Maximum boiler efficiency?

Redhawk24

We have an NTI high efficiency boiler we use for radiant floor heat. Is it more efficient/better to set the boiler at 120 degrees to warm the house for a longer period of time OR is it better to set it at 150 degrees for shorter period of time?

NTI TRX 150C
1100 sq. ft. w/ 18' ceiling
3 zones
Tile floors
6" cement slab

Jamie Hall

Way more efficient to run it for as long as possible -- all the time, if it will modulate that low -- and use the outdoor reset to control the boiler power and thus circulating temperature.

You should be able to be running in the 94 to 96% bracket with that boiler and radiant floor if you do that.

Use NO setback. With that much concrete you'd just be chasing your tail and burning fuel (might drop as low as 85% efficiency)

Redhawk24

Jamie Hall said:

Way more efficient to run it for as long as possible -- all the time, if it will modulate that low -- and use the outdoor reset to control the boiler power and thus circulating temperature.

You should be able to be running in the 94 to 96% bracket with that boiler and radiant floor if you do that.

Use NO setback. With that much concrete you'd just be chasing your tail and burning fuel (might drop as low as 85% efficiency)

Thanks for the advice! I'm still confused/unsure about setback and outdoor reset in terms of efficiency. Seems counter intuitive but I trust folks like yourself for their expertise. Thanks again.

Jamie Hall

The idea of outdoor reset -- which may take some time to get dialed in properly -- is to run the boiler just hard enough to supply water to your floor which is just warm enough to keep up with the heat loss of the house. What it does is increase the water temperature setpoint of the boiler as it gets colder outside. In a well insulated house with a reasonable amount of sun load it is possible to get it really really close -- so the boiler is always running as cool as it can (which increases efficiency) and also all the time -- which also increases efficiency. Often there is no need for an indoor thermostat at all.

The problem with setback is very simple, really. With a concrete floor such as you have, it takes a long time -- house or even a day or two -- for the floor to catch up to a change in the set air temperature on a thermometer Further, with a properly set outdoor reset in the system the boiler control will sense that the floor isn't heating up, as fast as it needs to to meet the raised thermostat setting and will boost the water temperature and firing rate to try and meet what the thermostat says. That is wasteful. Particularly since, if it is a daily setback, about the time the floor and the space get to the higher setting the setback kicks in.

Smart thermostats are even worse. Unless you strangle them they will continually try to adjust the space temperature for occupancy, and the poor boiler will just give up and run flat out.

psb75

You also want to lower the temperature of the supply water to the floor so that the return water to the boiler is below 135F so that you have the boiler condensing in order to achieve 95% efficiency. Longer run times, lower temperatures. The floor only needs to be a few degrees warmer than the desired ambient room temperature--if your system is set up and adjusted right.

hot_rod

Efficient and better? Hmmm. The most efficient in terms of energy consumption would be the lowest SWT and RWT. Plenty of condensing graphs out their showing that relationship.

However better or comfortable may not match up for you. In some cases the ODR can delay recovery in a shoulder season or rapid weather change. As @Jamie Hall mentioned, it takes time, maybe a heating season to get ODR dialed in to your liking.