My house has three hydronic heatin loops; two are shaped like a ladder. The baseboard radiators are plumbed with type M copper pipe.

I'm thinking about installing thermostatic radiator valves on the 'rungs' of the ladder; one loop will be left open.

  1. Will this cause excess pressure in the system, causing pipes to leak?
  2. Will this cause the pump to fail earlier?
  3. Any other concerns with doing this?

I'm curious as to why this isn't done more often, as the valves seem much cheaper than zone valves.

  1. Excess pressure?

    No. Residential circulating pumps can only produce limited head pressure.

  2. Early pump failure?

    No. Life expectancy of residential circulating pumps is not usually affected by changes in flow.

  3. Other concerns?

    Maybe, depending on your exact layout. Response time of wax motor thermostatic radiator valves is slow (20+ minutes) and restricted flow through partly open valves may distribute heat unpredictably where there are multiple radiators in series in a single branch (or ladder rung). From the Taco "Hydronic Zoning Seminar - Residential and Light Commercial Applications":

Series Loop Zoning

While thermostatic valves can be used in a multi-zone series loop system, electric zone valves or circulators are a better choice for series loop zoning. The reason: the modulating characteristic of thermostatic valve operation can result in unbalanced heating within each zone.

If thermostatic valves must be used for series loop system zoning, it may be necessary to balance radiation by adjusting baseboard dampers, or by removing radiation fins in a room that is overheating. An outdoor reset control may also be required.

Also, depending on the type of boiler you have, it may have a minimum flow and heat delivery requirement. Too few thermostatic valves open within a zone may cause the boiler to not work well.

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  • How do I tell if my loop is actually a 'ladder' (H shape), or if it snakes through each of the bedrooms (each on a separate floor, above each other) in a snaked 'S' configuration with a return buried in the wall? If all bedrooms are on one loop, I'd have to install a bypass pipe. – gbronner Oct 14 '15 at 18:13

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