Pay attention to the comment re: high efficiency gas boiler (condensing boiler) is not good match for hot water baseboard heating. It will work fine, but it will not be condensing, so it will run like a non-condensing boiler, which has an efficiency in the mid to high 80s (but you are paying for the 95% feature). Baseboard heating is considered a high temperature system and the temperature of the return water into the boiler is often higher than the dew point of the flue gases, so the condensing does not take place. I have read of a design where is a domestic water storage tank and heat exchanger is upstream from the boiler, it assures that the water temperature is low enough when re-entering the boiler, condensing can take place. Also you will need a drain pipe for the condensate, e.g., either a laundry standpipe or basement floor drain. I have oil and am considering gas, and a modulating condensing gas boiler (high efficiency) was recommended. I have hot water radiant ceiling heat, and I am trying to determine if this system is considered a high or low temperature system. Radiant floor is low; baseboard is high.

interesting... funny thing is in my area the Alpine was considered the high tech one with a decent upcharge. I'll tell you right now, I see no reason you'll be able to get the money back on the Lochinvar. you're comparing the Apline which is 95% efficicient against something that is even more efficient. 5% of your fuel is 25 gallons in oil, at $3 a gallon thats $75 a year. you're using gas now, so its not quite the same, but that is a huge price diff. I'm going with a Burnham Revolution 88%. all the plumbers I talked to didn't want to sell me the Alpine. mainly due to cost/benefit of going 95% and the fact that it was a 10 year warrenty vs lifetime. this is part of a whole heating system change. they would still install it if I insisted. but considered all the super high efficiencies a waste from a cost benefit perspective in the "normal" sized houses in my area as the cost was significant and the heating bills didn't justify the payback. again though, I don't have first hand experience with these yet, the work is still being done at my house as it was a full remodel, so I can't tell you operating costs.

Adapting a modulating, high efficiency boiler to an old system can be tricky. For starters, you need a heat loss estimate for the house and you need to determine your room heat exchanger capacity (radiators/convectors/hydrofoils). The latter needs to be done zone by zone if you have a zoned system. Is your baseboard fin-tube convectors, cast iron radiator or something else? Honestly, if you can use a low mass boiler, I'm biased to a system that uses the same boiler for DHW with an indirect tank. It simplifies and cuts down on maintenance costs. You can probably get a rebate on the indirect. Besides, are you gong to take a shower in a freezing cold house? With a small house and a low heating load, it can be hard to get a small enough boiler that produces enough DHW. (You might be thinking that argues for a separate water heater. Maybe, but there are other solutions.) Getting a system with lots of small zones matched to a boiler large enough to heat the whole house can be tricky as well. If zones are not needed for comfort, a good system designer will combine zones sometimes when installing a modulating, HE boiler. Probably the paramount thing to keep in mind that to get anything out of the condensing ability of a hight efficiency boiler, the return water needs to be quite low temperature. That is most easily attained with cast iron radiators and excess radiation capacity so water temp can be held at a minimum. If you have an old house with lots of window and insulation improvements, that is an easier target because radiators were probably not removed. If you have an old, leaky house with few improvements and with fin-tube radiators, it might not be worth installing a high efficiency boiler. They are more finicky, may not last as long, and you will get nothing out of it compared to a mid efficiency boiler.

My recommendation would not be install a combination boiler. In my opinion it is best to keep it simple and have a separate hot water heater. If you get a traditional 40 or 50 gallon hot water tank then you will have hot water during power outages and have an appliance that will last about 15 years. Are any of the contractors doing a load calculation? If you live an older house there is a good chance your current boiler is over sized. The boiler needs to also be sized by the number of radiators. The basement adds very little heat load if it mostly underground. You should not need to increase the size because you are finishing the basement. If you want good temperature control in the basement then make it a separate zone.