Need Help - 2 different sized gas boiler recommended by 2 plumbers

CCNJ

I live in northern NJ and asked 2 plumbers for estimates on replacing my peerless oil combi boiler with a Weil McLain CGa gas boiler and indirect 50 gallon water heater. One recommended the CGa 5 unit and one the CGa 7. My 1952 house is one floor with 1,800 sq feet. It is built over a full basement with 7 1/2 foot ceilings, 6 feet of the basement is underground. 700 square feet of the basement is finished with baseboard heating. I did major renovations 4 years ago with kitchen, 2 baths, large office fully insulated. I assume the rest of the is not but all rooms have new windows and was newly sided with insulation. Based on this info, can anyone tell me which boiler is best? I did a quick heat loss call using a simple form on the Weil McLain site and feel the CGa 5 is best but I would some other opinions. I know very little about plumbing but do know that an oversized boiler will be short cycling and costing more more in energy costs. I did consider the higher efficiency condensing boilers but chose against them for the external piping (I have a 2 year old SS chimney liner) and feel the cost savings will be slightly offset with maintenance and lesser life expectancy. Appreciate your experience and opinions

Ironman

You probably need less than 60k btus to heat your house.

Over-sizing a boiler is a huge mistake and shows a serious lack of hydronic knowledge by anyone trying to do so.

Slantfin has a free app that you can download and it will give an accurate heat loss calculation if you enter the correct data. There’s a link to it somewhere on this site.

Is the entire house heated with baseboards? If so, how many linear feet of elements do you have?

Ironman

@EzzyT is a contractor that serves N. NJ. You won’t find any better. I highly recommend that you contact him and forget the other ones.

EzzyT

@CCNJ the best way to reach me is at 2018878856.

CCNJ

Thanks for the comments. I have 108’ running feet of baseboard to heat the 1800 sq ft main level. I also have radiant floors in the 12x16 kitchen. The living room also has a gas fireplace so I mainly use that for heating that space in the winter. The 700 sq ft basement has 24 running feet of baseboard. I have 6 zone heating so not all are running at once. @Ironman - pretty impressed with your 60,000 btu estimate. My heat loss calc came to 62,000 btu but I wasn’t sure I was doing it correctly. Based on this I should be getting the Weil McLain CGa 4 model at 73 net mbh or no higher than CGa 5 model with a 98 net mbh. Is it better to be a little conservative and go with the CGa 5?

mattmia2

Is this boiler only doing space heating or is it also making domestic hot water?

CCNJ

I am getting an indirect 50 gallon tank that the boiler will heat

Ironman

Your radiant floor + your baseboards are only capable of about 71k btus output. Why would put in a boiler that’s much larger than that? Larger input will NOT increase the output of the system.

The fact that your system is over-zoned will only exacerbate the situation because all zones will seldom be calling simultaneously.

You only need the full capacity of the boiler on the coldest night of the year (design temp); at 35* outside, you only need half the capacity. The mean winter temperature for your area is probably 35-40*.

Unless you have an unusual domestic load, a 30-40 gallon indirect should be sufficient, but there’s nothing wrong with using a 50 gallon if you choose. If you have an unusually high domestic load, increase the size of the indirect, not the boiler.

CCNJ

@Ironman - Most helpful information. Really appreciate you taking time to explain it.

Hot_water_fan

62,000 btu heat loss implies you use about 900 gallons of oil per year. Does that sound right? 

CCNJ

That is a good question and the last piece of the puzzle. I need to call my oil provider to see how much oil I go through in a year. The existing peerless oil boiler has an AFUE of 83 (the new gas one is 84) but is oversized 155,000 btu/hr and also has internal coils to heat my domestic water so I burn oil all year round. The existing boiler is in good shape but I paid $3000 in Jan/Feb/Mar in oil deliveries. I know oil won’t be this high forever but I got to think there will be considerable savings over the life of the new boiler. My next research is how to calculate oil to gas savings using avg pricing to guide my decision. Any thoughts on this?

Hot_water_fan

@CCNJ knowing how much oil you used is another way of figuring the heat load, but remember to subtract out an estimate of the DHW portion. 

The $/MMbtu comparison calculations are easy. For oil: $/gallon x (1,000,000/138,000) / efficiency. 

For gas: $/therm x (1,000,000/100,000) / efficiency. For gas, ensure you’re counting all the variable cost: supply, delivery, etc. 

As for which will be cheaper in the future, that’s anyones guess. If you want gas for a stove, that’s the selling point. 

If you have AC near end of life, a heat pump is similarly priced to a new AC and can be cheaper than both gas or oil, but that’s a whole different calculation. 

CCNJ

@Hot_water_fan - great info, thanks. Really appreciate the oil and gas formulas. BTW - I already have a gas line coming into the house for my gas fireplace, stove and dryer so the plumber can just tap into the existing line. At this point it comes down to the savings associated with a more accurately sized boiler, moving to indirect heating, and moving from oil to gas.

Hot_water_fan

@CCNJ excellent - some utilities price gas at a high variable cost with a low monthly fixed cost, some flip that and some are high for both.