Sizing replacement hot water boilers. . .

Keith_13

We all know that when sizing out a replacement for a steam boiler, the proper method is to physically count each radiator section and compute a total EDR output. . . but when sizing a hot water boiler, what exactly should you be basing the model on? The IBR heat loss of the building? If a heat loss is not feasible, can you use the existing label method of replacement? Call me a moron, but I just wanted to be sure before I go take a look at a job tomorrow.

Thanks a bunch,

Dolph

Long Beach Ed

Lots of Work

Heat loss of the building... no other way.

Since the existing boiler is probably grossly oversized, when you are able to specify a smaller boiler your quote will be so low that it will make up for the three hours you spent doing the heat loss survey and calculation.

Right?

LB Ed

Brad White_24

Ed Nailed it -Again...

ASHRAE or similar bona-fide heat loss calculation.

If you do a name-plate replacement you are putting faith in a person you never met, an installer. OK, maybe a Dead Man, but still, who or what do you trust? Your own calculations.

Nameplate replacements also do not capture the envelope improvements the HO may have made over the years, taking a third or half off the capacity right there. Rightly or wrongly at least you will know.

No, you are not a moron, you asked a great question! If you did *not* ask it and went by the nameplate, all bets are off



Now, once you have the heat loss, you can also do a take-off of the radiation and compare the two. You are likely over-radiated if they insulated the house well, put in replacement windows and so on. The result is that you can predict what design hot water temperature will make that house heat beautifully! May even get them into a condensing boiler and you will be a hero.

Parades, accolades, groupies, all yours...

Roger Litman

Sizing hot water boilers

If you have a history of the fuel consumption in a cold period, you can get a very accurate heat loss of the structure by using the degree days and the consumption. You must have a cold period, a storage tank or indirector a separate hot water heater. The hot water consumption is usually of the order of less than 100,000 Btu per day, so it is usually negligible as well as other non heating uses. Calculate the usage during a period of time and extrapolate to a design day and you can calculate the actual house loss including infilitration. For oil with a design day of 0°F and a "K" factor of 5.3, the house loss is about 55,000 Btu per hour. All you are doing is taking the absolute consumption during a period of known temperatures and back calculatig the heat loss, assuming that the people keep the house at generally accepted conditions. BTW in Boston area the coldest day in the past twenty years averaged 2°. I have been using this system for twenty years and it is very easy and allows the correct size which is often smaller than most boilers that you can buy.

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