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Delta t ?

wrxz24wrxz24 Member Posts: 285
I have a tt 110 with a combination of baseboard heat and inslab radiant. Here is my question, when all the zones are calling, i am getting almost 30degree delta t. I assume because I am getting the return water from the baseboard at about a 20 degree delta when it is running by itself. The low temp zones when running by themselves have about a 10 to 12. The inboiler pump speed is on speed one. If all the zones are calling, is that limiting the flow to the boiler or is it just the result of low temp and high temp return water mixing together? I have taco 007 for each zone. Each radiant zone has a 3 way mixing valve.

For example, last night, outdoor temp was 26. The setpoint was at 144 and the return temp was 108 with all zones calling. The mix valves were at 100 degrees.

Thanks in advnce


  • ZmanZman Member Posts: 3,517
    delta t

    Not bad!

    Baseboard around 10 is fine. Radiant in slabs will often have a wide delta when they fire up. In a perfect world they will settle out in the 10-20 range.

    Keep in mind, the delta at the boiler is not the same as the secondary.

    How is it heating?

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • wrxz24wrxz24 Member Posts: 285
    Well so far

    Thanks, getting some pretty nice cycle times. The baseboard zone when it calls, i have been getting about a ten minute burn on low fire( from the ignition to when it stops firing) . I put a 7 minute block and post pump time on it. By the time the set point is reached or exceeded, I have about 10d t. The weather hasn't been real cold either so It should work real well when the colder Vermont weather hits. When all 3 zones call, the burn times are longer than I can wait watching the thing purr. Still trying to get the outdoor reset curve dialed in for the baseboard zone. Carl, thanks again for your help then ,now and I am sure in the future.
  • Steamfitter66Steamfitter66 Member Posts: 117
    Radiant floors

    In a residential application will typically come in 7 to 12 dt Baseboard at 20, your spot on see what it looks like when you experience close to design temp.

  • wrxz24wrxz24 Member Posts: 285
    Thanks, speaking of Delta T..

    My design temp for Barre, Vt is -11 so lets hope I don't get many days like that.  My outdoor reset temp seems to be a little higher than the actual temp though. So I am trying to figure out a way around that.  Set the the low end part of the curve up some?? open for suggestions if someone wants to chime in. 

    So, I am o.k with keeping the inboiler pump at speed 1 despite the high delta T when all zones are calling(baseboard, inslab radiant)?
  • Steamfitter66Steamfitter66 Member Posts: 117
    edited November 2013
    Indoor feedback

    will allow you to run the lowest temp possible.

    Its designed into the control strategy and is accomplished with thermostats that communicate space temp trends with a controller that then send a signal to the boiler control.

    Tekmar and Honeywell are most popular here, there are others as well.

    I will be trying this on a left over ps 60 going into a shop building w one zone and a 51K heat calc.

    Lower your upper curve temp and see how it responds
  • wrxz24wrxz24 Member Posts: 285
    Here is the problem I am having now with colder temps

    The boiler will not get to set point because of the high delta .  The target set point is 156 and the boiler is only making 142 and the return is 108.  I think the boiler is in protection mode due to the high delta T. 

    The boiler is piped pri/sec and the boiler pump is set to speed 1.

    I do have an system bypass from my old system and when I open that, the return temps rise as does the supply temp.

    Would setting my boiler pump to speed 2 to see if there is an improvement?

    If the baseboard zone is running by itself, the system works fine.  When the inslab radiant zones are running on the low temp curve, the system works great.  But when both are on at the same time, not so much.  Suggestions?  Thanks in advance
  • ZmanZman Member Posts: 3,517
    How much

    What percentage will the boiler go to?

    It is in the info menu.

    There is no harm in trying speed 2

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • wrxz24wrxz24 Member Posts: 285
    Does the boiler

    Need to be running to check?

    I will try speed 2 but hate to do that cause when the zones are running independent from one another, the system works great.

  • ZmanZman Member Posts: 3,517

    The boiler will tell you it's percentage of output. You have to check it at the time with the boiler running.

    It also may be a case of the cold slab drawing down the boiler when it first starts up.You may be at 100%

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • wrxz24wrxz24 Member Posts: 285
    Very well maybe

    I know when i open the bypass valve, the return temps go up and thus the supply temp can get to setpoint.

    Should i let some boiler water back into the return? Or set boiler pump to speed 2 to see if that helps?
  • wrxz24wrxz24 Member Posts: 285
    Firing rate is at 100% and

    The flame ionization is at 23.

    The boiler can not reach set point when both the baseboard and inslab radiant zones are calling.

    Any solutions? Does putting the inboiler pump on speed 2 pull more supply water back into the boiler? Or is the boiler undersized?
  • ZmanZman Member Posts: 3,517

    It is pretty common for cold slabs to overwhelm the boiler at startup.

    Your boiler may be undersized to the available radiation, I doubt it it is undersized to the heat loss. How is the the system performing? Are all the zones keeping up?

    You are going pretty far down the rabbit hole, which is not necessarily a bad thing.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • SWEISWEI Member Posts: 7,356
    Conventional zone valves

    (and circulators) fighting with outdoor reset control.  Again.
  • wrxz24wrxz24 Member Posts: 285
    The inslab radiant zones are

    fine due to the 120 degree water but my concern is the baseboard zone. What is going to happen when it is real cold and the baseboard zones need 170 degree water and the boiler can only make 120?

    Btw, I set the pump speed to 3 last night and that was maintaing a 20 degree delta t and the supply temps were starting to go up before going to bed. The boiler was on standby this morning when i got up.
  • wrxz24wrxz24 Member Posts: 285
    Can you explain please

    Should i consider a different inboiler circulator to maintain a set delta T?

    I like the idea of low return temps and the condensation but not at the expense of not reaching target supply temps..
  • ZmanZman Member Posts: 3,517

    How exactly are you mixing?

    For how long a period does this condition occur?

    If it only happens for 20-30 min at start up, I don't see it as a problem.

    Here is a great read

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • wrxz24wrxz24 Member Posts: 285
    edited November 2013
    3 way mix valve

    Supply + zone return= mixed temp. So the return piping goes to the mixing valve and back to the boiler.

    I believe it happens much longer than 25 minutes.

    Thanks for the link.

    This might be the problem I am having.....
  • SWEISWEI Member Posts: 7,356
    Conventional zone valves

    can be told only two things by a conventional thermostat.  Either "I'm too cold" or "I'm too hot."  That logic works (mostly) with a high water temperature and low mass baseboard emitters.  Not so much with high thermal mass emitters, of which a slab is the penultimate example.  This on/off control scheme breaks down when outdoor reset gets involved, since the fluid temp is closer to the space temp and therefore does not change the space temp quickly.  There are boost functions and other tricks to work around this, but the physics are working against you here.

    The best answer is to run two separate reset curves, with the lower one controlling a mixing valve for the slab.  Some boilers (Viessmann comes to mind) can run that valve from their onboard controls.  Lochinvar sells an add-on box.  For the rest, we buy a Taco iSeries-R valve or add external controls.  If you pipe the high temp zone ahead of the low temp zone, things generally work out better here.  Turn the thermostats all the way up (really) while you adjust the reset curves (this can take some time to get right) then lower them to perhaps 3ºF above the normal indoor space temp.  They will act as high limit controls.
  • wrxz24wrxz24 Member Posts: 285
    Well one problem is that my high temp

    Zone is the last zone down the line. The zones before that all have mixing valves set at 120 for protection.

    Today there was an overshoot in temp by 4 degrees tstat at 68 and temp was 71...sunny day... but the floor temp was at 73. I wonder if getting a floor sensor would help getting the flloor ready for the cold nights and have things get started when the floor starts to cool.

    I know with the things they are now, something has got to be done to get the supply temp up to heat the upstairs when both types of zones are calling.
  • Steamfitter66Steamfitter66 Member Posts: 117
    Overshoot will happen if

    You dont have indoor feedback but can be lessened by a tstat w PID.

    Is the pump for the baseboard on the return? If so why?

    Dont be overly concerned about short periods of unable to meet target. Its because of the slabs ability to absorb alot of energy.

    You most likely are over pumping your radiant and you most likely are using a cheap tstat.

    An integrated control system and system balancing would eleveate most of what your seeing.
  • wrxz24wrxz24 Member Posts: 285
    The pump on the baseboard is on the return

    Why? Not sure, the guy who put my initial system in,piped it that way.

    The radiant zones use a taco 007 with 3 way mixing valves.

    I guess, i am not sure what to do here. ,i just had the new boiler installed in May so really have no extra money to spend.

    I was thinking of buying a new tstat for the slab with floor sensor and hoping for a mild winter.
  • Steamfitter66Steamfitter66 Member Posts: 117
    Do you have

    a heat loss calc? It should be done prior to any installation.

    The reason a pump is on the supply is mainly to prevent air lock as you can push air with water and it minimizes cavitation.

    As far as thermostats I would recommend

    It has PWM logic to minimize overshoot.

    Focus more on space temp and dont get to caught up on minute by minute supply water temp.

    You may want to lower the mix temp on the slab and it wont rob as much from the other zones. typical slab supply with tile is around 85 to 100 depending on slab insulation
  • wrxz24wrxz24 Member Posts: 285
    I did a heat loss which came in at 90k for 3,400 sqft

    That includes the heated garage and future bonus room. I keep the garage at 50.

    I already have that tstat and it does work good but I do get a lot of solar gain so the house temp is often 3-4 degrees over tstat setting during the day but the slab has a chance to cool down. Thats why I think I could benefit with a slab sensor with the pulse width modulation tstat. So if the slab falls below a set point, the heat will start before the room temp drops at night. And when the baseboard zone calls for heat, the slab might be close to stabilized and the supply temp for the baseboard can be met.

    When the sun goes down and gets cold, that is when the problems start. I never had the low supply problem though with my 150k Smith boiler.

    The only reasonh I raised the temp of the mixing valves was to try and get the return temps up a bit. I figured that would help. I also set the inboiler pump speed to 2 and that helped. I know a cold slab really pulls the heat out of the water and the return temps are chilly.
  • wrxz24wrxz24 Member Posts: 285
    I tried something tonight

    I partially closed the flow valves on the return piping of the radiant zones to try and increase the supply temps.

    I was able to get a 20 degree delta t and the set point was met.

    So the radiant circulators were getting mixed temps from supply and return and then I slowed the flow back to the boiler.  Is this o.k to do? 
  • ZmanZman Member Posts: 3,517
    Flow control

    The correct way to do it is with the right size circulator  or flow control valves.People do what you are doing all the time, It is not that big a deal.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • wrxz24wrxz24 Member Posts: 285
    I thought so.

    It did help the boiler play catch up.

    Carl, any recommendations to prevent the slab return water from making it impossible to hit the high temp setpoint which happens when both inslab zones are calling at the same time as the baseboard zone. This happens usually at night after the slab has cooled and outside temps fall. Sunny days make it worse.

    Things I have tried and will try.

    1) increasing inboiler pump speed. (Tried)

    2) control the inslab return flow back to boiler (tried)

    3) buying new tekmar 519 with slab sensor to keep the slab charged eventhough the air temp is met.

    Any other suggestions? Thanks again

  • Steamfitter66Steamfitter66 Member Posts: 117
    I must be

    falling asleep when trying to post to this thread as the last 2x I've posted doesn't show up.

    By turning up the mixing valves for the slab you are inadvertently causing the issue to worsen.

    If you increase the temp you increase the amount of boiler water which increases the amount of return to the boiler from the slab.

    The Tekmar 519 will help with undershoot, overshoot would be helped by lowering your mixed temp and a control system with indoor feedback.

    If everything was fine with the last boiler than lets look at what has changed.

    1 Boiler output capacity 20 to 25%

    2 you have added outdoor reset....It causes recovery problems in some cases.

    3 Anything else that was repiped or added?

    I apologize I was not ignoring your posts
  • wrxz24wrxz24 Member Posts: 285
    no worries, I appreciate all the help and advice

    With the way my radiant zones are piped, I did no think the flow back to the boiler changed so I increased the mix valve temp so the return would be warmer.  I have since lowered it to 110 but do have these zones on the high mass reset curve which has 90-110 degrees of water from bottom to top. 

    The baseboard is also on its own curve which has priority hence the mix valves.

    Due to solar gain and slab cooling causing undershooting, I just ordered the tekmar 519 that has the slab sensor.  My plan is to drill a hole for the slab sensor.  Do I just put the sensor in the hole and call it a day or do I put it in a piece of copper? 

    My old boiler was a 150k input and 125k output piped direct.  It had a much larger heat exchanger.

    I don't think the OR was causing the problem because it was on 100% high fire and still wasn't getting the water to temp.

    I did partially closed the flow valves from the radiant zones and that allowed the boiler to reach its set point when all zones are calling.

    Thanks again for your help.
  • SWEISWEI Member Posts: 7,356
    Slab sensor

    Blow out the hole with an airgun, or vacuum with a computer vac.   Generous amounts of epoxy in the hole before you insert the sensor.  If it ever goes bad, you drill another hole.  Install the sensor in a location where morning sunshine hits the floor.
  • wrxz24wrxz24 Member Posts: 285
    edited November 2013
    Thanks SWEI

    The placement of the sensor is going to be under where my thermostat is already located. I think it does get morning sunshine too. What should I set the low set for the floor temp?This morning the floor was 76 degrees and tstat was at 68. Right now the floor temp is at 68-71 depending on spot and the tstat is still 68.
  • wrxz24wrxz24 Member Posts: 285
    edited November 2013
    Update on floor temp

    The boiler set point was at 106. Outside temp was 22. Floor temp in the middle of the house was 78. Towards the perimeter was 74. The boiler called for heat and ran at 18% for well over 15 minutes and it still may be running as iI got tired of waiting land left lol.

    Thanks for all the feedback and Happy Thanksgiving to you and yours!!

    Edit: at 29 minutes, the set point had been met and was on its 8 minute post pump cycle.
  • SWEISWEI Member Posts: 7,356

    Not sure how either the house or the zones are laid out, but if you can find a way to link all the loops subject to solar gain together and have them respond to that stat it will likely improve the overall system balance.  You'll have to tinker with the minimum floor temp setting on the 519 a bit -- I'd start a couple degrees above the lowest temp you're seeing now and see how it recovers.
  • wrxz24wrxz24 Member Posts: 285
    I have done that already

    Both radiant zones downstairs are controlled by that one tstat.

    I will try 73 and see how that works. Thanks.
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