-grade box. The boiler is a very new (2.5 yr old) Viessmann Vitodens 100 (the larger 118 MBH version, but even the smaller 91 MBH version is at least 2-3x oversized) modulating condensing boiler driving 88 ft of 3/4" fin tube and a 80-gallon indirect DHW tank (which is topped up with a secondary internal loop to solar, but that's neither here nor there).

    There are 3 space heating loops in the house -- 2 zones upstairs, 1 downstairs. The upstairs zones are plumbed with 3/4" copper, the downstairs zone with gray poly. Because of the gray poly and its permeability issues, the 3 space heating loops are isolated via a heat exchanger from the boiler. The primary line from the boiler goes through the heat exchanger, then the heating loop in the DHW tank before returning to the boiler. The 3 space heating loops are correctly plumbed in parallel with one another, and each with its own thermostat-activated switch. I have pipe insulation around most of the plumbing exposed in the mechanical room, which is located inside the house heating envelope.

    I've done a reasonably careful model of the house heat load, using wall area, ceiling area, floor area/perimeter, window area, door area, and measured air infiltration from a blower door, similar to the one here: http://www.builditsolar.com/Referenc...s/HeatLoss.htm (though ignoring internal heat generators). Modelled heat loss for internal/external temp of 68F/32F is 35kBtu/hr.

    I'm reasonably confident the results are real, as the results are consistent with what we are experiencing -- that the heating system is barely able to maintain a constant 68F when the outside temperature reaches 32-40F, and is unable to provide enough heat when the mercury drops even lower.

    But the boiler should have more than enough capacity, you say, and the Vitodens 100 normally runs at 160-180F, which ought to provide more than enough capacity, right? Unfortunately not. Well, I'm sure the boiler has enough capacity -- it's just that the heat isn't getting to the house properly. When not in DHW heating mode, the boiler never goes above 15% modulation. (that's with me sitting there staring at the boiler for 10-15 min or more)

    What happens when it's cold out is that the boiler will run for about 1-3 min, firing at 15%. The boiler temperature will crank up over this period from anywhere as low as 140-150F to up to a max of 190F, at which point the boiler rightly shuts itself off. The inline thermometer on the space heating circuit (almost) never goes above 150F, and usually is observed at around 140F. Based on a table quoted by Siegenthaler's book, this means that my 88 ft of fin tube only give me 26-31 kBtu/hr, at best. The ancient, inefficient boiler which we used to have driving the same fin tubes in the house had no problem kicking the house up to 75F or more even in the depths of winter. This was probably because the delivered water temperature going into the fin tubes before was no doubt much higher.

    Worse, I have a sneaking suspicion that the semi-short cycling has led to premature aging of the boiler. We've already gone through 2 blower fans and a bunch of gaskets and other miscellaneous parts -- $$$$ worth of parts and labor in the last 2.5 years we've had the boiler in.

    I say "almost never goes above 150F" because if I shut down the two larger loops (1 upstairs, 1 downstairs), I am able to drive the space heating circuit up to almost 165F (for a boiler temp of 190F), running just the one small loop. Otherwise, with either one of the big loops only on, the space heating circuit does not exceed 140-150F for the max boiler temp of 180-185F. When my installer came to troubleshoot during the middle of the day, he was able to see the circuit temperature go up to (reportedly) 170F, so he concluded everything was fine, and all I needed was to add some more radiator surface/length in particular rooms. (probably a Runtal or similar, though we didn't get into this detail) Ah, if only I could actually get 170F in the dead of the night, there wouldn't actually be a cold problem in the house with the existing baseboard...

    I should emphasize that I probably wouldn't have quirked an eyebrow if not for the fact that, despite not seeing the space heating circuit temperature go above 140-150F, the boiler never goes above 15% modulation for space heating. Although I am aware that I have a heat dump situation for at least the upstairs loops (a pass through the ceiling of a cold enclosed garage, though I have retrofitted some pipe wrap), I'd think that any heat dumped out into the cold garage would simply make the boiler work harder to bring the space heating pipe temperature up. This strikes me as a hydronic design fault of some sort.

    As I see it there are 3 potential remedies I can think of:

    1) Live with the lower temperature in the space heating circuit, and just add more radiator area/length/etc. ie, change existing fin tube for something else to compensate for the lower max Btu/hr due to the lower water temperature. This is what our installer is effectively suggesting, and is probably the lowest design risk. However, doing this does not address the semi-short cycling, nor properly answer why the loop temperature is so low. Also, to change out enough of the house that we can still maintain even just 68F at Vancouver's coldest (down to 10-20F) looks like in the neighbourhood of $9-10K for parts alone.

    2) Add a circuit for just the two upstairs (copper tube) loops, parallel with the DHW indirect tank, on the boiler side of the heat exchanger. The idea of this is so that we don't have whatever weird coupling/lagging effect is happening for at least the two all-copper circuits. Unsure if this is a feasible solution. If this idea worked, the hope is that the boiler would not run so short any more.

    3) Add a (grin) buffer tank on the boiler side of the heat exchanger to compensate for a bad design. Rough calculation based on house dimensions (23'x68' upstairs, 23'x53' downstairs) says there's about 7-8 gallons' of water in the space heating loops vs. maybe 0.5 gallon in the primary loop on the boiler side of the heat exchanger. Could this be contributing to the boiler cutting off before the heating loops come up to temp? Additional clue is that running just the short line upstairs (about 1 gallon, over about 30-40' of pipe) I can get to a much higher peak temperature.

    An additional, 4th choice to proceed might be to hire on a full-blooded hydronics engineer to check out the design and make recommendations.

    Obviously, I will continue to seal the house up to reduce the heating load, though my numbers say that after the current round of (relatively cheap) air sealing in the attic plane, etc, the next big heat loss offenders are spray foaming the garage ceiling including where the heat pipes run (and maybe replacing the garage doors), and then the big construction item of redoing the exterior wall insulation. So you can see why I'm looking for advice, as all the items above run in the $K to $$K.

    Thank you in advance for your help, and for getting to the end of this long post.
    Last edited by billdoors; 12-12-2011 at 01:31 PM.

  2. #2
    DIY Member Buffalobillpatrick's Avatar
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    "The primary line from the boiler goes through the heat exchanger, then the heating loop in the DHW tank before returning to the boiler"
    I don't think this is correct?

    Your solution #2 seems good.

    Sounds like heat exchanger is undersized or under-pumped.
    Get a low-cost IR themometer, put 2 wraps of masking tape, 1" wide around pipes so the emissivity will allow IR measurment & check all 4 port temps.
    Bare metal won't read correctly.
    Side-A is input from boiler, side-B is system.
    Mine measures 15-20* delta-T on both A & B sides.

    Get a new heating pro.

    BBP
    Last edited by Buffalobillpatrick; 12-12-2011 at 04:45 PM.

  3. #3
    DIY Junior Member billdoors's Avatar
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    Thanks, Bill. I'll try a conversation with my plumber again and see if I can get him to buy into doing the work.

  4. #4
    In the trades Dana's Avatar
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    What he said- you either don't have enough heat exchanger, or not enough pump (or both), or something limiting flow (kinked pipe, obstructions, ball-valve only 1/4 open, etc.)

    A heat load of 35K @ 32F outdoor temp for a 2700' house implies a lot of single-pane glass (or an open window or three. :-) ) Something closer to half that would be reasonable for a fairly tight house with code-min insulation. The builditsolar heat loss calculator is really crude there are better versions out there. Taco has a downloadable freebie on their website that is more like what the pros would use, but it requires a bit of user-savvy to not end up with a garbage-in-garbage-out problem. If you know how much gas you used in a typical year with the old beastie an estimate based on fuel use & annual heating degree and the steady state efficiency of the boiler would be more accurate. (When converted to BTUs per degree-hour, then multiply by the number of degrees below 65F for the heat load at that number.)

  5. #5
    DIY Junior Member billdoors's Avatar
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    Thank you for the reply Dana.

    The 35K comes from 2 major offenders: one is air infiltration (8.46 ach50 before I started air sealing), the other is the walls, which I've estimated at R10 based on construction and inspection (the walls get kinda cold in the winter). I'm hoping that when I'm done with the air sealing, I'll get at least part of the heat loss cut down significantly. FWIW, I found some pretty funny things hidden out of sight, eg, a 2'x2' square cut out of drywall and vapor barrier behind the dishwasher and never re-sealed so some electrician could fish wires. The windows are recently replaced with U-0.25 vinyls, so at least I only have 2 and not 3 major sources of heat leak. (new windows were prioritized higher as the old ones were in really bad shape; there is, though, a fairly large window area included) The house itself was a builder special from 1983 -- not as bad as some, but certainly not up to current code.
    The 35K figure also ignores solar heat gain and a few other things, but when I cross-checked against another modeling program by dumping published weather data into my spreadsheet, the numbers more or less agreed against my heating bill. With the single crude measurement, I was trying to figure out why I couldn't get enough heat out to the house at a given point in time.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    What is the heating transfer capacity of the heat exchanger you have? Could that be the limiting factor?

    Totally different situation, but when I added hydro-air, the heat exchanger I put in was only rated at (I think) 28KBTU, but it is enough to heat the house...slowly, if it is cold out, but it still will down below zero. I only run that when the in-floor heating can't keep up, plus, the two bedrooms upstairs only get heat from the bathroom slab in between them, the hydro-air ducts, or convection...but, I like them a little cooler.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #7
    DIY Member Buffalobillpatrick's Avatar
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    Default Hx

    I got this one off of ****:
    Oops!
    GEA Flatplate Brazed 30 Plate GBE 5x12 Heat Exchanger, 1" ports

    GEA is #1 & has a nice online simulator, but you must login & know pump flow, etc.
    http://flatplateselect.com/site/pub/...2fDefault.aspx

  8. #8
    DIY Junior Member billdoors's Avatar
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    I just spent a couple of hours with a stopwatch and notepad in hand not moving from in front of the boiler.
    First off, I need to correct my statement about short cycling. At least today (outside temp 37-42F over course of testing) firing time was longer, based on how many zones I had active. With all 3 zones on, firing time in steady state was about 15-16 min. With just the downstairs on, fire time was around 5-6min. Only having the short loop on would be what leads to the short cycles. Time between firing was a fairly constant 4.5 min.
    As might be predicted, running all 3 loops, the heating temp capped at 150F. Running just the downstairs loop (all in slab) max heat temp was 160F. Running short loop max heat temp was almost 170F.
    The heat exchanger is a Bell & Gossett BP 400-020 with a rated capacity of 60-125 kBtu/hr which should be more than enough even with what Dana pointed out was a pretty pessimistic instantaneous heat loss estimate.
    So I am leaning much towards trying to get the two upstairs copper zones plumbed on their own branch in parallel with the DHW on the boiler side of the heat exchanger. I suppose it is also possible that there might be some grit or blockage in the exchanger lowering its efficiency.
    One other question for you: if the problem with the cold house was just that there is not enough fin tube length for the operating temperature, I would've expected the observed heating circuit temps to all be higher, right? It 's the fact that the temp post-exchanger is so low vs the input that suggests a problem more with the heat exchange than the radiators. Does that reasoning make sense?

  9. #9
    DIY Member Buffalobillpatrick's Avatar
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    Yes & Yes, ......... (I got a too short reply error message, needed more characters)

  10. #10
    In the trades Dana's Avatar
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    Yes, if it was an insufficient radiation problem the radiation temps would spike higher, which points again toward the heat exchanger & flow (on either or both side s of the HX.)

  11. #11
    DIY Junior Member billdoors's Avatar
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    Thanks, Dana and Bill. I'm having some words with my HVAC guy shortly.

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