Boiler Sizing

[killick]

I have done a rough rule of thumb heat loss calculation on my house and come up with between 75,000 and 98,000 BTU's/Hr.I am going to have a detailed heat loss calculation done but was wondering how to size a gasification boiler with this information.I have read upsizing the boiler by 25-50% over the heat loss.This allows for quite a variation.Any opinions would be appreciated.

Earl Woodworth

 

[chew72]

Well first question is do you plan to have storage? If so work the numbers on how long a burn you want to do and how often vs money invested. If you have no storage do you have back up oil/gas boiler. If you do and had a boiler sized for your heat lost or +20%, on the few days your boiler isn't putting out enough then the oil or gas could take up the slack. No boiler, no storage, Then go 20-30% larger. The only down side with gasification seems to be the loss of efficiency with out storage and is has to cycle on and off.

If you dont mind me asking what part of Nova Scotia are you from? I'm from the south shore my self. Also what gasification boiler are you looking at and is there a seller in NS? I'm living in BC right now but plan on moving home and builing in a year or two.

Nick deMolitor

PS just my 2 cents see what other people have to say.

 

[killick]

Hi Nick,thanks for the input.I'm located about 5kms East of Mahone Bay in Martins River on the South Shore.Presently we have a separate wood and oil boiler system located in an out building about 50 ft. from the house with the heating water piped in underground.I would like to replace the wood boiler with a gasification unit.So far the only rep for this type of boiler I have been able to find is in Weymouth and would be dealing in Tarm.I have been in touch with a forum member in PEI, wantstoburnwood, who has an Econoburn which he is happy with. The Econoburn looks to be a quality unit with good reviews. I would plan to install water storage in the 800-1000 gal. range, unpressurised, with coils for heat and hot water. I may also go with some solar in the future.
Where abouts are you from?

Earl

 

[chew72]

East Jordan about 15 mins outside Shelburne and Lockeport.

 

[Gooserider]

Chew has it down pretty well... Essentially if you are running w/o storage, you want to be fairly close to your heat load estimate in order to minimize idling. If you are using storage, you almost don't care about the heat load, as you will mostly be working on heating the storage, and the storage will heat the house... You just need to size the storage to be able to supply the needed heat, and the boiler to be able to heat the storage faster than the house can draw it down... A larger storage system will give you longer between burns, but also take longer to charge, unless you use a bigger boiler.

Another factor is that the makers of fossil boilers seem to have a lot of models that are sized in fairly small steps, plus choices in burner setup that lets you get a boiler that is pretty close to your actual heat load. Since a fossil boiler will always put out it's rated / designed output, matching it to the load is the most efficient way to go... However because of the cyclic nature of a wood fire, a wood boiler does NOT put out it's rated power for a good part of a burn cycle. Especially if running w/o storage, it is important to use a real world estimate of 75-80% of rated power as what you will actually get on average. If using storage you can figure more like 85-90% of rated... Wood boilers also tend to come in a relatively limited number of models, so even with this considered, it is harder to get an exact match to the load...

My rough guide to sizing would be to figure either the house load (no storage) or storage load and add 20% - then look for boilers that will do AT LEAST that much rated. If a boiler in your preferred brand is right at the border on this calculation, strongly consider going to the next bigger size... In your case, using your 75-98K estimate, I would say that you should look for a boiler of at least 125K, and strongly consider looking for 150K, especially if your more detailed calcs show you getting on the high side of your estimate.

Gooserider

 

[killick]

goosrider,thanks it would appear I have to get the calculator out and put on my thinking cap.Is there a calculation to determine the sizing based on heat loss for the house and a specific amount of thermal storage?I'm trying to get out of loading a furnace at 3 AM as well as reduce my overall wood consumption.
Chew,my grandparents were from Port Mouton.

Earl

 

[flyingcow]

Just curious how many BTU's is your oil boiler, and your wood boiler?

 

[killick]

flyingcrow,the wood boiler is 200,000BTU's I believe.I can't find the rating on the oil boiler.The plate lists five different possibilities but doesn't identify which one mine is.They are both New York Boiler Corp. of Sussex New Brunswick.The wood boiler is basically a box with a chimney,not very efficient.

Earl

 

[Gooserider]

goosrider,thanks it would appear I have to get the calculator out and put on my thinking cap.Is there a calculation to determine the sizing based on heat loss for the house and a specific amount of thermal storage?I'm trying to get out of loading a furnace at 3 AM as well as reduce my overall wood consumption.
Chew,my grandparents were from Port Mouton.

Earl

This gets a bit tricky, as there are several variables that can enter into sizing storage, although it seems most folks end up with 500 to 1,000 gallons. The heat loss of the house is one factor, and you also have to look at what your "usable temp range" is - how hot can you get it on the high end, (safe to figure 180*F for non-pressurized, 190 for pressurized) and how cold can you draw it down and get satisfactory results in heating the load. This will depend on your heating system and how well it's setup - Baseboards tend to need very hot water, often in the 140-160 range. Infloor radiant can sometimes go down as low as 100. If using the storage tank to heat your DHW, you might need 120*... The difference between high and low times the weight of the stored water is how many BTU's you can store (1 BTU changes 1lb of water 1 degree F)

The basic idea is to decide how many hours you want to be able to run off the storage, and how many hours at a time you can burn conveniently. Your objective is to start a burn when the storage is at it's low point, and have the fire finish burning just as the storage is reaching it's maximum. (You can load extra wood in during the burn) An ideal scenario is start a fire when you come home from work, maybe throw an extra load in just before you go to bed, and then build a second fire just before going to work in the morning...

Most boilers seem to run about 4-6 hours on a full load of hardwoods. If you figure two fires and three loads of wood per day, that means you need about 10-12 hours of time for running off storage, and about the same running off the boiler and charging the storage - Thus I would figure that your boiler needs to be able to supply your full house load, plus 10% of your storage capacity at a minimum...

The other angle to figure as a double check, is that the above numbers assume "design day" loads, or your coldest anticipated weather. However in shoulder season, or especially if you are burning for DHW in the summer, you might not have ANY house load while burning, so you want to size the boiler to be able to heat the storage to max on about one load of wood...

Gooserider

 

[killick]

Gooserider.Well that certainly is a lot to chew on.I really appreciate you taking the time to reply to my questions.Back to the calculations and research.

Earl