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Community and Q&A

Boiler Combustion Air & Indirect HW tank question

humm9er | Posted in Energy Efficiency and Durability on

Hi Everyone,

I live in MA (5A) and have a system 2000 oil boiler, currently pulling combustion air from my basement, which is a combination of poured concrete and fieldstone. Boiler vents through the chimney.

I have airsealed the attic, rim joist, and done other insulation work. Solid but still (always?) room for improvement.

1) I imagine that allowing the combustion air to be pulled from the basement pressurizes the house and exacerbates stack effect? I was thinking of moving my combustion inlet outside the house, but want to be sure that doing so makes sense and that I’m not overlooking any pitfalls. Will moving the combustion air inlet outside the house save energy?

2) I have an indirect 80 gallon bradford white HW tank. My hot water is scalding hot. Adjusting aquastat on tank has no effect. I had an HVAC tech look at it, and he suggested that adding a mixing/tempering valve at the tank would be a good idea. I understand the benefits to kids/seniors in preventing the scalding hot water from getting to the faucets. Will adding a mixing valve to my tank and lowering the outlet temp save my significant energy? I’m a bit perplexed that that’s the “only way” I’m being told to manage this issue.

Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    When combustion appliances use indoor air for combustion air, they don't pressurize the house -- they depressurize the house. Pulling air through the appliance tends to draw outdoor air into your house through cracks in the thermal envelope.

    If your basement is large and your boiler isn't confined to a small mechanical room, and your boiler is running well (without evidence of problems), you don't necessarily need an outdoor air duct to provide combustion air. Modern codes often require a "proximity duct" to convey outdoor air to a point near an atmospherically vented boiler like yours, but these outdoor air supplies were rarely installed in older homes. If you have any doubts, consult a service technician and ask for a combustion safety test -- or consult your local code authority.

    It sounds like the aquastat on your indirect water heater is broken. I advise you to replace it.

  2. Expert Member
    Dana Dorsett | | #2

    Pulling combustion air from the basement DE-pressurizes the house, but not by much. It's much less than say, a bath fan and WAY under that of a clothes dryer. Most ducted combustion air into boilers is 3" or smaller in diameter.

    Ducting in combustion air is fine, and generally a good idea. Be sure to insulate the duct to at least R2 or it'll be dripping with condensation from the room air much of the winter.

    A thermostatic mixing valve or tempering valve on the output of the indirect is more than just a good idea, it's required by code for all water heaters in Massachusetts! That has been the case for quite awhile now (more than 5 years, maybe even 10) , but it's possible the indirect was installed prior to that requirement.

    But figuring out why it's not responding to the aquastat setting would be useful. IIRC the System 2000 always purges heat from the boiler into the indirect after a space heating call for heat is finished- it could just be that the recent space heating loads have kept the temperature inside the tank higher than the aquastat setting.

  3. humm9er | | #3

    Thank you both.

    Martin, I did replace the aquastat with no improvement.

    HVAC company said the System 2000 is so efficient I'm not wasting much oil keeping the HW tank hotter than I'd like. Not sure if that's true...if it is, and if adding a mixing valve will save me a bit of energy loss, I'm game to add the mixing valve. Not too worried about people being scalded.

    Yes, I meant de-pressurize the house. Sorry guys. What I meant was, by de-pressurizing my basement and encouraging makeup air, does that in theory add to pressurization/air loss at the top floor ceiling plane enough to justify moving the combustion inlet outside? Just want to be sure I understand if that's why it's generally a good idea as Dana says.

    Energy Kinetics (System 2000 manufacturer), encourages, but does not require external combustion air. And yes, I would have the boiler combustion tested if I moved the combustion inlet outside.

    Thank you!

  4. Expert Member
    Dana Dorsett | | #4

    A mixing valve on the output of the indirect delivers zero reduction of standby losses, but would reduce some of the distribution losses of potable hot water distribution plumbing.

    If the distribution plumbing is insulated to a code-min R3 even that savings is minimal. Reducing the tank temp from the 160F or whatever to 140F (or less) would deliver bigger savings.

    It's possible/likely that the boiler's operating temperature is set higher than needed to cover the heat load even at recent cool temperatures, and that with the higher duty cycle and potentially higher numbers of burns during these cool outdoor temps the boiler purge of heat into the indirect is keeping it hotter than the aquastat setting. (But you or a halfway competent tech should be able to test that thesis.)

    If your System 2K has the outdoor reset option, setting it up properly would reduce the boiler temp to the minimum needed, and there would be less heat to purge into the zone or indirect at the end of a burn. If it doesn't have the outdoor reset function, run a fuel-use based heat load calculation, and measure up the radiation or baseboards to determine the necessary water temp to cover the load at the 99% outside design temp, and set the boiler's high-limit to 5-10F above that temp. For the load calc, see:

    Measuring the radiation is covered somewhat here:

    There are multiple versions controls for this boiler, so it's hard to make generic recommendations that are true or appropriate for each model or generation. When all else fails, read the manual (and make sure it's the RIGHT manual for your controller.)

    The smaller System 2Ks are set up to use 2" PVC for the combustion air intake port, the bigger ones take 3". There are "equivalent length" limits of 20 equivalent feet for those intake runs. As noted before, insulating the intake duct will be necessary to avoid condensation during cold weather. See the discussion starting on p.7 of this randomly chosen System 2000 manual:

  5. humm9er | | #5

    Thanks Dana! I have the EK2, with 3" combustion port. Just so I'm sure I understand, moving to external combustion air will reduce stack effect? Minimally it sounds like but I want to be sure I understand.

    Agree on the standby temp for the bradford white HW tank. At the end of the day I don't like the idea that the tank can't be set to call at a lower temp. This has been an issue since I moved in -- not related to outdoor temps. Doesn't this sound more like a tank issue than a boiler issue?

    Thank you again.

  6. Expert Member
    Dana Dorsett | | #6

    With sealed combustion (in which even the dilution air for the exhaust is also coming from the combustion air port, not the room, which is the case with the System 2000 boilers) it takes the boiler completely out of the stack effect equation, and those air volumes are not depressurizing the house even slightly. When the combustion & dilution air are taken from conditioned space it depressurizes the house slightly.

    There are DIP switch programmable features in most System 2K controllers that may be defeating your tank aquastat. I'm not going to pore through the manuals to find it, but YOU should. For instance, if happens that it's set up that the call for heat is only started with the aquastat, but the burn continues until it reaches the boiler's high-temp or something you'll have scalding hot water at times no matter how low you set the aquastat on the tank. This IS a soluble problem, and it's a control problem.

  7. humm9er | | #7

    Awesome Dana thank you, I will read that manual cover-to-cover and figure it out, and will move combustion inlet outside.

  8. user-2890856 | | #8

    System 2000 is just that , A SYSTEM . It thermally purges heat left in the boiler as opposed to leaving it in the heat exchanger and being lost through the exhaust . What you need is a DHW mixing valve which will only use a smaller portion of the stored water mixed with cold to acheive your desired set point temp , Maybe 120-130* . You will leave the SYSTEM intact to do what it is designed to do while in a sense incrrease your usable DHW . Alll contactors are not alike in as much as they really mostly have no clue about what they are intsalling and why . I will say that the Marran family takes as much care vetting as they possibly can but at the end of the day many terrible contractors still make it through to be an EK qualified contractor . Care to ask how I know /

    By all means move the intake to outdoors and perform new combustion analysis . System 2000 depends on the sum of it's parts and controls to achieve system efficiencies as opposed to mythical efficiency numbers pthers are based on and rarely achieved .

    I will qualify what Dana stated also , RTFM

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