Replacing indirect water heater with gas fired heater?

Discussion in 'Water Heater Forum, Tanks' started by MHM, Aug 28, 2011.

  1. MHM

    MHM New Member

    Messages:
    1
    Location:
    Upstate NY
    I've got a 9 year old boiler (Crown) and indirect water heater (40 gal Bradford White) system. The domestic water heater is leaking into my boiler loop causing my boiler pressure relief valve to leak. I've read in these forums the efficiency advantages of the indirect water heaters. However my concern, is are they really over-all more efficient? My boiler / water heater is in a small room ( 4.5' x 10") with an outside air intake directed to the boiler. The room is warm to hot all year. Especially in the summer. So it seems to me that while the indirect water heater is quite efficient and has very low heat loss, I'm still heating up this large boiler all year and have a lot of heat loss from the boiler and the pipes in the summer. It's a 4 zone system, including the water heater, so it's quite a mass of pipes in the room giving off heat even though I've got them insulated. Would I be better off staying with this system or should I consider switching to a separate natural gas fired water heater. I understand that the separate heater would be less efficient then my boiler but it sure seems like I'm heating up lot of water and loosing efficiency the extra pipes in running the indirect water heater.

    If I stay with the indirect water heater, is there any recommended boiler and water heater temp settings to maximize the efficiency? I'm assuming that the boiler temp setting should be some minimum temp above the indirect heater setting.

    Thanks,
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,183
    Location:
    New England
    There's a lot of different boiler designs...it sounds like yours has fairly high mass and isn't designed for cold starts (i.e., it maintains a set heat band whenever on). The newest, low-mass boilers are generally designed to shut down when there's no call for heat. Mine, for example, in the summer often only runs once a day after the morning showers, then cools off and often reaches ambient temps before it needs to turn on again.

    There's a tradeoff between response, longevity, and reliability for the settings. Many older boilers cannot handle low, condensing, return water temps. So, to keep them from self-destructing, you have to maintain some minimum temperature, often in the 140-degree range or so. You could reduce the low-temp setting during the non-heating season, and may also be able to reduce the upper bound. Most gas-fired WH are not all that efficient - your boiler is generally better. You are taking a hit if it is trying to maintain heating season temperatures during the summer, though.

    To maintain the same hot water supply, you'd need a bigger tank if you go direct fired...indirects take advantage of the entire boiler's heat capacity and can recover much quicker so they generally are smaller. You may find that alone a consideration for your small space. Also, you have to consider the additional flue - you may not be able to connect the new WH's flue into the boiler's. Indirects come in at least a couple of different designs: jacketed and coils. Either can develop leaks. There are some, though, that have very long warranties. Mine is a SuperStor Ultra, mostly SS, and I expect it to last a very long time.

    One of the pros will be familiar with that model and be best able to offer some practical suggestions, but my gut feeling is that another indirect may be the better long-term choice, especially if you eventually consider a new, more efficient, low-mass boiler in the future as well.
  3. Dana

    Dana In the trades

    Messages:
    2,933
    Location:
    01609
    Any post Y2K gas fired boiler is almost certainly cold-start tolerant, and doesn't need to be maintained at any temperature.

    As long as the boiler controls aren't configured to inhibit the pump that drives the indirect loop until the boiler is up to temp, setting the indirect to call for heat when it drops to 120-125F (or higher) ensures that even on cold start the boiler is pre-heated by the thermal mass of the indirect, and spends very little time burning in condensing mode. As long as the loop water coming back from the indirect, entering the boiler isn't below 125F for an extended period very little bad can happen to the boiler, but it needs to be above 130F for most of the burn to protect a terra cotta lined flue from some amount of condensation. (A standalone HW heater in the same flue would have the same issue, only worse... google "orphaned water heater". )

    The Bradford White indirects come pre-set to 120F, with a 5F differential (see p. 27) which means it kicks on a call for heat when the temp in the tank drops to 115F, and turns off when it reaches 120F. But the differential is (apparently) adjustable, as well as the max temp. Set the max temp on the indirect to ~140F and the differential to 15F-20F (they don't really explain how to do that in the manual, but it may be in the Honeywell documentation for the controls.)

    The bigger the differential, the longer and more efficient the burn on the boiler, but the higher the storage temp on the tank, the higher the standby loss from the tank. But the standby losses of a 140F indirect will be far below that of a boiler maintained at 140F, since it's much-better insulated.

    Even with a fairly massive boiler, the summertime water heating efficiency of the indirect won't be much worse than an atmospheric drafted HW heater. During the heating season the higher duty cycle the boiler has due to the additional load of the indirect only increases the net efficiency of the system as a whole.

    With 4 zones and an indirect it's probably worth retrofitting an Intellicon 3250 HW+ to control the thing, unless you have an even smarter zone-controller already in place. (An Intellicon retrofit can be a DIY project for those with electrician-skills.) It's a ~$200 piece of hardware from internet sources, but typically $500 or more to have installed by a pro. With the indirect set to 140F, the boiler's output will typically be ~ 160F at the end of a burn using dumb bang-bang aquastats, but the Intellicon anticipates the end of the call for heat (based on the temperature changes of the return water "learned" from prior burns), and will kick the burner off early, which allows heat to be purged from the boiler into the tank, parking it at a lower temp than it would have had earlier. This limits the amount of heat abandoned in the boiler on each call for heat, thereby reducing the boiler's standby losses, even during the heating season.

    This type of control increases efficiency during the heating season by inhibiting the burner until the return water hits the programmed low-limit on a new call for heat, purging any remaining heat stored in the boiler into the system before firing up. This can dramatically improve the part-load efficiency of multi-zoned systems, since no one single zone is much of a load for the boiler, resulting in short burn cycles and high standby temps unless controlled more intelligently. The more oversized the boiler to the actual heating loads the bigger difference this method of control will make. But even if the boiler were perfectly sized for the whole house design condition, the average load of one zone when it's 40F out instead of -3F is likely to be less than 10% of the boiler's output- it's by-definition well-oversized for the individual zones.
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