3 questions

DD_5

1. Does a primary/secondary loop design protect against short cycling in most cases? 2. Also, how much can you over size a boiler? 10% of the heat load....30? What's the limit?

3. Does a modulating boiler control short cycling completely if the total load falls within the output range of the boiler?

BobbyG

I'll give it a shot

Q.1 I don't think so.  but of course there are things to consider.  (is it modulating boiler?) it may give you a little more mass, but in the grand scheme it won't add very much time.



Q.2 Again it depends on what type of boiler (modulating vs. on-off)  If it is a modulating boiler and the load is withing the lower firing range, then shouldn't be a problem. On a on-off boiler and it is over-sized for "design heating outdoor temp" then it will short-cycle even worse for most of the winter (design conditions are rare in winter - 80% or more of heating load is approx. only 10% of the time)



Q.3  depends.  if there are any small zones and they are the only ones calling and that load doesn't fall within the firing rate of the boiler, then you could have short-cycling.



Hope that helps.

Jean-David Beyer

Q3 applies to me.

I have a mod|con that can fire between 16,000 BTU/hr and 80,000 BTU/hr (input). My upstairs zone can only consume about 6,500 BTU/hr when it is 0F outsice, but my design day is 14F. It is piped primary-secondary, but that probably does not affect this. The boiler holds 3 quarts of water, and the primary piping is only a few feet of 1 1/4" pipe, so a quart or so more?



I have done several things to mitigate short cycling. I adjusted the reset curve for the upstairs zone so it puts a minimum of 110F water up there, even if I could otherwise use lower temperature water on warm days. This raises the heat loss of the baseboard up there, and causes the thermostat to be satisfied quicker than I would need.



I then adjusted the hysteresis on the supply water temperature for that zone to be -8F top +7F instead of the default -5F to +5F.



I then reduced the maximum firing rate from 94% to 55% for this zone, so it takes longer for the boiler to get from the minimum to the maximum supply temperature.



I wish they made a smaller boiler, or one with a greater modulation range. My 1200 square foot Cape Cod could be satisfied with about 35,000 BTU/hr when it is 0F outside. Design day is 14F, and I have seen it go just under 10F outside last year for a few hours.

BobbyG

I understand

On those jobs with microzones, sometimes people use a buffer tank where the boiler maintains the tank temperature.  If a microzone calls for heat, then it pulls off the tank and the boiler might not even fire.



What brand boiler are you using? just curious.  I'm guessing a Weil-McLain Ultra?

Jean-David Beyer

If you mean me, ...

you guessed right. I have a W-M Ultra 3. I suppose you guessed by all the things you can do from the front panel, and that you can have different reset curves for each priority.



I like its control because each of my zones can have different reset curve. One is in-slab radiant, and one is (oversized) baseboard.  So I do not need any temperature control valves. This would not work for everybody, but because of the low demand in the "microzone" (two rooms), it works for me.



My guess is that a suitable buffer tank would help, but it would be a pain to install. I will know better after my new settings have been through a winter.