Which new boiler to choose?

bjs317

I have two HVAC contractors recommending two different size propane boilers. I am replacing an old oil boiler, and there is an existing indirect water heater which I will keep. House is about 2500 sqft w two heating zones, 2.5 baths. We are in Connecticut. #1 is recommending the Lochinvar Knight 155, while #2 recommends the Lochinvar Knight 199. #1 says the 199 is overkill and will lead to higher fuel use and cost, while #2 says the 155 is too small and the house will be cold.

Any thoughts are appreciated, thanks.

jesmed1

Take 15 minutes and find your heating bills from last winter. Find the bills from one cold month and calculate the number of BTU's your house used during that month. Then download some weather data for your area to find the "heating degree days" for that same period. Then do some basic math and find your actual heat loss. This article tells you how.

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

As a sanity check, you can also consult these general guidelines for BTU/sq ft needed for houses of varying vintage and construction. If your house is well-built and well-insulated, you might only need 20 BTU/sq ft, or 50,000 BTU/hr. If it's leaky, you might need 40 BTU/sq ft, or 100,000 BTU/hr. But unless you live in a seive, I'd be surprised if the Lochinvar 155 wasn't sufficient.

https://inspectusa.com/blog/typical-heating-cooling-loads-in-older-homes/

As a point of reference, I live in a 100-year-old building in Boston with a heat load of 20 BTU/sq ft. It has been modernized and insulated and is reasonably airtight, so even old buildings can have low heat loads. But they can also have high heat loads if they haven't been upgraded. YMMV.

GGross

If neither is giving you a load calculation they are both guessing. For the sake of simplicity the smaller boiler will output between 50-60 btu per square foot of your home depending on how efficient it runs. This is about double a high heating load in my region (northern MI). They are almost certainly both oversized.

Another check you can do, if you have baseboard count up the linear feet of fin tube element you have (don't count the dummy cover only sections) multiply the total linear feet by 540, that is essentially the most heat you can transfer through emitters to your home

If I had to take a guess, I would guess anything between 80,000-120,000 output would be more than adequate, since these are modulating boilers I would be OK with something in that range, if it were a cast iron boiler or one that didn't have a high modulation range I would want to get more exact on that size.

KC_Jones

To add to the above, my 120+ year old house with original single pane windows and basically only attic insulation is just barely hitting 40 btu/sq ft. If you have anything built in the last 40-50 years you should be significantly better than that.

Shooting from the hip I'd say both contractors are over sizing the boiler, by a fair margin. I wouldn't let them put in anything over the 110, and it's quite possible the 85 is big enough.

The contactor needs to do a heat loss of the building, that's the only way to size the boiler properly.

Hot_water_fan

Both sounds massively oversized, but we won’t know until you do the fuel use heat loss. It’s a bit insignificant though- they both have a 10:1 turndown ratio so the smallest Knight (the 55)  and the 199’s minimum fire is only 11kbtu apart. 

hot_rod

what size is the current boiler
how is your dhw load
A 110 might be adequate, but the 155 would recover the indirect faster. If that is a concern

EdTheHeaterMan

When I taught a Hydronics one day seminar, There was a point in the class where we went over how to do a Load Calculation. I handed out a Form 1504WH and told the class to go home and fill out the form based on their own home. Then compare it to the actual heater in their home to see how much the heater was oversized. The next slide in the program indicated that you may have a home that only requires 38,000 BTUh for a small bungalow but the heater may need to be larger in order to accommodate the domestic hot water needs. This sounds like this may be the case for you.

The load calculation for your home is the best way to determine what you actually need as far as the size boiler you require. As @hot_rod said above, the heater you need for space heating may be too small to recover your Indirect water heater as fast as you would like, so you should consider the 155

The fact that your space heating can modulate from as low as 15,500 BTUh input all the way up to 155,000 BTUh input means, if you home only needs 40,000 or 60,000 BTU on the coldest days, your boiler will only operate at that lower number for space heating, all the way down to 15,500 when it is milder outside.

You can also do the Degree Day Load Calculation method above to get close to the actual need, but remember that gas bill info will not have the oil usage included in there. You need to get your total gallons used in a year from your fuel dealer to do the degree day method.

Some folks don't read the whole post before they give advice. I know... Im one of them!

Hot_water_fan

The typical gas tank water heater is what…40kbtu? The typical electric tank is 15kbtu. These boiler options are gigantic compared to the options the majority of Americans get by with! It’s like having a 600HP minivan.