Boiler Sizing with Indirect

Chas_4

Sorry for the double post, not enough coffee yet.

tccoggs

Boiler Sizing with Indirect

I'm getting bids right now on replaceing my existing Utica 175k input boiler with a smaller unit as it seems to be very oversized for my house. It is currently coupled with a Superstor 45 gallon indirect. The unit is downfired right now, for 175k in, it supposed to be 1.35 nozzle and it is currently using a 1.00.

After using Slant Fins calc, I came up with 49k btu heat loss and the house has approx 55k of baseboard radiation. The house is 90's construction, pretty tight, anderson winodows, etc.

The Superstor is only 2 years old, so I plan on keeping it, but after having 3 bids on the job, I got a good spread in the suggested boiler size by the 3 different plumbers. I won't get into brands, but they are all cast iron muti pass oil units 86% eff.

Plumber 1 has quoted a 91k unit
Plumber 2 has 112k
Plumber 3 has 140k

Why the big spread?? They all took measurements of the house (Approx 1700 sqft + half finished basement) and did some number crunching, so who's math is off here??? Its just me and my wife in the house, so the hot water need isn't off the chart.

Thanks in advance for the input

Robert O'Brien

Your math

is correct.As a wise man once said " the urge to oversize is stronger than the sex drive" Truer words were never spoken.Keep looking.

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tccoggs

Robert,

I kinda figured that since the largest unit was quoted by an oil company

So do you typically just size the boiler to the heat load only and not up the boiler size at all for a Indirect?? Is there a percentage factor??? Even Slant Fin's software doesn't add anything for the indirect.

Robert O'Brien

In your application

just size to the heat loss.With priority,the entire boiler output will be directed to the indirect on a DHW call.I'd recommend ODR as well

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Empire_2

Agree with all coments:

The only "Fudge factor to be used is if,....Well you know how the average degree day's is say, 20* and you get the cold snap from hel;,......-5* for a week, that's the only consideration as far as the buffer zone goes.

Chas_4

If her math for the heat loss is correct, it does seem so based on square footage, the smallest oil boiler she'll be able to have installed will be oversized. Even on a day that's colder than design there'll be more than adequate reserve.

As far as domestic hot water, unless it's a significant part of the load, don't add anything for it.

Dick Charland

There are indeed cases today where the load calculation for DHW will be higher than the heat load. It is important to check what the DHW requirements are and then take a look at the indirect and what it will need as far as input to meet the output specs stated in the literature. Minimum boiler output to achieve 1st hour ratings in many cases will be much higher than heat load, e.g. 40 gallon Superstor requires 141,000 BTU's. Not complicated, simple physics.