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Add a high efficiency boiler that provides base heat, but leave existing boiler(s) for coldest days?

JimEngr | Posted in Energy, Heating & Insulation on September 23, 2010 11:56am

I’ve seen a big residential building heated by a gas boiler with three sections, about 300,000 BTUH each section. They all work, but aren’t very efficient – maybe 70-80 AFUE. They are wired so that one comes on in milder weather, the next comes on when the outside is colder, and the third comes on when lots of heat is needed. The residents would like to find a more efficient system to reduce the heating costs

Is it possible to add a high-efficiency boiler to the system that would be wired so that it comes on first and provides heat pretty much continuously throughout the winter, with the existing boilers coming on as needed to provide enough heat on the colder days? The system should provide some significant savings without replacing everything at once. (Later – when funds permit – another high-efficiency boiler could be added.)

Does this present any problems with the plumbing and heating code? Is a residential high-efficiency boiler designed to run continuously for a few months?

Thanks for your help!

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  1. User avater
    kurt99 | Sep 24, 2010 09:18am | #1

    Some Thoughts

    I am not an expert but I will throw out some ideas to start the discussion.

    • High efficiency boilers are most efficient when operated at low temperatures.  Once the inlet water temperature rises to the 130 - 140F range, they no longer condense water out of the flue gas and thus act as expensive conventional boilers.  Conventional boilers do not like to operate cool because they are not designed for condensing flue gasses which causes corrosion.  What temperatures does this system operate at?

    • How are these boilers plumbed?  In parallel l or series?  With on pump or a separate pump and check valve with each? If they are in paralell with one pump, the new boiler and the old boilers may have greatly different internal restrictions to water flow and thus there could be a balance problem with too much or not enough flow through each boiler.

    • Stack losses.  If water flows through the boilers that are not in operation because there is only one pump, the old boilers will loose heat up the stack if there is not an effective flue damper.  This condition will not change from the current condition, but may be the first place to look for efficiency gains.

    1. JimEngr | Sep 24, 2010 01:48pm | #3

      "High efficiency boilers are

      "High efficiency boilers are most efficient when operated at low temperatures.  Once the inlet water temperature rises to the 130 - 140F range, they no longer condense water out of the flue gas and thus act as expensive conventional boilers.  Conventional boilers do not like to operate cool because they are not designed for condensing flue gasses which causes corrosion.  What temperatures does this system operate at?"

      This system has an outside temperature sensor (mechanical thermostat with an outside sensing bulb and water sensing bulb) that regulates the maximum water temperature - the colder outside, the hotter the water. Another thermostat turns on the third boiler section when the outside temperature is lower.

      "How are these boilers plumbed?  In parallel l or series?  With on pump or a separate pump and check valve with each? If they are in paralell with one pump, the new boiler and the old boilers may have greatly different internal restrictions to water flow and thus there could be a balance problem with too much or not enough flow through each boiler."

      All three are in parallel with a single circulating pump.

      "Stack losses.  If water flows through the boilers that are not in operation because there is only one pump, the old boilers will loose heat up the stack if there is not an effective flue damper.  This condition will not change from the current condition, but may be the first place to look for efficiency gains."

      Good point - I don't think that there is any active damper in the flue, and the chimney is about 50 ft high, for quite a stack effect.

      Thanks again for your help!

      1. Clewless1 | Oct 01, 2010 03:28pm | #12

        This system has an outside temperature sensor (mechanical thermostat with an outside sensing bulb and water sensing bulb) that regulates the maximum water temperature - the colder outside, the hotter the water. Another thermostat turns on the third boiler section when the outside temperature is lower.

        You are describing an outside air reset control ... which is from an energy standpoint much more efficient than simply turning the boiler on/off based on a constant supply water temperature setpoint (e.g. 180 degF). During milder weather, the house doesn't need the hotter water.

        Ideal is a boiler that runs continuously ... just exactly meeting the thermal load. This is theoretically done w/ the reset controller modulating the suppy water temp to just that needed to offset the heating load ... which would keep the boiler running longer (you then have no standby losses).

        Not sure if your existing system is as inefficient as you think it is. Surely a condensing boiler would work efficiently, but it is also quite expensive and you may save a small percentage of your fuel bill. I'm not advocating that you don't follow through, just that you fully realize that your goal may not be what you think it is.

    2. NRTRob | Sep 29, 2010 02:38pm | #10

      I see this misconception a lot

      And it's far from the truth.

      Mod/con boilers are of course much more efficient when they can condense.

      But they bring several other features to the table that are at least as, and probably more important than condensation when it relates to efficiency.

      1.  Modulation:  Less cycling.  Produce only the heat you need as you need it.   Less air needed for combustion = less heat wasted.  

      2.  Sealed combustion:  no need to reheat air pulled into the envelope by negative pressure.  this too is big.

      3.  Low exhaust temperatures on top of the lower airflow at lower burns.

      4.  Outdoor reset built in.  Even if you need 180 max, you run cooler 99% of the time and you'll probably still condense 50% of the winter's heating hours.

      5.  Purge control is often built in for DHW.  this is a very big deal.

      In short, they are a significant leap forward for efficiency on ANY hydronic system. 

      1. Tim | Sep 30, 2010 09:57am | #11

        I disagree

        that "condensing boilers operating at low (water) temperatures operate most efficiently" is a misconception or far from the truth.

        You own examples contradict the first statement. Boilers do not condense (the flue gases) at high return water temperatures, so by your own statement "...are of course much more efficient when they can condense",  they (mod/con boilers) are much more efficient when operating at low temperatures(i.e. condnesing). Not a misconception at all.

        Modulation helps, but not significantly, in the the overall thermal efficiency of the heat transfer. Cycling, while important to reduce for equipement longevity ond thermal comfort has very little impact on thermal efficiency. Modulating boilers operating at low fire are more efficient (all other things being equal) than on high fire. OTOH, there a few if any condensing boilers that do not modulate. Impossible to separate the two.

        Does a modulating unit provide less heat delivered to the space than a non-modulating unit, for the same heat loss? Not if a thermostat of some sort is involved.

        Sealed combustion is also a great feature, but has little impact on thermal efficiency of the boiler. The consequences of pulling a space neagtive is a very subjective and poorly documented. Mostly anecdotal.

        Low exhaust temperatures are only possible when the unit is operating at low supply and return water temperatures. Great supporting information to futher dispell the misconception misconception.

        Outdoor reset is not only a feature avilable on mod-con boilers, but I agree is a significant fuel saver.

        Purge control? You'll have to explain this one. A big deal, you say. I'll wait to see.

  2. Tim | Sep 24, 2010 10:32am | #2

    Your questions raise

    more questions. To answer a few....

    Is it possible to add a new boiler and control it as the "lead" boiler? Absolutely. Any competent heating contractor should be able to do this, if not, then they would not fall in the category of "competent".

    Does this present problems with Codes? Only if done by anyone that does not know and understand those Codes. In concept, there is no potential violation to any code I have have ever read, and I've been exposed to many. Upgrading equipment to newer and more efficient equipment on a piecemeal basis is common and acceptable.

    Is a residential boiler designed to ....? Yes. There are obviously varing levels of quality in the residential and every other market. This is what boilers do and are expected to do - run continuously during the heating system. Most equipment, especially heating equipment, works best and lasts longest when operated continuously as opposed to cycling.

    Personally, I would recommend replacing the modular boiler bank with a single modulating condensing boiler, that meets the heating load of the buidling (based on an accurate heat load calculation). Basic modern heating system design features would easily pay for itself in a few years (outdoor temperature reset, variable flow/capacity, zone controls, etc.).

  3. Shacko | Sep 24, 2010 02:13pm | #4

    Boiler

    One thing I would check on is your insurance input, some insurance companies are more restrictive then the local codes.

  4. 50_Pascals | Sep 24, 2010 04:38pm | #5

    Simply tweek the existing system?

    Hi Jim,

    My first advice is to do a search for Henry Gifford or his company Gifford Fuel Saving.  He is expert at hydronic heating systems for large buildings.  He usually wrings considerable savings out of poorly designed systems while using the original equipment.

     

    Next, you still haven't told us what is the supply and/or return water temperature for this system.  That's important.  Also, what type of heat and number of units.  For example, hot water baseboard, or old fashioned cast iron radiators.

    I guarantee that there's a good amount of energy to be saved with the control system.  Sensing outdoor air temperature is a good idea for determining supply water temperature, but I do not like it in your application.  Because assumptions are made as to the heat demand of the building with outdoor temperature.  In general, it's not a bad idea, but there are opportunities for savings there.

    I'd suggest staging the boilers based on the supply or return water temperatures.  If the temp trends too low, kick the next boiler on.  The supply temp can be determiend by an outdoor air sensor.  But the sensor lacks ability to know whether it is sunny or cloudy, or whether people are home, or even want more heat.

    And finally, it is also possible to tweek the gas supply pressures to reduce boiler output.  This would be done as a result of some study of the system operating characteristics.  For example the second and third boilers may be dramatically oversize and short cycling.

    We cut 40% off of a customers fuel bill for a very large house just by tweeking the controls, and by changing some of the near-boiler piping to provide some mixing for the return water.  We also made the boiler MUCH MUCH happier.  We only adjusted gas pressure for clean burn, but I know it could have been reduced further with a bit more time (we were on site quite late and had to get home in a snow storm).

    A study of the existing system is in order.  You can do a lot with what you have.  It's not always about spending money.

    -Rob

    1. JimEngr | Sep 25, 2010 12:00am | #6

      More building info.

      Henry Gifford - his web site is http://www.energysavingscience.com/ and has a bunch of interesting calculations. I'll check further on that site to see the other ways to save energy. (Imagine - reducing the energy usage of the circulating pump!)

      The building was built in 1917 or so as a retirement rest home for a religious group, was foreclosed on (twice) and my friends bought it in 1995, after it had been standing empty for several years, to convert it to a cooperative with about ten living units. The existing gas boilers were certainly installed before 1990 or so, but I don't know when. The heating is done with radiators, so the water temperature is probably closer to 140 F than to 190 F (as in baseboard). I don't think that there are any thermostat controls on individual radiators.

      My initial suggestion to them was to install a very-efficient baseload boiler (my own guess as to what might help with the fuel bill) and they asked me for more information. This forum has been a wonderful source of information! Many thanks to you (you all? y'all? ...this is Minnesota, after all :o).

      The existing controls do modulate the water temperature, but they are certainly technology from decades ago. There is probably a newer control system that is available now, but I'm not sure if the owners will decide to go that route ($$$?).

      You mentioned mixing some of the return water - with the heated water output of the boiler? That would make the boiler run with hotter water - is that desirable (in an older boiler?).

       - Jim

      1. DanH | Sep 25, 2010 07:57am | #7

        Seems to me that in a building built in 1917, wit a boiler only 20-odd years old, you should first look in other areas for heating efficiency -- insulation, weatherstripping, etc.

  5. rich1 | Sep 25, 2010 10:21am | #8

    TRV on the rads.

    500,000 BTU condensing boiler.

    Leave the other boilers on line.

    Use a Grundfos Alpha modulating system pump.

    Make sure the piping in the boiler room is done right.

    Use a boiler to make dhw using and indirect tank.

    Use a tekmar control to run the whole works.

    Do all that and I'll bet the old boilers hardly ever run for space heating.

    Do like Dan says and they will never run.

    I've got a six story, 35 unit building, built in the 60's, that runs on 500,000, and my design temp is colder than yours.

    1. JimEngr | Sep 27, 2010 12:22am | #9

      Many thanks for all the help!

      Thanks to all of you for your help. I will forward your comments to the group, and they can decide what is the best direction for them to take.

       - Jim

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