Boiler sizer-

jabedin1

New here. i currently have a natural gas boiler for my 4000 sqft house. 3 zone

Hydrotherm boiler input 180t btu, output 144 T btu, net rating 125 T btu/hr

Model HC-180. Serial 2550

im trying to replace with Nat Gas Weil Mclein natural draft gas boiler(no hot water). Which CGI series should I use? Plz help

Jamie Hall

I'm assuming hot water heat?

You need two numbers to start: the actual heat loss of the house at design conditions — there are a number of ways to get that — and the actual amount and type of radiation installed in the house.

The first number will give you the boiler power required. The second number will tell you whether or not you can use a condensing type boiler or need a conventional one, such as the Hydrotherm you are replacing.

Why are you considering replacement?

jabedin1

yes its baseboard heater for heating the house. the boiler is old and i would like to replace it , dont want to get stuck with in the mid winter failure. and i would like to replace with gas fired conventional Weil Mclein type natural draft gas boiler. I personally dont like condensing type. has too many issues with such. I honestly dont know heat loss math. thank you

Jamie Hall

Fair enough on the math, @jabedin1 . However, a good contractor should be able and willing to do the math to size the boiler. Just replacing like for like is easy — and may not be the correct approach. See what you can find in that regard.

PolychromeUganda

If you have been in the house for a while you probably have some experience with what percentage of the time the boiler is firing during the coldest weather. If it’s firing 75% of the time or more during the coldest nights, don’t reduce the size. If it’s firing continuously and the room temperature remains below setpoint increase the size.

If you’re new to the house, I suggest having someone clean it and adjust the burners. Some of the cast iron boiler designs have a very long lifespan, some fail very predictably at 7 years or 14 years, etc. The boiler has been there for quite some time, it may be one of the designs that lasts indefinitely. Since you would replace it with a non-condensing boiler, there isn’t a significant efficiency gain or impressive technology update. Don’t express an urge to replace the system to your heating professional at this point. Many heating professionals will interpret that to mean that you’re hoping they can find something that helps you justify the expense of a new system to yourself even if they might otherwise say it’s aging well and will last at least another 5 years. If there’s any sign of a boiler leak or burner defects or other significant problem they’ll tell you.
Every boiler manufacturer periodically loses control of the quality of their grey iron casting line and the boiler sections have excessive internal stresses and/or material consistency issues. The result tends to be boilers that leak and fail in much less than 20 years. Sometimes just 2 to 5 years. Since your objective is reliability my suggestion is to keep an eye on the casting you have and when it needs to be replaced look for one from a manufacturer that hasn’t had any models with problems in the last 10 or 15 years. Don’t consider a sheet steel boiler instead of cast iron, there are a different set of problems that limit their working life. There have been good welded steel plate boilers made or imported from Europe in the past. I’m not aware of any being widely marketed for gas burners right now, but I don’t follow it that closely.

jabedin1

thank you soo much. your information and words are so priceless.🙏

JUGHNE

I changed out that same boiler for someone, only it was 100,000 Btuh input.

It is notable that that boiler is rated for 100 PSI.

I did that change out because the old boiler was stuck in a closet with water heater on a carpeted floor.

The carpet was burned and after pulling the old unit out there was a hole burned in the floor……hopefully not your situation. The pump was not accessible being in the back corner, the exp tank was the same. The age was with the 1960's house.

I did a simple heat loss on the house, (1960's double wide with hydronic baseboard heat….how about that?) Old school, one page, heat loss called for 63,000 required. It came down to the fact that he had only 100' of 3/4" BB connected in a single loop.

100' of 3/4" BB at 550 BTUH per foot meant the BB could only deliver 55,000 BTUH at 170 average water temp.

So a Buderus/Boch GC 124/19 85% AFUE rated at 74,000 input was installed. Approved for combustible floor installation, has riser legs and also 2" concrete block platform was installed under it.

The homeowner did call the first winter that it was not quite warm enough in the house (usually runs temp at 74 or so). So he turned up the water temp for that time.

jabedin1

thank you for your detailed valuable experience. I always hated the Heat loss calculation lol i guess i will do it for myself.

EdTheHeaterMan

Weil McLain CGa is the simplest offering the Weil McLain offers today. It is basically the same 76% efficient CG boiler they introduced in the 1960s with a few tweaks to make it more efficient. To get to the minimum 80% AFUE they added electronic ignition to eliminate the standing pilot, and added a vent damper to close off the vent connector to the chimney to keep the heat in the house during the off cycle.

That said, your Hydrotherm was probably installed when plumbers thought "Bigger is Better" and is probably oversized. In my opinion, you have 2 choices. Keep the boiler you have as long as there is no signs of leaking boiler sections. the parts in that boiler can be replaced as needed for a lot less than a new boiler install. The moving parts like a circulator pump, gas valve, limit controls and other interesting things can be replaced with new upgraded stuff.

The other option is to get a new boiler that will not offer you any real savings over the Hydrotherm. The efficiency will go from 77% to 83%, which will save you about 8 cents on the dollar of gas usage. But getting the proper size, like perhaps the CGa 6 at 166,000 BTU input, if it is large enough may add a few more thermal efficiency points to reduce short cycling. Then you may get your total savings over 10%. That choices is not going to break the bank in fuel savings. but the load calculation will let you know that you do not need to use the CGa 7 at 200,000BTU input (because they don't make a 180,000 BTU input boiler.

If you go the new boiler route, the CGa 5 may even be enough for you, Have your contractor do the load Calculation for you. You may be surprised.

jabedin1

that is one of most helpful comment i got. you are right looking into it CGA 7 would be too big. I will go with your tip, stay with whatever I have since i have no leak, I do maintain my boiler each year with cleaning, keep it as long as I can, CGA 6 I would go it just in case.

Thank you again. You all are so healpful