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Cold Tenants

Pilot RE
Pilot RE Member Posts: 11
I manage an apartment that won’t get warm enough when the

temperature drops to about 15F or lower. It’s a single zone forced hot water

system with a gas boiler. I’ve checked the boiler, measured the heating

element, and done a heat loss calculation. Everything seems to be working

correctly, and the boiler and element properly sized for the load. Structure and system information is below.

System: Vaillant GA92-080 boiler (80,000 Input; 68,000 DOE;

59,100 Net) with a Taco 007-F4 circulator and 85’ of 3/4" element in a

single zone. The thermostat is in the first room on the loop and reads about

65F when it’s 15F outside. All of the radiator dampers are open and the element

reasonably clean, unobstructed, and in decent condition. The piping in the

basement has foam insulation. The boiler cycles on/off every 1.5

minutes when the thermostat calls for heat.

Structure: Built 1900, has original plaster walls (un-insulated),

original hardwood floors (un-insulated) over an unheated basement, vinyl

replacement windows, original doors, and a heated apartment above. The building's

footprint is 1436 sq/ft with 162 linear feet of exterior walls. The apartment

is 1231 sq/ft with 9’4” ceilings, about 130 linear feet of exterior walls, 39

sq/ft of exterior doors, and 180 sq/ft of windows. The average basement

temperature is in the low 40’s.

With a design temperature of 0F I calculated a heating load

of 48,000 btu, and at 180F average temperature, the system should put out

51,000 btu. Where is this going wrong? Since I don’t live there, I have to

trust (and generally do) that the tenant is giving me accurate information.

Thanks for any suggestions.


  • Ironman
    Ironman Member Posts: 7,352
    Boiler Cycles Every 1.5 Minutes?

    Then it's short cycling. It should run a min. of 10 minutes.

    Is it cycling on limit: little or no circulation. Check the circulator. Is there air binding? What's the Delta T at the boiler?

    Is it cycling on the thermostat: set the heat anticipator or replace the t'stat and set the cycle rate to 2 per hour.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • heatpro02920
    heatpro02920 Member Posts: 991
    short cycling

    Did you say the boiler cycles on and off ever 1.5 minutes? I would address that first.

    What is the boiler temp set at?

    Is the 85' of baseboard a constant loop starting in the first room ending in the last?

    Im thinking fix the short cycling problem, and make sure there is no air in the loop that may be stopping certain area to get hot water....
  • icesailor
    icesailor Member Posts: 7,265
    85' of active radiation?

    Its hard to DX something as complex as your problem. There are a lot of variables. If you are saying that there is 85' of active baseboard, how long is the peripheral piping, the piping between the heat emitters and the supply and return. From the boiler and back? There used to be listed maximum lengths for copper tube. 85' of active radiation seems like a lot for a 3/4" supply loop. You must check the outlet boiler temperature with a somewhat accurate thermometer and check the return temperature. Just because the boiler thermometer may say that it is leaving at 180 degrees, doesn't mean it is 180 degrees when it gets to the first emitter. Then, there is the loss to the outlet of the last emitter. To run 85' of active element on a single loop is usually considered wrong. It should be split. The supply and return in a unconditioned space that is 40 degrees needs to be covered and insulated. I once used a candy thermometer that I wrapped towels around to hold it against the pipe to get an accurate reading. I have better methods today.

    I check these applications with an infra-red thermometer gun. Shoot the baseboards on the outside tops where the hot water first comes in, and leaves. Then, go to the last unit. Check the difference. You will be surprised. Check above the baseboard heaters and a foot or so above the heater. If there is a radical drop, you are loosing heat into the wall. Shoot the ceilings, walls and floors. Check the differences.

    Is the boiler cycling every 1.5 minutes because the thermostat is cycling, or the differential of the high limit of the boiler is being met and needs to be farther apart. Some high limits don't have adjustable differential controls. Even if you have a 20 degree Delta T, the last baseboard will be using 160 degree water and that emitter must be sized at 160 degree water.

    It sounds more to me like an under radiated zone than anything else. I have always found that if you have a zone that heats to set temperature when it is 20 degrees outside with no wind, but won't go above 60 degrees when it is 28 degrees outside and the wind is blowing 20 MPH and gusting to 30 MPH, you have an infiltration and lack of insulation problem.

    I was once faced with your problem and the owner felt I had installed an undersized boiler. The boiler was cycling with a 20 degree Delta T and you could see the supply and return rise and fall behind the boiler going on and off. There was nothing wrong with the install. The heat loss and infiltration was out of control because of recessed ceiling lights in a ventilated cathedral ceiling, and not all the outside knee walls being insulated and wall boarded with doors installed.

    In my opinion.
  • Steve Whitbeck
    Steve Whitbeck Member Posts: 669

    First thing is to test the thermostat. Remove the thermostat and jumper the wires to see if the boiler runs constant.

    If it does - does the thermostat have an adjustable heat anticapator. If it is a  mechanical model look for an ajustable slide that is numbered up to 1.0 - slide it all of the way toward the 1.0 end.

    If it is an electronic model look in the install manuall to see if you can adjust the cycle rate. Adjust it for hot water heat or for a furnace of 90% efficiency.

    If the boiler doesn't run constant then what temperature is the boiler when it turns off and on?

    It should turn off at 180* and on at 160*.

    If that is happening then you are not pumping the proper amount of water.

    You have too much baseboard on a 3/4 inch loop ( rule of thumb is 75 ft max. )

    If the problem is lack of proper water flow and the pump is bad I would install a pump with higher head than the one you have now. I would put on a Grundfoss UPS 26-99. It has a three speed motor so it can be set to the flow you need.

    This bigger pump should help with the TOO long of a loop problem.

    And the new pump will stop the short cycling problem - IF the original pump is bad.
  • Pilot RE
    Pilot RE Member Posts: 11
    Thanks for the feedback

    Thanks for the feedback. Some clarifications: The burner cycles on/off at 1.5 minute intervals and shuts off on high limit (190F) when it is running full tilt and can’t satisfy the thermostat, while the circulator runs continuously during a call for heat. The burner fires longer on an initial call (i.e. in warmer weather) but I don’t know how long it runs in those conditions. The baseboards are in a constant loop from start to finish.

    Using a Fluke 62 thermometer on the outlet and return elbows in the back of the boiler I got Delta T around 20F with 190F out and 170F returning on average, which seemed to correspond with the boiler’s temperature and pressure gauge. The burner comes on when the return read 165F; occasionally there were cooler pockets of water and the return dropped to 155F and the outlet to 170F or 175F. These readings were taken on a slightly warmer day (mid 20’s) while the burner was cycling in 1.5 minute intervals but eventually satisfied the call for heat. I wasn’t able to get reliable looking readings on the copper and I’m still not sure what’s a good emissivity setting for copper pipes.

    Monday morning I’m going to open the circulator, inspect the cartridge, and purge the system exactly as Taco recommends (the technique I’ve been using is similar). The system does not have an air scoop or bleeder valves installed. The circulator runs quietly, I don’t hear any air in the pipes, and there’s never been an appreciable amount of sediment or rust in the water. I’ll also measure the piping in the basement on Monday. A significant amount of work went into sealing gaps and reducing infiltration in the basement, which has always been cool and drafty. It remains cool, but is much improved and far less drafty. Infiltration in the apartment is on par with other buildings I manage of that age, if not a little better due to the windows.
  • Steve Whitbeck
    Steve Whitbeck Member Posts: 669
    Poor flow

    IR thermometers don't work well on copper pipes.

    Put a piece of black electrical tape on the pipe and try again.

    Also get us some readings off of the pipes at the mid point between two radiators.

    I think that You aren't getting the proper flow.

    The fact that the boiler is getting up to 190* tells me the boiler output is not the problem. If the building heated properly in cold weather last year and not this year - then the problem is flow.
  • Steve Whitbeck
    Steve Whitbeck Member Posts: 669
    edited February 2013
    double post

  • icesailor
    icesailor Member Posts: 7,265
    Lack of insulation:

    Whenever I have seen a loop or a zone with a 20 degree Delta T and the burner was cycling on high limit but the circulator didn't stop, and it was cold outside, there was a heat loss problem.

    If the circulator isn't pumping maximum flow, the return water will be colder.

    I don't know where you are but if it is warmer and not windy or it is cold and not windy, and the heat will go over 70 degrees, and tomorrow, it is windy and the zone won't get to 70 degrees, you have an infiltration problem.

    The guy last week with the garden hose on one side didn't have flow and he has cool water on that side. There is nothing wrong with the circulator. You have to fix the heat loss to the outside. You don't have enough heat for the extreme conditions.

    Take the thermometer gun and shoot the floors, ceilings and outside walls and windows. Do it when it isn't windy and do it when it is windy. You will see the difference.
  • Steve Whitbeck
    Steve Whitbeck Member Posts: 669
    still think it is flow

    The readings he gave may have been when the pump is running. Or there could be no flow when he took the readings. And without black tape on the copper pipes the readings are suspect.

    He may have a pump shutting down on thermal overload.

    If the system worked before and now it doesn't - heat loss isn't the problem unless something was changed in the building.
  • Pilot RE
    Pilot RE Member Posts: 11

    I stopped by the apartment twice Monday to observe and take measurements, once in the morning, and once in the evening (three times actually; I’ll come back to that). The tenant sets the temperature to 64F during the day and to 71F after 4pm. I happened to arrive just before a call for heat in the morning. The pipes were around 145F, the burner came on, ran for 12-15 minutes, and I observed a very even temperature gradient around the loop and fairly consistent delta T  at the boiler of 25F. The return was right around 165F. After the call for heat was satisfied, I cooled the boiler, drained it, inspected the circulator (no signs of damage or wear on the impeller, spun freely, etc.), refilled and purged the system, had a snack, and then took some more readings while the boiler satisfied another call for heat with exactly similar performance.

    The boiler was running when I arrived in the evening (after 6), and continued to run, with the burner cycling on/off every 1.5 minutes, for about 30 minutes before satisfying the call for heat. I got a confusing set of temperature readings and fluctuations. There were spikes up to 205F+on the outlet, readings on the return of 188F while the outlet was 180F, as well as 175F on the return and 195F on the outlet, occasional drops into the mid 160’s, and so on. I’d say the delta T was around 20F, but it was all over the map. I don’t know what to make of that. I took readings at several points around the loop to make sure I was getting a good overall picture each time.

    The weather in this part of New England was 28F with winds 20 g25 in the morning, and 28F with winds
  • Pilot RE
    Pilot RE Member Posts: 11
    (2nd half of update)

    The weather in this part of New England was 28F with winds 20 gusting 25 in the morning, and 28F with winds 8 in the evening.  There’s about 120’ of pipe in the basement connected to 85’ of element. The previous tenant was a very long term resident who never mentioned problems with the heat to me. I would like to believe that’s because everything was working properly, but it’s entirely possible the system has never worked correctly.

    Trip #3: The tenant called later that night with no heat. The pilot and circulator were on, the burners off.  I connected a jumper wire to the thermostat terminals and the boiler fired, (emerg. switch off), then I tightened the wires on the thermostat, removed the jumper, (emerg. switch on), and the burners came on. As far as I know it made it through the night. Maybe the thermopile or aquastat is going bad? I don’t see how jumping the thermostat or tightening the tstat wires should make a difference if the circulator was running. I’m also not sure if this is related or coincidental to the other problems (or why this half of the update went missing).
  • Pilot RE
    Pilot RE Member Posts: 11
    New information

    I may have discovered the root of this problem. I was reading Dan’s book “Classic Hydronics” (p. 60) regarding a circulator pumping toward the point of no pressure change, and realized that that is how this boiler set up. Would re-piping the fill valve and expansion tank into the return (and adding an air scoop) seem like a logical thing to try first? 
  • heatpro02920
    heatpro02920 Member Posts: 991
    Delta T circualtor

    I would throw in a $150 ciculator, do the headloss and pick a 00? DT model. that is capable to give you the gpm you need for your load...

    and when you are installing it flush the system out and install the sensors....

    Its a solid loop 120ft of tubing {3/4"?} then 85ft of board then back to the boiler there is over 200' of pipe, is this 1 floor?

    What is the psi in this boiler?
  • Pilot RE
    Pilot RE Member Posts: 11

    Correct 120’ of 3/4" tubing plus the 85’ of radiator. The apartment is on the first floor and several of the radiators are not on adjacent walls, so the pipe drops down into the basement from one radiator, then over and up to the next, and so on. The boiler itself is located closer to the center of the building and there’s a good 25’ of tubing just to get the outlet and return to the first and last radiators respectively. The boiler pressure is 15PSI. I’m certainly considering a different circulator.
This discussion has been closed.