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I'm posting this for Bill Brooks.

DanHolohan Member, Moderator, Administrator Posts: 16,551
<span style="color:#1F497D">I use a quietside qmv-9 mod condensing boiler with an output of 55kbtuh-115kbtuh. The boiler has 2 distinct hx, one for the indirect dhw, the other for my radiant heat which currently fires 3 zones.</span>

<span style="color:#1F497D">The main lv. Area, garage, mer/shop.  Heat loss was calc. using rfh warmsource 2.08a. and results in 48.5kbthu for the house.</span>

<span style="color:#1F497D">Main lv.area =14.25kbthu, garage=10.7kbthu, mer/shop=7.7kbtuh.  I suffered from short-cycling, so 3yrs. Ago I installed a 40gal. buffer tank which increased firing rates to well over 10min.  I have no issues with this decision.</span>

<span style="color:#1F497D">The

first 2 winters were typical with several design days. At 2deg, the

main area was 67-68deg. As predicted.  The stat (lux pro air sensing) works ok but not overly speedy.  Last winter was very mild but nevertheless without complaint.  All was good. 


<span style="color:#1F497D">This

summer the flow sw. sensing call for heat by the dhw failed and allowed

copious amounts of water and (glug,glug-air) to invade the boiler

interior piping before I could get things isolated.  Needless to say,

the whole radiant system was pretty well drained with tons of air.</span>

<span style="color:#1F497D">Obviously

I chose to do a power purge of the main lv.area first since its supply

and return manifolds are below floor level.  I am using rehau Pro

balance manifolds with stop-cocks on the return. These units can be

completely isolated from the rest of the system and purge each loop independently

from the other.  By connecting a hose to the return manifold and

immersing it in a partially filled pail of water I was able to

watch for air as it was moved out.  I pressurized the circulator header

and valved off the other circulators so that I would get a complete purge

thru this entire circuit only.  All of the loops had gobs of air and

took a while to get everything out (so I thought).  I opted to do

nothing with the

other zones until cold weather as their supply and return manifolds are

above floor level and easier to move out the air if necessary. Turns out they’re ok anyway.</span>

<span style="color:#1F497D">When

weather got cold I was surprised to note the main lv. Area stat took

forever to get satisfied, in some instances firing all nite.  I did not want

to fool with the plant going into winter so I chose to monitor all

zones, measuring temps. And looking at floor temps.  Also observe the performance

of the plant in general.  Using a snap-on pipe thermometer as an

indicator,  I noted the buffer tank discharge as 120deg.most of the


<span style="color:#1F497D">Oddly

the garage and mer/shop zones were always satisfied with very short

firing periods needed.  The main lv. Area is heated by ½’pex supplied by

¾’pex back

to the boiler.  Porcelain tile covers 1 ½’ liteweight  maxon concrete. 

During firing periods I measured 70-72deg. Floor temps which are

6-8deg. cooler

than previous years.  Also the space feels noticeably drafty.  I had to

run the gas logs most of the day to cut the chill and even had to call

up the a/c airhandler (on its own stat) with electric strip heat inside when it was below 20deg.    ABSOLUTELY UNHEARD OF!</span>

<span style="color:#1F497D"> The

boiler and buffer tank appeared in order with discharge temps. Usually

around 120deg.  I decided to check temps. Further down the line to see what

temps. Were actually being sent to the main lv. Area zone.  Between the

buffer tank discharge and this zone’s circulator is roughly 5’of ¾’


<span style="color:#1F497D">And

a Honeywell supervent in between.  HORROR OF HORRORS I measured

typically 80-85deg. Going to the floor and sometimes as cool as only


<span style="color:#1F497D">What

is causing this temp. drop of 40deg. Before it even gets to the

floor??  Is the supervent clogged causing a temp/pressure drop,  or is

there another

slug of air lurking in the circulator headers?   Also I am not using

outdoor reset and the circulators are not modulating type.</span>

<span style="color:#1F497D"> I

want to restate that the plant has never suffered problems in the past

until this air situation. In the past even in bitterly cold weather

walking in bare feet

was always very comfortable!  I’ve been trying to think this thru

without outside assistance.  The smile has long since left my face!</span>

<span style="color:#1F497D"> I’ve enclosed 3 photos which hopefully get transmitted and clarify the plant layout:</span>

<span style="color:#1F497D">1.       </span><span style="color:#1F497D">Shows

the top piping  of the buffer tank. The 2 pipes toward the back are the

main radiant zone returns mixing with the heated water from the boiler.</span>

<span style="color:#1F497D">The pipe in the foreground is the heated buffer water going to the circulator header. The boiler directly fires the buffer.</span>

<span style="color:#1F497D">2.       </span><span style="color:#1F497D"> Shows the relationship of the circulator header to the boiler piping. (note supervent)</span>

<span style="color:#1F497D">3.       </span><span style="color:#1F497D"> Continues

with the remainder of the piping relationship to the dhw,etc.  the

direct firing of the dhw and buffer are seen in this pic.</span>

<span style="color:#1F497D"> Any thoughts or suggestions would be greatly appreciated.</span>

<span style="color:#1F497D"> </span><span style="color:#1F497D">bill</span>
Retired and loving it.


  • Paul Pollets
    Paul Pollets Member Posts: 3,658
    edited February 2013
    Near Boiler Piping

    From what I see, the boiler is not piped for primary/secondary. I'd re-pipe an 11/4" loop around the boiler with (2) closely spaced tees for each zone, as required. You'll then get the required flow rates without creating a "thermal restriction" which is present in the existing piping. Or, install a hydraulic separator between the zones and the boiler.
  • Zman
    Zman Member Posts: 7,588

    The first part of your post reads like an air problem. Aerated water is put in in the summer and left to deaerate  where you least desire.

    The horror of horror of the cold supply manifold temp smells of a bad or non existent check valve. Is it possible that under certain circumstances, one of your zone circs is pulling water backwards threw a another zone(cold slab)?

    I would start by just feeling the supply and return pipes during different operating conditions. I bet there are times when things that are supposed to be hot are cold and vise versa.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
This discussion has been closed.