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Thread: Boiler bypass using mixing valve??

  1. #1

    Default Boiler bypass using mixing valve??

    Hi, This spring I plan to tackle a redo of my boiler plumbing, adding some radiant heat in addition to the existing baseboard. The current boiler is a Laars MinithermII -160KBtu model. I was planning on keeping this boiler and replacing a few worn out parts. After doing some study, I know now that the current installation (done before I owned the house) is a total mess - there are 5 zones, one 007 pump on a single loop, no bypass. I replaced the heat exchanger a few years back - no surprise that it failed.

    In the redo, I plan to put in a dedicated boiler loop and then run 3 separate loops off of that for hot water, baseboard and radiant.

    My question has to do with the bypass recommended for low temp returns - which I will now have with the radiant loop. The Laars docs recommend one of several bypass schemes to route hot water back to the intake for low temp returns. I am thinking that doing the bypass with manually adjustable valves may work OK for the radiant, but then be inefficient for the other loops, since they don't need hot bypass. There is one scheme that uses a mixing valve - see picture attached. Anyone have insight or experience with this way of doing a bypass for low temp return? I know the mixing valve will be pricey since 1.25, but seems like a good way to do this?? Thanks, Dave from Santa Fe, NM
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  2. #2
    Like an engineer alternety's Avatar
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    A few points. It is generally recommended that the pump be placed to pump away from the boiler.

    You may want to examine the cost/benefit of replacing the boiler. A modern condensing boiler (gas does this, not sure how clever oil units are) will modulate the water temperature based on outside temperature and required use (floor radiant, water/air exchanger, DHW). No tempering valves needed. ADDED - this implies a very smart control system. It is probably required in a moderate system to provide water at a temperature for the highest temperature heating load (with reasonable smarts, the system will deal with DHW as a separate temperature) and use a mixer to reduce temperatures for lower temperature requirements. water to air is the highest temperature, baseboard next coolest (maybe the same, try it) and in-floor is the coolest. To do things properly, you need the design heat load for each of the heating loops and the water temp necessary to deliver the required BTUs.

    When heating DHW, the other uses are not activated. IF you are concurrently using baseboard and in-floor radiant the water temperatures will generally be different for effective use. Try with one temperature and see how it feels. If it is not good, then a mixing valve can be used to use higher temp water for the baseboard and lower for the floor. The DHW should get only the hottest water. The DHW tank should be kept around 140 degrees to kill things like Liegonella. Either the output of the DHW tank, or each individual hot water faucet, must have anti-scald mixers. Mixing in the faucets keeps the pipes cleaned of bacteria. The suggestions to run a cool DHW tank (e.g. < 140 degrees) to save energy is dangerous. The water becomes an excellent breeding ground for bad bacterial things.


    This approach works best with the coldest possible water returning to the boiler. They are designed to do this. You can get 98-99% fuel efficiency this way. The primary/secondary loop approach you have shown is a good one.

    If the heating loops are individually operated rather than than all on or all off, a pressure bypass on the heating loops would be useful to maintain flow control. If there is a big mismatch between pump capacity and required pressure/flow the pressure bypass would make your pump happier.

    If you are switching individual loops, you may also need to consider a buffer tank to avoid short cycling on small loops.

    The best way to deal with pressure in a loop is to use a loop pump that maintains a constant pressure. I tried that, but it was before they were selling the required combined control/pumps and my attempt to do it with a pressure sensor and a variable speed pump failed because of excessive noise in the pump. Should not have been an issue, manufacturer swore it was not an issue, but in my opinion their variable speed pump made more noise than was acceptable in a residential environment.
    Last edited by alternety; 03-04-2009 at 10:47 AM. Reason: More clarity on water temps

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