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Viessman microload strategy

RPK Member Posts: 95
I was just reading through the Vitodens 100 Application Guide and noticed something interesting. On page 38 there is a piping design for extracting heat from an indirect tank to satisfy a microload. The diagram shows a towel warmer as the load. Has anyone ever used this strategy?

On the plus side, you don’t need a buffer because indirect serves double duty.

One downside is that it requires another circulator.

I don’t see why the mixing valve shown in the diagram is necessary (unless lower water temps were required for a radiant floor or something).

Harvey Ramer suggested using a TRV for small radiant zones in a thread about buffer tanks. The TRV is piped such that flow is available to the small zone whenever any other zone is flowing, but the small zone can’t call for the boiler to run. This sounds like a great solution and avoids the extra circulator. Warming the towels might be hit-or-miss if it was used in that application though.

How do you control your hydronic towel warmer?



  • SuperJ
    SuperJ Member Posts: 605
    edited March 2018
    JUST RE_READ the manual.
    I see the novel thing is the load is drawing off the lines to the indirect. Good use of check valve to keep the DHW priority pump off. So the panel rad zone can run until the tank is drawn down to initiate a DHW call. That is clever I guess, it gets around a buffer tank nicely. You are going to sacrifice some boiler efficiency since the DHW is typically maintained with a high boiler temp for fast recovery. (so not much condensing).

    I still really like buffer tanks on low mass boilers for houses that are full of microloads. The Viessman example is good if you just have one or two (like that bathroom example). A low mass condensing boiler is a waste if you running most of your zones off what is effectively 180degF water.

    IGNORE EVERYTHING BELOW (I missed that the tank was an indirect tank):
    I think Viessman is basically pointing out the boiler's job is to maintain the tank temp (not the zone), and stat's job is to run the zone (whether it be a pump or zone valve). That way with a reasonable differential on the tank the load will be big enough to prevent short cycling.

    If your doing standard P/S with a delta P pump, you could just have the secondary pump, pumping out to the zone valves (or have zoning by pumps pulling off the primary as shown).

    The towel warmer is just a microload example Viessman chose, it could've been a small infloor zone, panel rad, fan coil, whatever. Whether it has a pump, or valve the main point is, it is not capable of pulling enough btu's to keep the boiler running.

    The mistake made traditionally they want you to avoid is starting the boiler to satisfy the microload (don't tie that stat directly to the boiler). In this case the could care less what the zone is doing, it's just keeping the tank warm by firing up a once or twice an hour with a nice long run cycle.

    IMHO, the mixing valve is a distraction in the drawing. It is specific to the towel warmer, it's just to keep the surface temps reasonable for something people are likely to touch. If there wasn't a mixing valve, a single secondary pump with a zone valve would be simpler and more efficient electrically.
  • unclejohn
    unclejohn Member Posts: 1,778
    I tried it once with a smart tank from TT and had no luck. The water on the boiler side of the smart 40 was only about 6 gallons I think and even though I had only one bathroom rad connected to it it ran constantly and never really heated a small bath.
  • RPK
    RPK Member Posts: 95
    Unclejohn, what do you think the problem was in your case?