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  1. #1
    Join Date
    Nov 2012
    Posts
    6

    Inefficient (expensive) In-Floor Hydronic RH - Incorrect Boiler Piping?

    Hey all,

    Sorry for the novel, but I want to provide as much info as would be helpful.


    Background:
    I have a 17 yr.old home which I have owned for 2 years. It is two-story with in-floor RH on both stories (in-slab on the first floor, gypcrete on the second). Avg outside winter temps are high/low 45/20. I am a DIYer, knowing just enough to get myself into trouble, an engineer by trade. The only work I have done on the RH system is to replace a Honeywell zone valve and a backflow preventer valve, both faulty when the home was purchased but discovered several months after purchase.


    System:
    The boiler is a Weil Mclain CGa ('95, original to the house), 81% AFUE. The hot water outlet goes through a lever-style shut-off and directly spits into the 3 zone supplies, which disappear into the wall. On the wall adjacent to the 3 zone supplies are the 3 zone returns which each run through a zone valve and then union into a single supply feeding the TACO circulator via a lever shut-off valve, which then goes directly into the boiler supply. Tee-ing into the boiler supply after the circulator is the expansion tank with a spigot which receives cold water supply via a lever shut-off valve and the backflow preventer valve, which drains to the exterior of the home. I can take pictures and attach them this evening, if needed, but I don't have any right now. The system if visually pretty simple. All pipes are wrapped in foam.

    There are three zones, one for the upstairs (8'-10' cathedral ceilings), one for the two downstairs bedrooms and bath (8' ceilings), and one for the main portion of the downstairs (ceiling and dining room with 8' ceilings and the family room with fireplace and vaulted ceilings to a 2nd-story loft).


    Problem:
    Expensive $500/month propane bill ($2.50/gallon for propane). 4 space heaters typically only run up the electric bill $150/month.

    Additionally, when we've run the RH, the home doesn't usually stay warmer than 65*F. We have had a home energy audit done, which indicated the slab was not insulated, but the rest of the house was well insulated and there were no leaks in the RH system. 3 or so local plumbers have been to look at the system and (after rather un-thorough exams) all concluded we should just replace with a new high-efficiency boiler (95-99% AFUE, $10k install).

    Boiler manual indicates (many times in bold print) the minimum return water temp should be 130*F. My understanding is that RH is best at 90-120*F supply (70-100*F return), with 140*F max and exceeding that risks damage to flooring. The manual indicates for low-heat RH applications, the piping should feature a feedback path and mixing valve to ensure the return water temp is >130*F, but **our system has no such feedback**. Additionally, the temp. control for the boiler only goes down to 140*F (it has historically been set at 140, which the boiler temp. gauge confirms), so I can't even set it for optimal RH temperatures. When the boiler runs, it seems to run forever (like all night, I don't suffer from short cycling), and the PSI seems to maintain in the normal range (14 or so).


    Questions:
    1. Is a new boiler really the solution or is the problem just RH. Space heaters work well and for 10k we might rather DIY install a solar water system large enough to run the main zone when solar production is normal and keep the heaters for the bedrooms (we live in the SW desert, a near-perfect solar environment).

    2. Is our boiler piping correct for low-temperature RH w/ Weil Mclain CGa? Could fixing this, along with a new, efficient, low-temperature-friendly boiler produce substantial savings? We would switch back to the RH if we could get the cost down to avg. $250-300/month.




    I'm not an expert, but I appreciate your constructive advice/expertise. I'm beginning to trust the opinion of contributors to this forum more than our local plumbers.



    Thanks,
    JoRyTe

  2. #2
    Join Date
    Feb 2008
    Location
    Altmar, New York, United States
    Posts
    5,116
    i would get another pro in ti have a look. there are man y variables that we can't see from here. you may not need a new boiler. we are not allowed to give diy advise here, sorry.

  3. #3
    Join Date
    Nov 2012
    Posts
    6
    Thanks for the response. I'm trying not to step on any DIY rules. I guess my long-winded question is: based on your experience working with RH applications, does this level of performance seem normal, or should one normally be expecting a lower bill? Is it definitely a problem that the system was installed as per return temps above 130*F while actually being used in a configuration w/ return temps lower than 130*F? Is this just the flat-out wrong boiler to have been chosen for low-temp RH?

  4. #4
    Join Date
    Feb 2008
    Location
    Altmar, New York, United States
    Posts
    5,116
    if it is only used for infloor i would not have used that boiler. i would have gone with a condensing boiler. i would try to get the return temp up. also i would have someone do a combustion test. this could tell some things as well. is this boiler set up as a cold start or does it maintain temp constantly?

  5. #5
    Join Date
    Nov 2012
    Location
    Upstate New York
    Posts
    6
    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

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    Last edited by jpsmith1cm; 11-26-2012 at 08:54 PM.

  6. #6
    Join Date
    Nov 2012
    Posts
    6
    Quote Originally Posted by snupytcb View Post
    is this boiler set up as a cold start or does it maintain temp constantly?
    I'm not entirely sure. The behavior of the boiler, once it is turned on, is to always read 140*F. I'm pretty sure it heats the water and maintains temperature whether any zones are calling or not.

  7. #7
    Join Date
    Sep 2002
    Location
    Northeast Ohio
    Posts
    4,855
    Three zones as in only three loops total? What size is the piping? My first thought is that if it won't heat any better than that set at 140 degrees, you have a major issue and for darned sure it won't heat set to proper slab radiant temps. Pics do help and remember the rules are specific about ANY PRICING and NO DIY ADVISE.
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  8. #8
    Join Date
    Nov 2012
    Posts
    6
    Quote Originally Posted by heaterman View Post
    Three zones as in only three loops total? What size is the piping?[/B]
    Yes, three zones with independent thermostats, each zone covering about 600 sq ft. The piping to each zone is copper 1/2", at least at the point it enters the wall. I believe it breaks out to Pex or the like once it starts running traces in its zone, but I can't actually look in the gypcrete to verify. We do, however, have occasional wall box accesses to some pex piping that I assume is for the RH as there would be no other reason for pex to be running at those parts of the walls. I'm not even sure why they would run part of the RH in the wall for access, perhaps for diagnostics?

    I don't believe the quality of work to be very high, since the slab wasn't even insulated, though that might have been the standard practice 17 years ago.

  9. #9
    Join Date
    Feb 2009
    Location
    Portland OR
    Posts
    2,029
    No way to really say if the costs are high without knowing your load calculation(manual J) and location.
    If you have a house with little to no insulation and leave windows open during the day your doing well, if you have spray foam and .3 U-value windows you have an issue.

    First propane is very expensive, in some areas of the country as much as electric per BTU of heat, you might want to look at a ground soruce heat pump or daikin Altherma air to water heat pump.
    Check out my YouTube channel - http://www.youtube.com/user/skyheating1 We have customer testimonials, product reviews and more!
    Like us on FACEBOOK if you like our advice here!

  10. #10
    Join Date
    Nov 2012
    Posts
    6
    Quote Originally Posted by SkyHeating View Post
    No way to really say if the costs are high without knowing your load calculation(manual J) and location.
    If you have a house with little to no insulation and leave windows open during the day your doing well, if you have spray foam and .3 U-value windows you have an issue.

    First propane is very expensive, in some areas of the country as much as electric per BTU of heat, you might want to look at a ground soruce heat pump or daikin Altherma air to water heat pump.

    Fair enough. I can add that there is no interior insulation. I haven't probed exterior walls to see if they are spray or roll-in, but I would suspect roll-in. The windows are not spectacular (double paned but not insulated, aluminum framed so they transfer thermal very easily). The house meets code, but is not full of extras and upgrades. We actually put thick plastic over our windows and the difference is quite remarkable. With the plastic the energy audit indicated the home was extremely tight (borderline too tight, he said). Insulation was identified as adequate.


    However, I'm concerned that there might be some inherent inefficiency resulting from using a boiler that appears to target high-temp water heating apps in a low-temp in-floor situation. Perhaps by swapping boilers would I expect not only the 15% AFUE efficiency improvement but also added improvement of using a boiler intended for low-water temps? I can't even run my current boiler at the most efficient RH temps. Is there a rule of thumb for what improvement one would expect by dropping the temp from 140 to 110 or lower, assuming the RH system can actually heat at those lower temps, as @heaterman commented?

    Additionally, could the fact that my system is installed according to Fig 12 of in the installation manual (http://www.weil-mclain.com/en/assets...1-009_0107.pdf) for multi-zone high-temp applications instead of Fig 14 for multi-zone low-temp applications be causing some major inefficiency that might explain over-costly operation? If I replumbed it according to Fig 14, would you expect it to buy me anything in terms of operating cost?


    Thanks.

  11. #11
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,925
    JimBE323

    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

    You can find the rules for posting and qualifications here.

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  12. #12
    Join Date
    Jun 2007
    Location
    Minneapolis, MN
    Posts
    761
    First lets determine the cost-per-therm of available fuel in your area. If electricity costs less then an electric boiler would be the perfect solution to your problem.

    You have a low efficiency boiler and a high efficiency condensing boiler will certainly cost less to operate (if properly sized and installed).

    Low efficiency, cast iron, atmospheric boilers such as the Weil McLain CGa should not be operated with return water temperature below 130F since sustained flue gas condensation will corrode internal heat exchanger surfaces and burners making a potentially unsafe fire condition i.e. CO and greatly shorten the life of the boiler.

    The fact that the house will not satisfy the thermostat indicates that you may have a very rare condition - an undersized boiler. It may also be that the boiler is under-fired for some reason. This is where skills come into play.

    You are correct in assuming that a condensing boiler will save fuel. The AFUE isn't as accurate as one might assume since the condensing boiler will recover much more energy in low temperature applications often saving 25-50% on fuel bills by virtue of the sealed combustion, premix burner, ODR, and recovering latent heat normally driven up the chimney.

    An uninsulated slab is not the end of the world but I would want to control the temperature or the water going into the slab and use a weather sensitive control to modify it for better comfort and lower fuel consumption.

    Both electric and propane condensing boilers are available with weather sensitive controls - out door reset.

  13. #13
    Join Date
    Nov 2012
    Posts
    6
    Quote Originally Posted by BadgerBoiler MN View Post
    First lets determine the cost-per-therm of available fuel in your area. If electricity costs less then an electric boiler would be the perfect solution to your problem.

    You have a low efficiency boiler and a high efficiency condensing boiler will certainly cost less to operate (if properly sized and installed).

    Low efficiency, cast iron, atmospheric boilers such as the Weil McLain CGa should not be operated with return water temperature below 130F since sustained flue gas condensation will corrode internal heat exchanger surfaces and burners making a potentially unsafe fire condition i.e. CO and greatly shorten the life of the boiler.

    The fact that the house will not satisfy the thermostat indicates that you may have a very rare condition - an undersized boiler. It may also be that the boiler is under-fired for some reason. This is where skills come into play.

    You are correct in assuming that a condensing boiler will save fuel. The AFUE isn't as accurate as one might assume since the condensing boiler will recover much more energy in low temperature applications often saving 25-50% on fuel bills by virtue of the sealed combustion, premix burner, ODR, and recovering latent heat normally driven up the chimney.

    An uninsulated slab is not the end of the world but I would want to control the temperature or the water going into the slab and use a weather sensitive control to modify it for better comfort and lower fuel consumption.

    Both electric and propane condensing boilers are available with weather sensitive controls - out door reset.

    Excellent insight. Thanks for the reply! I'm particularly interested in the discussion on what savings I would feasibly realize by switching from the CGa to a condensing boiler (one plumber tried to sell me on Lochinvar Knight boilers, which I just happened to see installed in an episode of "This Old House" last night, and seem to be well respected). Also, electricity in my area is tiered, but starts at a reasonable $0.09/kWh and goes up to about $0.15/kWh. I don't know the math off the top of my head, but I believe this is reasonably competitive with $2.5/g propane per BTU. While I would like to go all electric, I live in the mountains with frequent power outages and it is easier to run the small electric load of the propane system off a battery backup for sustained periods of time than the full load of an electric boiler. It's also because I live in the mountains we don't get natural gas, or this would be no issue as NG is so cheap.

    w.r.t. boiler sizing, the PexSupply.com BTU calculator estimates about 45 kBTU for our home. Our model of CGa is sized for 88 kBTU output. I believe the calculator is underestimating a little and my wife (the source of all truth) reminded me last night that our current boiler has, in fact, kept our house properly warm in the past, we just stopped setting the thermostat so high because of the overbearing propane cost. So, there is potential for a solution to our RH problem to heat our house appropriately. I plan to test this again in the depths of this winter, otherwise we'll ditch the RH and install a good quality wood-burning stove.

    If we end up replacing the boiler, I will ensure to get and install an outdoor reset.

    I'm still baffled why the home builder would have installed the system in such a way. I'll have to turn the temperature up to 150*F whenever I run it in the future and hope our flooring does not get damaged. We only have carpet, tile, and linoleum (which we'll eventually replace w/ tile), but I guess the subfloor on the second story could risk damage?

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