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panel rads, constant circulation, short cycling, outdoor reset, trv's, buffer tanks, mod cons, varia

CapeCod Member Posts: 67
Hello all,

You were very helpful in the past with my dilemma (SWEI, Karl, Chris, etc.) so thought I would give you an update after I just had the work done that you proposed.  It took a lot longer than anticipated to find a competent contractor in my area.  Here is the link to the old post:

<a href="http://www.heatinghelp.com/forum-thread/143729/Short-Cycling-Triangle-Tube">http://www.heatinghelp.com/forum-thread/143729/Short-Cycling-Triangle-Tube</a>

Sorry for the long post but it might be helpful to someone...got some questions on the end if you want to give me some more feedback.

Old System was set up for direct pumping using only the

boiler circulator and with with two zones, upstairs and downstairs.  Cycle times were running average of 10

minutes. But in spring / fall they were more like 3 minutes.  Control strategy was “thermostat + outdoor

reset” on Triangle Tube with panel rads.  Low water panel rads are installed.

New System changed to primary / secondary piping.  Added Wilo variable speed circulator for

secondary. Added 3 additional radiators. 

Changed towel warmer control scheme to come on with programmable

thermostat.  Removed zones and control of

remaining rads via TRV’s.  Cycles times

are now more in the 15 to 20 minute range with 30 degree weather outside.  Control strategy is now “constant circulation

+ outdoor reset”.  I did have to put on

the 15 min call block feature on to allow the boiler to achieve longer burn

times.  Otherwise, once the boiler

reaches steady state and shuts off it doesn’t take much to raise it a few

degrees before it shuts off again.

I am happy with the comfort level and it looks like short

cycling has been reduced.  I am going to

keep my eye on this as we head into spring. 

With just towel warmers or one or 2 radiators calling for heat there is

still the possibility of short cycling.

You all suggested a buffer tank as well….the contractor and

I decided to try just changing to primary / secondary to see if that was enough

and then go from there.

In constant circulation mode with no central heating call

blocking, the boiler overshoots temperature setpoint by 6 degrees before

shutting off and fires again after dropping 6 degrees.  There is no way to modify this per

TriangleTube.  Referencing  formula for buffer tank using the formula

(desired on time * (difference of min boiler firing and smallest zone) / (delta

T * 500):

Question 1: is my

delta T in this equation equal to 12 degrees (6 degrees of over / undershoot in

each direction)?  No way to modify this because it’s already

locked in as part of the boiler algorithm?

Question 2: Using an

electric water tank as a buffer tank, is there anything else needed other than

putting it in line with the return to the boiler…no other fancy controls?  I am coming up with a tank anywhere from

30 to 50 gallons depending on the numbers I put in.  The boiler would be forced to bring the whole

tank up to 6 degrees above the setpoint every time it fires which should

provide me plenty of water to pull from if just the towel warmer calls for


Question 3:  Will buffer tank allow me to run at lower

temperature?  As it stands now, the

boiler shuts off for 15 minutes between firings.  If I have more water in reserve, I would

think I can run longer but at a lower temperature and remove the call blocking

feature I have set up currently.  Then I

would have constant circulation of (nearly) constant temperature water.  I would think this would also improve the

comfort factor slightly.  I am running a

majority of the time outside the condensing mode so maybe I can justify cost of

boiler with decreased operating costs?


  • Ironman
    Ironman Member Posts: 7,376
    Buffer tank

    Take the minimum firing rate of the boiler and subtract from that the minimum load. The difference is the amount of btu's that the buffer tank must be sized to. You can over-size the tank and get longer run times, but don't under-size.

    Your system is probably designed for a 20* delta T. This has nothing to do with the programming in the boiler, but the sizing of your emitters, piping, circulator, etc.

    The three main variables in the formula are btu difference, minimum run time and system delta T. Minimum run time should be at least 10 min. Delta T we'll assume the standard 20*. You'll have to supply the btu difference. With these parameters, you'll need one gal. of storage for every 1000 btu's of difference between the minimum firing rate of the boiler and the minimum load. If you have a 30k btu difference, you'll need at least 30 gal. of buffer. The formula will confirm the same. I just gave you the short cut.

    As far as piping goes, it depends on how your pri/sec piping is done. Is the primary piped where the loop goes from boiler supply to boiler return and the system connects to it through the sides (bull) of the closely spaced Tees? Or is it the reverse where the boiler injects into the primary through the bull of two closely spaced Tees? Is there an indirect, and if so, how is it connected?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • CapeCod
    CapeCod Member Posts: 67
    Primary / Secondary & delta T

    Interesting...didn't know there was more than one way to do pri / sec piping.  What you described as the "bull" looks like my system.  Closely spaced tee's off the side and then right after that is the Wilo circulator followed by the manifolds that feed each panel rad homerun style.

    Sorry, still not getting the delta T thing.  I know it depends on a lot of factors but I am not confident the theoretical calculation matches reality very well.  At least not enough to blindly plug in the formula to size a tank.  For example, the Wilo circulator can be adjusted from 16 ft. hd down to 4 ft. hd.  I started out with 16 and now am playing around with 4....the slower the water moves around the circuit the wider my delta T will be right? Plus, the rads are balanced so each of my 7 rads will have different delta T's.  Seems like a lot of variables to put any kind of faith in.

    But at the end of the day, I am trying to understand what is going to tell the boiler to start heating up the water in the buffer tank in constant circulation.  In looking at the heating operation, it appears it has a sensor telling the boiler to fire when the temperature drops 6 degrees below the setpoint and that same sensor tells it to shut off when it exceeds setpoint by 6 degrees.  So if this sensor could be moved to the buffer tank, firing would be controlled by the buffer tank temp and the delta T (at least that's what I am referring to it as) would be 12 degrees because it would have to raise x gallons of water 12 degrees.
  • Ironman
    Ironman Member Posts: 7,376
    Delta T

    As you've observed, a DESIGN delta T is seldom the ACTUAL delta T of the system. However, the actual will come closer to the design as it gets colder out side and the demand for heat increases.

    Don't confuse the differential and temp overshoot parameters of the boiler with the system deltaT.

    Unless you have radiant floor heat, 20* is the standard DESIGN delta T for most American hydronic systems and it will more than suffice for sizing your buffer tank. Your over-thinking this and causing yourself more concern than is necessary. Again, if you want longer run times, up-size the tank; it won't hurt anything.

    Regarding your piping: you are saying that the system connection comes out of the side (bull) of the Tees and the boiler water goes straight through the run of the Tees? What about an indirect? Do you have one?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • CapeCod
    CapeCod Member Posts: 67

    Thanks, I do tend to over analyze this stuff but I do like

    to understand it.  I think we are on the

    same page with the piping.  Maybe a

    picture would help but if you look at the boiler on the wall, the primary loop

    looks like a big “O” of 1 inch copper pipe hanging down.  On the left side of the “O” are the closely

    spaced “T’s”.  The top tee is is the

    supply to the rads, the bottom tee is the return from the rads.

    So, you just stick that tank in there on the return side and

    let the boiler do its thing?  Simple as

    that?  It basically would act like a big

    fat pipe.  I am thinking there is

    something like 10 gallons of water flowing around in a circle on the primary

    side.  If I have just a towel warmer

    calling for heat, there will be a couple gallons of “colder” return water

    mixing in with that loop and pulling down the temperature.  The buffer tank will only hold down the

    temperature longer then and oversizing the buffer tank will just make it run

    longer which is not necessarily bad but has to be balanced by standby losses.
  • CapeCod
    CapeCod Member Posts: 67

    forgot to answer the question about the indirect...i have a prestige excellence with has the indirect built in the boiler.
    PLUMMER Member Posts: 42
    Buffer and differential

    Is there an adjustable boiler differential? Where is the 6 degree overshoot being read at?

    Buffer, if you do use an electric tank just because those are 1" taps on the element holes doesn't mean you can't use larger pipe. I see better stratification when piped with 1.25"&1.5" from boiler to buffer. Getting an accurate tank temp reading for the boiler and or how to control boiler and buffer temps will be a plan.
  • Drewgrizz
    Drewgrizz Member Posts: 21
    tank on primary

    Can a electric water heater be put in the primary circuit as a battery or does in need to be on the return side of the secondary circuit?  I am building a very similar system except I am using a old school style boiler and a Taco 4way I series mix valve with the outdoor reset.  I am thinking about adding a 40 or 50 gallon GE electric water heater into the system to boost efficiency.  I did add a tee and a zone valve/aquastat/heat exchanger off the secondary loop for a hot tub which will probally boost efficiency enough.