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3x Oversized ModCon with Concrete Slab, New Home

I'm designing my new ICF home. 2900 ft2 slab heat 3-zones with master/slave zone synchronization.

I did a ResCheck DOE heat loss estimate with my proposed plans & got UA = 400

Location is Cripple Creek Colorado so my design Temp is about -30*F, thus at 70* indoor, my Delta T is about 100* 400 x 100 = 40,000 BTU/hr.

I have a new in box Buderus GB142-45.

Looks like this boiler is about 3x oversized after being derated for 10,000' altitude.

I did the math on my slab, 40yds concrete x 3,800#/yd (with rebar) = 152,000#

With a Specific heat of .2 it will take 30,400 btus to raise the slab 1*f

So at -30* my boiler duty cycle will be about 33%, at freezing (this is a guess) possibly a 10-15% duty cycle?

So my question is: Should I sell the Buderus & buy smaller ModCon OR will slab act like a large buffer tank & Buderus will perform fine with low duty cycles?

Thanks BBP


  • Jean-David BeyerJean-David Beyer Member Posts: 2,526
    will slab act like a large buffer tank & Buderus will perform fine with low duty cycles?

    I am not a pro, but I have a slab with radiant heat in it. I used to have a boiler that was about double the needed capacity (70,000 BTU/hr). My house might need as much as 30,000 BTU/hr in the very coldest days (way under the design temperature). It would cycle very rapidly: 30 seconds on, 45 seconds off. This because the slab could not accept the heat as fast as the boiler produced it. The boiler was rated at 70,000 BTU/hr input, and the slab is about 32 feet by 28 feet. I do not know how thick it is, but it could be six inches or a little more. That boiler must have been pretty tough because it was still working when it was 60 years old, and I set the aquastat to run at about 130 to 140F. The biggest problem is that the time it takes to change the slab temperature is so great that they system was unstable. The main symptom was wide temperature swings.

    When the thermostat called for heat, the boiler started heating the slab. After about 4 hours the slab warmed up to produce significant heat. The trouble is that when the thermostat was satisfied, the slab heated the house for another 4 to 8 hours, so the temperature went way over, by 5F or so. So it took quite a while for the house to cool down. When the thermostat again called for heat, the slab was way under temperature so the cycle started over again.

    Converting to a modulating boiler with outdoor reset fixed all that because I never had to heat the slab to its former temperatures, and the slab almost never got hotter than necessary. So now I can hold +|- 1F easily.
  • GordyGordy Member Posts: 6,788
    edited September 2011

    Is the slab for your new ICF home insulated? May seem like a stupid question....but you would'nt believe. Also slab insulating detail at edges. What is it?

    Also concrete is 150 lbs cubic foot or 4050 lbs CY. 27 cf in a yard. So total slab is 162000 lbs.

  • james patrickjames patrick Member Posts: 70
    edited September 2011

    Thanks Gordy, my source was wrong, 162,000# x .2 = 32,400 btus to raise 1* F

    Additional 10,000# doesn't change my question.

    Yes, plans are to use SPF 2# foam at R30 under floating slab with extra perimiter foam on outside of ICFs, below grade. Have bought 4,000' O2 barrier PEX, each of the 20 loops will be 200'

    Am using a constant circulation Grundfos 15-58 Brute pump in each of the 3 zones.

    Miniature 15 watt zone injection pumps 3gpm each. Slab overshoot protection sensor (Fenwal) & an air thermostat in series in each zone.

    Great-room / kitchen is largest zone with the most windows, this will be the master zone in my zone synchronization. Each bedroom zone will be slave, & can only call for heat if the master is also calling for heat. Through a latching relay design, a bedroom zone can continue to recieve heat after the master is satisfied.

    If I sell the Buderus, I will loose about $300 due to shipping.

    So what's wrong with a low duty cycle?

  • GordyGordy Member Posts: 6,788
    edited September 2011

        If your reference to (low duty cycle) is the cycling of an over sized boiler. The problems are that in a frequent cycling condition the boiler does not really get to peak operating efficiencies, and the frequent cycling is hard on the boilers components which shortens the life span of them compared to a properly sized boiler not cycling as much.

       The object of mod/con boiler sizing or any other type for that matter, is to pick one closest suited to your specific requirements. Many brands with different ranges out there some will only get close to the requirements needed. You must also keep in mind what brands are specific to your region.Iin other words who is familiar with the boiler your purchasing to get parts, and service when the time comes.

     There are ways to minimize short cycling through buffer tanks being implemented in the initial design. If you are confident on your heatloss calcs. I would sell the 45, and down size the 300 dollar loss in shipping fees will be recouped in fuel savings.
  • james patrickjames patrick Member Posts: 70
    edited September 2011
    duty cycle

  • james patrickjames patrick Member Posts: 70
    duty cycle

    I was thinking along the lines of 20 minutes on & 40 minutes off at design temp -30*

    This would be what I would call a 33% duty cycle.

    Depending on how I setup the ODR, I think I could have pretty long burns with this much mass absorbing the BTUs.

    Planning for a 10* boiler Delta-T

    Thanks, BBP
  • NH03865NH03865 Member Posts: 38
    Fact Checking

    You have calculated that the heat load of the house is 32,400 BTU with a design temperature of -30 degrees.

    The minimum output rating of the boiler significantly exceeds the 32,400 BTU required at the design temperature.

    At minimum output from the boiler and at design temperature (which only happens a few days a year) the boiler will operate with a 33% duty cycle.  This means that the longest duty cycle the boiler can ever achieve is 33%.  At temperatures warmer than the design temperature, the duty cycle will be shorter.

    I don't see what help ODR will be, it cannot make the boiler operate below the minimum output rating.
  • james patrickjames patrick Member Posts: 70

    Either I am off-base in how this large slab & this boiler will behave,


    You fellas are misunderstanding what I am trying to say:

    The large thermal mass of the slab should act as a giant buffer tank, allowing long infrequent burn cycles.

    If not, why not?

  • GordyGordy Member Posts: 6,788
    Thermal mass as buffer verses flywheel effect.


     Usually with large mass such as your slab the system experiences" fly wheel effect". Basically what happens is that your slab once up and running will experience lag in response time to changes in the heat load.

    Examples are many such as.

    Sharp changes when an excessive temperature drop, or increase is experienced from a cold, or warm front happens.

    Solar gains from the day causing slab temp increases that carry through the night.

     A concrete slab takes a long time to heat up, and cool down the response is not instantaneous such as with sandwich, or over the top types of RFH.

    The fact that your slab is insulated, or will be does help tame the fly wheel effect some what in that you are not also trying to heat the earth, and the earth is not trying to cool the slab.

    The point is with the boiler size you do not want the boiler minimum modulation to be the maximum heat load requirement your home will ever see. The max design day only happens a few days in a heating season the rest of the time you are above a design day temp so now you are short cycling the boiler because it is already modulated down to its minimum on the coldest days.

    A mod/con is fitted best clawing up hill to maintain the heat load. When sized properly at design day the boiler should be running flat out at highest modulation ratio. From there as loads decrease the boiler is allowed to modulate down to meet the lesser load, and still run for extended times.

    Calling the slab a buffer tank is kind of a wishy washy to rely on as a buffer component in a radiant system. its hard to control to achieve a fixed outcome to design in to a system. Usually there is control strategies to tame the fly wheel effect of a radiant heated concrete slab..

     The benefit of stored heat usually comes in to play when you have a power outage, and the slab is going to take a while to cool down. So you have some stored energy to get through such a dilemma

  • NH03865NH03865 Member Posts: 38
    Take it to the extremes

    I often find it helpful to take something to the extreme to understand the concept and then back off from there.

    Let's say that I will heat the slab to 90 degrees and then let it cool to 50 degrees before I add more heat into the slab.  You are then correct, you will achieve nice long burn cycles on the boiler, there will be a long time that the heat in the slab radiated into the house, but you will not be very comfortable for most of that process.  As I reduce the difference in the allowable temperature swing, I get more comfort but shorter burn times for the boiler.  I eventually end up with let's say a 1 degree difference between the max temp and min temp of the slab.  I then have nice comfort but much shorter cycles on the boiler.

    Remember, you are only putting enough heat into the slab to meet the heat load of the house.  That means that at the design day, you are putting 32000 BTU into the slab and on warmer days, much less.  The boiler you are trying to use will not run very long before it has supplies this much heat into the slab.

    Keep in mind that however you deliver the heat into the house, you still only need to provide the amount of heat necessary to satisfy the heat load at that time.  Whether I use a concrete slab, radiant panels or scorched air, I am still only providing enough heat to satisfy the heat load.  The boiler you are trying to use will always short cycle when trying to satisfy such a small heat load
  • james patrickjames patrick Member Posts: 70
    Call me Convinced

    Thanks for the advice, I see that you are correct. I figured that this was the case, but wanted to be talked into it. I really will not take a beating on the Buderus as they have gone up in price since I bought it.

    The TT Solo 60 that I am fighting with in my current house would be about the right size, if I could just get it to work at high altitude. BTW after repeated clocking of how long it taked to heat up my Lochinivar 80gal DHW tank, it seems to only put out about 35Kbtus on high fire.

    I also like what I've read about the Knight Lochnivar WHN085 with the Fire Tube heat exchanger, I'm waiting for a quote on one with the High Altitude version controler.

    Thanks, BBP
  • GordanGordan Member Posts: 891
    Only 35kBTU/hr on high fire?

    I've got mine hooked up to a 26 gallon reverse indirect, and I can take an endless shower with a 2.5 gpm shower head without temperature drop. There's no way that that would be possible with 35kBTU/hr. You're at a higher altitude so some derating can be expected, but not that much. I guess what I'm trying to say is that either your measurement method is flawed or there's something wrong with your particular PS60. Measuring the time required to satisfy some aquastat with an unspecified differential, and inside some tank with an unknown average temperature, strikes me as an inexact way to figure output, to be kind. A flow meter, while monitoring the return and supply temps on the boiler's display, would get you close.
  • james patrickjames patrick Member Posts: 70
    edited September 2011
    estimate BTUs on high fire

    What I have:

    Using the TT supplied DHW sensor on the outside steel surface of the middle of the 80 gal tank, covered by fiberglas & access lid.

    TT display on info mode item 3 is DHW temp.

    So with stable tank (no recient flow via draws) I record the temp, boiler fires for DHW cycle & runs for X time, after set point is reached, boiler stops burning, I have the parameter that controls how long the DHW pump continues to run set to max which is about 5 minutes, temp of tank continues to rise some few degrees, after tank has stabilized I record the temp, & do the math based on 80 gal x 8.3# x temperature rise to get BTUs added to tank, do the math with the burn time.

    I know that some more heat has been produced, tank steel, water filled copper pipes, etc. I havent factored this in as I don't know the mass involved, I could guess, but haven't yet.

  • james patrickjames patrick Member Posts: 70
    edited September 2011
    TT 60 BTU output @ 8,500'

    I just tested it again.

    TT 60 DHW cycle ran on high fire 42 min.

    I know that it's on high fire by watching the Fan RPMs as display don't show level of modulation as some higher priced boilers do.

    Tank Temp went from 102 -> 130* stabilized on both ends. = 28* rise

    42 min. is .7 hrs, Reciprocal 1 / .7 = 1.4286 ( 1.4286 x 42 = 60 )

    I guessed at the tank & pipes being equivalent to 136# of water + (80gal x 8.3# = 664#) = aprox. 800#

    800# x 28* = 22,400 btus, 22,400 x 1.4286 = 32,000 btus/hr

    Results are pretty consistant.

  • ChrisChris Member Posts: 3,056

    An awful long run time to satisfy the tank. What's the temp of the hot water leaving the tap as compared to the sensors reading?
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • NRT_RobNRT_Rob Member Posts: 1,009
    the real answer

    is maybe.

    1. you have to look at the smallest zone, not all your concrete, first of all.

    2. within that zone, you have to look at your emitter to see how much heat you can dissipate as well. If you can't dissipate at least the min mod of that boiler in the smallest zone, you have a cycling concern. even if the zone takes 100kBTU to heat up one degree, if you only have the ability to dump 3kBTU/hr with the installed tubing, you can't use it as a buffer.

    If you balance buffer, emitter capacity and temperature rise you *could* buffer out a boiler with a large mass. but if you are only going to take a $300 hit.... get the right sized boiler.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
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