Condensing Boiler Sizing

Ed Goldner, NJ

I'm a service Tech for a HVAC contractor in Nothern NJ. I am considering a condensing boiler (Munchkin) for my home and there seems to be a difference of opinion on sizing based on the existing HWBB radiation. Keeping in mind that the new boiler will turn down instead of the constant firing rate of the present boiler. How do I correctly size the boiler for three unequal size zones especially for milder weather. I am also considering adding an indirect WH if the boiler works out. I would be looking at the T50M, T80M or 80M.

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I am confused by the question. You are unsure about sizing, but you have already decided that you want an 80MBTU boiler? You want to size based on mild weather? You want to size based on existing radiation?

Hot water boilers are not sized based on ho much radiation you have. They are sized on the basis of maximum BTU needed by the house...on a design day, with all zones calling for heat.

Chuckles_3

Correction: I see now that you are trying to choose between 50 and 80.

Ed Goldner, NJ

Clarification

My house has 3 existing zones with Slant-Fin Fineline 30 3/4 baseboard radiation. Zones have 53', 35' and 22'of baseboard.Based on 600BTU per liner ft @ 180F water = 66,000 btu total which is exactly the I=B=R rating of the existing boiler I installed 12 years ago. My concern is the low end firing rate of the condensing boiler at mild ambient conditions with , lets say only 1 zone with 22' of radiation calling. The present boiler with a constant firing rate cycles more frequently. With the Munchkin turning down and its low water content I am concerned about short cycling.

jalcoplumb_2

Size by doing a...

heat loss calculation on the whole house. Just because you have 66,000btu baseboard @180* doesn't mean on a design day you will need that.

You might only need 45000 btu, or 70,000 btu.

You don't know.

Do a heat loss and then pick the boiler based on that.

You should have less short cycling with the Munchkin. If you are really concerned look into a buffer tank.

Chuckles_3

As I said, the size of radiation is irrelevant in sizing a boiler.

Of course, on a mild day with only one zone calling, you will have short-cycling. Since the Munchkin modulates, you will have much less short cycling than your old CI boiler, but still you will have some short cycling on warm days. If you are trying to "solve" this problem by making the boiler too small to heat the house on a design day, that's silly. And there is no reason to get a boiler that is bigger than is needed to heat the house on a design day. S waht's your boiler sizing question?

The Munchkin also has an adjustable difference between "on temperature" and "off temperature", which reduces short cycling in low-load situations and where the pipes, radiation and boiler don't hold much water. You can set it at 30F, or even 50F(!)

Ed Goldner, NJ

I remember finding

I=B=R calculation sheets when I first moved in 31 years ago, unfortunatly I worked at a Ford assembly plant back then. Based on performace below +10F I need every inch of existing baseboard. I mentioned a buffer tank to the factory rep. for this area and he said it would make no difference. That is my main concern short cycling during mild weather.
P.S. I can't believe Dan Holahan is still around. I attended steam and hydronic seminars by him back in the 1980's. It was the best 2 classes I ever attended and has been extremely helpful over the past 20+ years

Chuckles_3

> I=B=R calculation sheets when I first moved in

> 31 years ago, unfortunatly I worked at a Ford

> assembly plant back then. Based on performace

> below +10F I need every inch of existing

> baseboard.


If you have inadequate radiation, turn the water temperature up. That isn't a boiler sizing problem; as long as the boiler is able to heat the house, it will be able to produce the water temperature needed to heat the house (within limits, of course, you don't want to boil the water).

Why do you want a condensing boiler in this situation?

Ed Goldner, NJ

I have adequate

radiation. I currently have a 12year old Slant-Fin boiler W/ standing pilot and 82% afue rating. It also has a Honeywell reset T/stat.At below +10F 180 water is sufficient to heat the house and at 0F I may have to increase the reset set point slightly to get 200 water.But, the sole reason I am considering changing out the boiler is the new technology. When I inquired about the Vision I reset control I was told that I had to take a Factory training course and while talking to a factory rep the subject of sizing came up. He said there were some unique sizing considerations with the Munchkin and mentioned something about sizing to a single zone, which didn't sound right to me because it seemed to undersize the boiler at design conditions. When I put boiler sizing into the search engine I saw the name Dan Holahan
and here I am.

Tony_8

I would

add some bbd to keep your water temp requirements down, at least 180 or below on the coldest day. Then size your new boiler to the WHOLE system. I don't know what that rep is talking about as far as sizing to one zone, but I think that would be a fatal mistake.

Get a condenser and enjoy the savings ! Don't let anyone tell you it won't work.

Ed Goldner, NJ

Thanks

Thanks for all the input, the DH legend lives on!

Mike T., Swampeast MO

Until they can modulate down to zero, you're always faced with cycling (possibly very short) when loss is less than the low-end modulation rate. Since low-end modulation is a simple factor in all the boiler's I've researched, it's all the more reason to NOT use more boiler than you need.