Newbie Question: Ideal setup for Prestige Solo 110

Yes, have it serviced. Get them to take flue gas samplings with a combustion analyzer. Measure the flue temperature, and check water supply temperature at various points in the modulation cycle of the boiler and post them here.

As per above, you will only get the maximum efficiency out of your boiler if it's properly tuned (i.e. a tested with a combustion analyzer by a qualified professional) and if your outdoor reset (ODR) curve is dialed in correctly. So first, you want to make sure you have an outdoor temperature sensor. Then you program the boiler controls. Cast iron radiators can be awesome when paired with a modulating/condensing boiler.

The simple way to describe it is that you tell the boiler what water temperature it needs to produce in order to send just enough heat to your radiators to keep the living space as warm as you like it at any given outside temperature. For example, in my situation I can keep my house at 68F when it's 4F outside by telling the boiler it needs to send out supply water at 120F. That means that 120F water is going out to the radiators, and because the radiators are releasing heat into the living space the water returning to the boiler is at something below 120F (this is generally known as the Delta T).

The boiler supply temperature goes up and down depending on how cold it is outside. The concept is based on the premise that there's a fairly linear relationship between how cold it is outside and how much heat your house loses per hour.

The modulating part of a "modcon" boiler is that it will adjust the flame up and down depending on how hard it has to work to meet the supply temperature the curve is calling for. Modcon boilers achieve their maximum efficiency when the return water is as low as possible and when the boiler is operating in the lowest third or so of its maximum capability.

Once you have your combustion tuned properly, the temperature of the water returning to the boiler is the key to maximizing efficiency. It's worth experimenting with by making various adjustments to the ODR and see how it performs. This is definitely something you can do yourself.

One of the other contributors here suggests turning your thermostat up to 80F and then adjusting the ODR curve up or down until you get just the right water temps to keep the house at your desired indoor temperature. Then you set the t-stat at or just above your desired temp and it becomes a high-limit switch, and the ODR curve controls the heat.

The most important thing about modcons is that they won't ever operate in condensing mode (above 90% efficiency) unless your return water is around 130F or lower -- the lower the better. So if it's sending out water at 180F (which might be the default factory setting) you'll never see return water as low as 130F and your boiler operates around 85% efficiency -- no better than a 'regular' boiler.

It's great you're asking these questions and that you have a plumber who at least understands that modcon boilers can and should be tweaked to maximize efficiency based on your particular circumstances. The best thing is to educate yourself or have someone show you how to adjust the key settings because it's an iterative process and you probably don't want to pay for a bunch of service calls.

Of course, the boiler may or may not be the problem. Your heating bill is driven by a number of factors besides the efficiency of the appliance.