Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Boiler and Indirect Questions

Pulse Member Posts: 78
edited July 2020 in Radiant Heating
Planned Boiler:
IBC SL 85k - actual output of 79k BTU max and 10.1k BTU min
Planned Indirect:

Heat loss:
design temp = ~30k BTUs @ 63 delta T (70 indoor temp & 7 outdoor temp)
~10k BTUs @ 15 delta T (70 indoor temp & 55 outdoor temp)

Other specs:
~2000 sqft of heated space in the state of PA
2.5 baths
Plan to use panel radiators @ a design temp of 140 degrees

#1) Based on my low heat load, I should not create heating zones, instead keep it one zone for the entire house?
#2) If the outside temperature does reach 55 degrees or more, I should turn off my heat to prevent the boiler from short cycling(see heat loss above)?
#3) Is it OK to use my boiler strictly for my indirect while I prepare for the hot water heat conversion, even if this becomes a long period of time?
#4) Is best to keep my indirect @ 140 degrees?
#5) What temp water does the boiler output when heating the indirect to 140 degrees?
#6) When an indirect takes priority, that means the boiler will stop heating the home for however long it takes to meet the DHW demand?

55 gal tank * 8.34 weight of 1 gal * 10 degree on/off diff = 4587 BTUs

79000 max BTU output / 60 minutes = 1316 BTU / 4587 BTU = 3.5 minutes of firing time
10100 min BTU output / 60 minutes = 168 BTU / 4587 BTU = 27.25 minutes of firing time

When my indirect is just maintaining temp, based on the above formula, it will require 4587 BTUs. My boiler will automatically find some happy medium between 10k and 79k BTUs, so it is not short cycling?

#7A)..and a side question..what is the ideal run time for a boiler?

52 degree ground water temp
140 DHW temp

88 degree delta * 8.34 = 733 BTUs required per gal
79k BTUs / 733 / 60 = 1.79 GPM of potential endless water?

So, when there is an actual demand on my DHW from say a shower, which will be 2.5 GPM @ 10 minutes. How many BTUs is my boiler outputting? ..and how long is it actually running?


I am just trying to fully understand how everything works. Thanks for your time and help.


  • flat_twin
    flat_twin Member Posts: 350
    edited July 2020
    1 Fewest zones possible is best. A desire for cooler bedrooms or a room with large solar gains might be reasons for a separate zone.

    2 The outdoor reset programming will dial down the supply water temp as outdoor air temps rise. At some point you may be creating more btu than can be used and shorter cycles (not necessarily short cycling) will result. As long as the run time is 7-8 minutes or longer it should be OK.

    3 Yes you can run only as indirect water heater. From mid May to Oct, it's only function will be to make domestic hot water.

    4 Yes, the aqua stat should be set at 140 to kill Legionella. Use a mixing valve to reduce the temp at the faucets to 120.

    5 I set the supply temp for indirect at 160. Others may set it higher. Quick recovery wasn't a high priority with us and lower supply temps build less minerals in the hx. You should use water treatment like Sentinel in your boiler water to help scrub O2, reduce mineral build up and control ph.

    6 Yes the indirect priority means there is no space heating while making domestic hot water. Our 40 gal indirect recovers in 10-11 minutes.

    7 The boiler will fire and slowly ramp up the fire to the preset supply temp for the indirect. If you monitor, you'll see the supply and return temps rise until the set temp is reached at that point the boiler will reduce fire to maintain that temp until the aqua stat is satisfied.

    7A "Ideal" run time for space heating is continuous running at the lowest fire possible to keep the thermostat happy. One tactic to set the outdoor reset is to set the thermostat to 80 so it can't be easily satisfied and then program the boiler supply water temps to just maintain your desired indoor temperature. It may be something like... supply water is 85 degrees when it's 60 outdoors. And when it's minus 20 outside the supply water will be 140. I actually did this for a while with continuous running as the goal but cold windy days with overcast skies caused the indoor temp to sag a bit too much so I added a couple degrees to the supply water temp to account for those days and lowered the t stat to 71. Our thermostat stays at 71 all thru the heating season. No night time setback.

    8 Again, the boiler will ramp up to the set supply water temp for DHW and maintain that supply temp until the aqua stat is satisfied. I can't speak to the math you provided. Our boiler is set at 70k btu max. It runs briefly at that maximum until 160 is reached and then slowly ramps down to about 25% until the aqua stat is satisfied. 10 minutes is typical. I watched it while a shower was in progress and the indirect makes water faster than the shower can use it. I have a temp gauge just before the mixing valve to verify this.

  • Pulse
    Pulse Member Posts: 78
    edited July 2020

    First off, thanks for the detailed response.

    I think I need to better understand how a boiler works..

    If I want a SWT of 140 degrees or any other temp, how does the boiler decide how many BTUs it is going to output to produce that result?

    Does the boiler decide it is going to stop ramping up at say 25k BTUs and just sit @ 25k achieving its goal, but at a longer duration? ..or does it instead go all the way up to the potential max BTU output, then slowly back down? Based on your response I would assume its the latter, but if you wanted a continuous run that doesnt seem like the best way to go about it.

    ..and does this routine change between heating and DHW?
  • flat_twin
    flat_twin Member Posts: 350
    edited July 2020

    The boiler doesn't decide based on btu. Temperature sensors on the supply and return side of the heat exchanger are what the boiler looks at with regard to how much fire is needed to maintain the programmed SWT. The amount of fire is generally expressed as a percentage rather than btu. Some mod con boilers have variable programming for the ramp up and ramp down. Not sure about the boiler you mentioned. Others have set ramp programming likely based on the relationship of the supply water sensor temperature and the target supply temperature.

    My boiler only runs at high fire during calls for DHW or when it first fires up for the heating season. Even then it isn't for very long. Even before reaching the target temperature the boiler starts ramping down in anticipation of hitting the target.

    Regarding continuous or nearly continuous running. Outdoor temperatures rise and fall throughout the day. An outdoor temp sensor sends that info to the boiler and it adjusts the SWT accordingly. Any ramping up and down is very subtle. Actually when a call for DHW comes, that's when you really see things change. Heating SWT target may be 100 and now the boiler has switched over to DHW mode and is asking for 160 degrees. Heating pump stops, DHW pump starts (or it could be valves opening and closing, depends on your control method) and the boiler ramps up the fire to 100% until it nears 160 SWT. Then it ramps back down maintaining 160 the entire time until the aqua stat on the indirect is happy. Then, space heating resumes with a SWT target of, in this example, 100 degrees and the boiler goes back to monitoring the outdoor temp and adjusting the SWT per outdoor reset programming.

    Here's a chart I made of my own outdoor reset curve. You can see for any outdoor temp within the range, what the SWT of the boiler will be.

  • Pulse
    Pulse Member Posts: 78
    edited July 2020

    Got it!

    Going back to question #1 & #2

    The outdoor reset will lower SWT as the days get hotter, however, my estimated heat loss on those days remains the same. In this case my boiler will be producing excess BTUs. At this point do I have to think about getting some type of buffer?


    I attached the heat loss spread sheet I made. If you do not mind, take a look. I originally planned on 4 zones:
    Zone 1 - Kitchen/Dining/Living room/Foyer
    Zone 2 - 3 Bedrooms (w/TRVs in each room?)
    Zone 3 - 2 Bedrooms/Offices (w/TRVs in each room?)
    Zone 4 - Basement

    Based on my heat loss is there anyway to do zones or is not efficient?
  • flat_twin
    flat_twin Member Posts: 350
    This must be new construction. 30k btu for 7 degree design day in a 2000 sf home sounds like it's extremely tight and well insulated. I used the Slant Fin app to calculate heat loss and that was after the fact just to verify what was going on with my own 185 year old home and 90 year old heating system which is on one zone and works quite well with very little temperature variation throughout the house. One of the pros here would be better able to go into details about combining your zones in a way that would minimize the chance of short cycling during the warmer shoulder seasons and whether a buffer tank might be needed. Panel rads sound like a good way to go for their mass and that certainly helps with smaller zones.
  • hot_rod
    hot_rod Member Posts: 22,100
    That boiler turns down to 10K, it can run a 40° delta down to 2 gpm, it could be ideal with properly sized panel rads running a 30 delta. It probably has other short cycle functions, delay or a ramp function? If cycling proves to be un-acceptable a small buffer could be added.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Pulse
    Pulse Member Posts: 78
    edited July 2020
    It is not new construction, 1979 within an addition in mid-late 80s.

    I need someone to look over my heat loss, but I am fairly confident in it.

    "it can run a 40° delta down to 2 gpm, it could be ideal with properly sized panel rads running a 30 delta"

    Can you explain this?..I dont understand the two different deltas

    EDIT: nevermind, boiler delta is indoor/outdoor, panel is SWT/RWT
  • hot_rod
    hot_rod Member Posts: 22,100
    you could supply the panels with 140, return 30° lower. So the distribution runs a 30 delta.
    Lower SWT requires larger panel size.

    Page 9 shows a derate formula. In this example a 120° SWT was used.https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_6_0.pdf

    Runtal or Myson have a small fan module that goes below panel rads to increase output. With a bit of convection you might not need to upsize to get the BTU output you need.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream