Understanding heat flows in high mass heating systems in old houses

dcaheat

Any idea why Honeywell warns against setting the anticipator lower than 0.3?

mattmia2

I suspect it could burn out the resistor depending on the current draw of the control it is connected to.

jesmed1

@dcaheat, it sounds like your fundamental problem is that you have a combination of oversized boiler and probably a lot more radiation/thermal mass than the house really needs. By the time the thermostat senses sufficient temperature increase to satisfy, it's too late, and the residual heat causes a large overshoot.

So IMO, yes, a smaller boiler would help solve the problem. A smaller boiler would take longer to raise the water temperature, and the thermostat would satisfy at a lower water temperature with less residual heat to cause overshoot. But before you change to a smaller boiler, there are still some things you can try with the thermostat.

One thermostat setting that will help you fix the problem is the "swing" setting. This is the setting that lets you tell the thermostat how far to let the temperature drop below the setpoint before calling for heat.

A higher swing setting means your boiler will have to run longer to heat the house back up to the setpoint, and longer boiler cycles are contributing to your overshoot. So you can compensate by setting the swing lower. OTOH, you don't want the swing set so low that the boiler short-cycles. You need to find a happy medium in which the boiler runs long enough to be efficient but not so long that it overshoots, and short enough that it reduces overshoot, but not so short that it short cycles and runs inefficiently.

If your new thermostat doesn't allow you to adjust the swing setting, I suggest getting one that does. It can make a big difference in how long the boiler will run each cycle, and thus help reduce overshoot. When I moved into our 4-unit condo building, I found our oil boilers were short-cycling and running inefficiently. I found that one of the reasons was the Honeywell thermostats that had a fixed, non-adjustable swing. I changed them out to Lux TX100 thermostats that have adjustable swing settings, and with some experimenting found that a swing of 0.5-0.75 degrees was optimal for our setup. With that swing setting, our boilers would run for about 45 minutes per cycle every 3-6 hours, with no overshoot. The Lux TX100's have been simple, accurate, and work very well for us.

You have a different boiler and a different amount of thermal mass, so your optimum swing setting will vary, but the priniciple is the same. You say your boiler now runs 30 minutes per cycle. A lower swing setting would make it run, say 15-20 minutes per cycle.

But before you get yet another new thermostat, one other setting you can try changing: the "temperature display offset." That's Installer Setup Option (ISU) #1420 on the Honeywell T1 Pro. By changing that offset, you are adding or subtracting the number of degrees by which you want to "bias" the sensor in the thermostat. This allows you to make the thermostat stop calling for heat, say, 2 degrees before the setpoint is reached, so that if you typically get 2 degrees of overshoot, your 2 degree offset will compensate for the 2 degree overshoot, and you'll end up at the desired setpoint temperature.

There's another thread running here discussing a similar overshoot problem in a steam heating system with cast iron radiators, in which the owner has found that changing the temperature display offset on his Honeywell thermostats have helped correct the overshoot. You can read about it here:

https://forum.heatinghelp.com/discussion/194473/honeywell-focuspro-th5320-thermostat-with-eim#latest

Jamie Hall

Not all thermostats have a swing setting, although there are a few brands which do. On thermostats with a real anticipator, lowering that may help as it will heat the thermostat faster and shut it off sooner. On thermostats with a cycle per hour setting -- very common now on digitals -- try various settings for that. You may find that more cycles per hour will actually work better.

hot_rod

Another option that solve a number of issues is a 4 way mixing valve.
Run it with outdoor reset to prevent over-shooting the supply temperature, and having excessive temperature in the radiators flywheeling away. Comfort increases when you modulate SWT and lessen on off cycling.

The second feature is it provides boiler return protection. A 4 way valve has two mixing points, so it can modulate supply temperature and protect the boiler.

It needs to be a low pressure drop boiler, most cast boilers are, piping needs to be sized generously. Keep the valve close to the boiler to keep piping resistance low.

If you do that, usually a single circulator will properly run the entire system. With cast radiators and large diameter piping, it is an ideal candidate for 4 way mixing and control.