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Boiler water temp not matching aquastat setting

Homegk | Posted in Mechanicals on

I have a hydronic heating system- mostly cast iron radiators and a couple cast iron baseboards and 3 copper fin baseboards. Aquastat is set to 190*. Water temps only get to 150* before the boiler shuts off. Radiators throw heat, but the baseboards need hotter water to be effective IMO. In any event, I’m not sure where to look first. Can this possibly an issue with my programmable thermostat? The room reaches the temperature it need to, to satisfy the thermostat. Thanks

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  1. user-2890856 | | #1

    How many zones , how is this system piped , are you comfortable , thermostat location ? What type boiler are you using , is it mod con technology or cast iron boiler ? Lots of questions to give a qualified answer .

  2. Expert Member
    Dana Dorsett | | #2

    The fact that it's meeting the thermostat demand means you don't need more than 150F water to meet the loads you're seeing.

    If you have a lot of radiation and a smaller boiler you could be seeing a balance point of boiler output BTU-rate to radiation output BTU-rate at temps below 190F, which is USUALLY the case for single-zoned systems.

    Most baseboard (fin-tube or cast-iron) will deliver 350-400 BTU/hr per running foot with 150F entering water temps. (see: )

    You'd have to measure up the radiators (see: ) , but they'll put out about 90-100 BTU/hr per "equivalent direct radiation" (EDR) square feet with 150F water.

    With a programmable thermostat try setting a deep setback temp or bumping the temp up by at least 5F using an over-ride function to give a long recovery ramp, then observe the boiler operation. If it's cycling on/off during the recovery ramp before the thermostat is satisified there could be something set up incorrectly on the boiler's controls. Some thermostats over-anticipate and may cut out early then re-start the boiler but that's a less likely scenario.

    To help explain what's going on we'd need to know the boiler's make, model, and input & output BTU numbers, the total type/size of the radiation (if zoned, break it down per-zone), and the make model of the thermostat(s).

    Without this information it's sort of like trying to figure out why your car only reaches 35mph despite the fact that you still get where you're going on time.

    With the boiler OUTPUT number, estimate the temperature at which the baseboard + radiator EDR is equal to the boiler output. At normal room temperature ranges your boiler temp would not be able to exceed that temperature, no matter how long it is firing, since the radiation is dumping heat into the house as fast as the boiler is putting it in.

    Fin-tube baseboard heat transfer is almost 100% via convection, and it has very little thermal mass, which makes it less comfortable than radiators or cast iron baseboard. But you can still heat your house with it at 100F water temps if you have enough of it. Even at the same heat rate cast iron baseboard is a much more comfortable proposition- it has an order of magnitude more thermal mass than fin-tube, it is still emitting SOME heat even an hour after the thermostat was satisfied, with a good fraction of the heat being direct radiation.

  3. Homegk | | #3

    It's a Gas Weil McLain boiler, model P-VHF- 5. 133k BTU/HR Input; 117k Output.
    Thermostat is a Honeywell programmable- I can't find a model # on it for the life of me.
    Only 1 zone and not sure how to answer about the size of the system but I have 6 cast iron radiators, 2 copper find baseboards on the main floor, 3 cast iron radiators and 1 copper fin on 2nd floor, and 2 right foot cast iron baseboards in the basement.

  4. user-2890856 | | #4

    Well, you state the home is satisfying , so , are you comfortable ? Your bigger problem , right now is more likely to be low return water temps which will destroy your boiler . Non condensing boilers should not condense . If you are not condensing than you are moving too much water too fast . I suggest boiler protection through use of a 4 way valve near the boiler . Can you post a picture or 2 of the near boiler piping and how the emitters are piped off of the loop ? This would be helpful . Your boiler is most likely to be shutting down not on limit but by the switch opening because the house is satisfied . This is not a problem but offers an opportunity to make your certainly oversized boiler and radiation work better for you while using less gas . Don't be concerned that the baseboard or radiators don't feel hot enough for you , worry about if they are heating the space and keeping you comfortable

  5. gusfhb | | #5

    odds are the heavy radiators satisfy the thermostat before the boiler reaches temperature. It is likely it has always been thus, and is likely that way in many houses with heavy radiators. i would leave it as it is.

  6. wjrobinson | | #6

    Outside setback if installed could be why but lower temps add efficiency. Or you need a new aquastat or temperature gauge. Call your HVAC people as there are quite a few possibilities and a site visit is what I would have to do.

  7. Homegk | | #7

    Very cold day today. When I got home and checked the boiler, it was reading 180*. I also found about a gallon of water in the bucket I keep below the drain pipe. I added some water to the system yesterday with the fast fill valve to raise the pressure a bit bc it was running at about 10psi. Now it's running at 24/25psi while the boiler and pump are on. The drain pipe is dripping slowly into the bucket. Should I be concerned? The rooms with the baseboards are up about 4* despite being so cold today.
    Here's requested pic of the boiler. Thanks

  8. Homegk | | #8

    Front of boiler:

  9. gusfhb | | #9

    I do not understand what is going on with your boiler, lot of RTV around, drain from the stack........

    stop messing with the pressure, you are going to crack your boiler.

    go to

    pressure is related to how high you have to pump the water. if it is a single story, I do not think you need 25 PSI

  10. Expert Member
    Dana Dorsett | | #10

    The number of radiators or baseboard without their sizing isn't enough information. You have to actually measure the individual radiators using the guidelines in that link to come up with the EDR square feet to run any analysis.

    If it's fin-tube baseboard (or 9-10" tall cast iron baseboard) we can estimate on just the running length.

    If the system is running 25psi @ 180F and the high-limit is 190F it's likely that you'll trip the 30psi overpressure limit on the pressure relief valve. It could be that the expansion tank was not correctly charged, not correctly sized for the system volume, or failing (less likely, but possible.)

    The drip into the bucket means the valve seat on the overpressure valve may have grit or corrosion. With gloved hands and protective mask/clothing you might try opening and closing the PRV with the lever a few times to see if you can get it to seat better.

    Bleeding water from the system to get it down to 20psi when operating at temp may work just fine. When the system is cool it may drop to 12psi, which is also fine. 10psi is a bit on the low side, but still enough for most 2 story houses.

    If the boiler is running 150F most of the time, the return water will be above 130F which is fine for a gas-burner. It has to drop to 125F or lower to do much condensing on the heat exchanger plates. It's customary to plumb in a "boiler bypass" or "system bypass" branch on high-mass radiation systems to protect the boiler from protracted operation in the condensing temp range, but it's no clear from the picture if it's configured that way.

  11. user-2890856 | | #11

    Rick ,

    Is your expansion tank between the boiler and that oversized pump ? Looks that way from the pictures but i cannot tell for sure . If so it needs to be moved , that or the pump . You should always pump away from the expansion tank . Possibly that tank has been compromised due to that type of issue and will also need replacing . If the tank cannot handle the volume of the hotter water it will relieve through T&P valve just like it is doing . When tank is replaced if in fact that is the problem tell your plumber to raise the charge from the default 12 PSI to the cold fill pressure before filling the system back up with fluid . That circulator would do much better on the supply side of the boiler also and the expansion tank can stay where it is . Don't forget air elimination prior to the circulator , we don't want cavitation .
    Dana , the condensing temperature of NG is 131* F .A non condensing boiler is just that , non condensing and if you allow any condensing you will , not maybe , rot the heat exchanger . Up until the point where we started advertising outdoor reset we did not see the issue quite as much as now , lately it is a problem because even 90% of plumbers have no idea what that temp is . i see boilers all the time with the limit set to 140* , luckily most of these men don't know how to size a circulator either and are returning 135* water except from a cold start . then there's that little problem of delivering hardly any energy to the space with a 5* Delta . This is the reason why there are 4 way mixing valves which insure boiler protection .

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