Do condesning boilers reach max efficiency only in winter?

silky said:

Hey all,

I had a condensing boiler installed last year and everything seems fine so far. I understand that condensing boilers reach maximum efficiency when they run for prolonged periods....better to heat water at a low temperature over a longer period. In this light, are condensing boilers more efficient as the temperature drops since they should be on for longer periods?

My boiler is controlled by an outdoor thermostat and the pumps are controlled seperately through the indoor thermostat. That is, I set the indoor thermostat to 73 and until the temperature reaches 73 the pumps won't shut off but the boiler may be on or off. Is this a good way of opearting the system?


thanks

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Efficiency due to long run times on heating equipment is because of less start-up sequences mainly. With mod-con boilers this isn't as much an issue as it is with say warm air furnaces. The circulator starts immediately and even the very first bit of energy that the burner produces is being transferred to the liquid (almost).

I did read your other posts about this system. I'm not quite sure I understand what you're meaning by the boiler is controlled by the outdoor thermostat. If you mean the boiler output temp is controlled by the outdoor temp, then ok, this is the outdoor reset function. As far as the boiler cycling on and off when the thermostat is still calling for heat.... it could mean that the input of the boiler can not modulate down far enough to match the heat loss of the zone(s) being heated. In this case the boiler will cycle on and off until the zone thermostat is satisfied.

As far as if this is a good way of operating the system.... It's the way it's operating and if everything is set up right it may just come down to the minimum firing rate of the boiler is still too much input for what your smallest load can use.

I'll describe the system a bit.

The boiler is a slant-fin NG B200. There are 2 zones. One zone does just the basement, the other does the rest of the house. Each zone has its own thermostat.

The boiler has an outdoor reset so the water temperature is relative to the outside air temperature. The thermostats control the pumps to their respective zones but do nothing to the boiler. So, I have both zones set to 73*. Until the thermostats read 73 the pumps will continue to pump. When the temperature reaches 73 the pumps stop but the boiler may still run depending on the outdoor air temperature.

In short, boiler temperature controlled by outdoor thermostat, pumps controlled by indoor thermostat.

My original intention was to run the system 100% from the outdoor reset to maximize efficiency. The problem is that the installers configured it for the indoor thermostats, not the outdoor reset. I tried to use outdoor reset alone and leave out the indoor thermostats but when I did that the boiler would run but no hot water would pump.

I imagine to run it purely as outdoor reset I would have to hard-wire the pumps to the boiler so when the boiler runs the pumps run. I wasn't prepared to mess with the boiler wiring so I decided to just set the indoor temperature with the thermostats and the boiler water temperature with the outdoor reset.

I think the cycling of the boiler was an old problem. That was part of the reason why I switched to the outdoor reset. I also had some plumbing problems I straightened out: I think the manifold couldn't flow enough to feed these two 4 inch trunk lines that ran to the far end of the house. Because of this I believe water was not flowing there but was short-looping back to the boiler. Essentially the newly heated water was skipping radiators and flowing directly back into the boiler. I fixed this by adding a T-fitting before the manifold and feeding these trunk lines directly. After I did that the radiators on the trunk lines functioned perfectly and the boiler would no longer peg at 180* all the time.

Now the boiler seems to run almost continuously (which I believe is a good thing). With the cold weather upon us it is rare to not see the boiler's thin white exhaust.