Home>Discussions>INSULATION & HVAC>Can someone help me on the correct btu on my new oil burner?
12 posts / 0 new
Last post
Tommyg's old house
Can someone help me on the correct btu on my new oil burner?

Old furnace finally went out after almost 30 years last winter. Had a Crown oil boiler furnace as the replacement.I have baseboard heat. My homes heated living space is about 990 square feet,my ceilings are 8 feet high. No insulation in the walls(which is in the process of being taken care of)and the windows are single paned awning with another single pain insert on the inside of the home. The windows will get replaced in the spring with a much more energy efficiency. The attic is well insulated and the crawl space below the house,I'd say four feet from the ground is covered with a believe a 6 millimeter tarp and stones covering the whole living space of the home with fiberglass insulation under the flooring in the crawlspace. My boiler is a Crown,model (TWZ100bott1psu) The heating capacity is 120,000 BTU/HR and a Net Rating of 104,000 BTU/HR. I was in a jam as the old furnace went out during the coldest days of winter and could only get this one guy in on short notice to replace with this model. He told me the specs were fine for the size of my home. My question is, are they? Would have liked to have had a more efficient model put in, this boiler has an efficiency rating of 84.5%.Keep the heat on consistently at 68 degrees,no lower than 65 when not at home or I drop it at night before bed. Had an oil delivery approximately about one month ago to top it off and have just under 3/4 oil left in my 275 gallon tank,more towards 5/8 according to the reading.I live in North West New Jersey. Some very cold days so far the last couple of weeks. Is my model correct and do you think that that amount of oil burned in this period of time is ok? Thanks all for the info, Tom.

Re: Can someone help me on the correct btu on my new oil burner?


A cursory heat loss computation based on the info you provided in your post would seem to infer that the Crown is almost twice as large as it should be.

The upside is that Crown makes very good boilers with an excellent reputation, and it's very hard to come up with anything more than a ballpark figure with the info provided.

Generally, a very tight house with full insulation in walls and ceilings and double-pane windows needs ~30 or 40 but/hr per sq.ft.

But even at 60 btu/sq.ft. you should only need ~59k btu/hr for that house.

You will have to do a heat loss calculation at the sites below to get a better idea of what should have been installed.

That being said, the boiler is already in there, and you'll have to live with it until you can arrange for a more efficient system.

You can always sell the older unit when the time comes.

For the new boiler consider a gas-fired condensing boiler (if natural gas is available in your area).

If not, consider an oil-fired condensing boiler, or a 3-pass boiler; all of these have >90% efficiency ratings.

There are several things you can do to improve things with your present boiler.

You mentioned having insulation blown into the exterior walls, and adding new 2-pane windows, both of which are excellent ideas.

If the present boiler has a coil heater for the hot tap water, also plan on having an indirect hot water heater (40 gallon Megastor) included in your upgrade.

This will save money on fuel and provide all the hot water you need.

You can also have your service person de-rate the oil burner nozzle from 1 gpm to 3/4 gpm (if this model allows for such a derating).

Check the owner's manual for this, it should be listed in there.

Start your free HLC's with the bgm site, and progress to the free slantfin site.

The slantfin HLC is much more comprehensive and will take ~1/2 hour for it to load in your system; click onto "heat loss software".


Tommyg's old house
Re: Can someone help me on the correct btu on my new oil burner?

Thank you for the reply. I remember my old furnace, it was a Rheem. I had service information dating back to 1980 on it.It was replaced in February of 2007 by my current boiler. You state that my newer furnace is twice the size of my home. The specs on my old Rheem furnace had a btu rating of 165,000/120,000. My house did always feel somewhat warm but it was kicking on and running so much moreso than my present furnace. Oil and propane are the only two options in my area. This newer one really doesnt seem like it kicks on all that much. It kicks on also when the temperature on my electronic aquastat goes below 160 degrees and stops at 180 degrees. The installer said that that was the proper setting for it. I have a Crown oil burner model number TWZ100, D.O.E. Heating capacity BTU/HR is 120,000, IBR net water rating is 104,000, burner capacity (GPH) is 1.00, Tankless heater (GPM) is 3.25. Would that be the right setting as for the high/low? I want to maintain the maximum efficiency of my unit at all times. Also, being how my furnace heats my hot water would it be ok, say if I went away on a vacation for longer than one week at at time (warm weather of course) to shut the furnace down so that it wouldnt kick on at all? Would I be doing any harm to it while trying to maintain a fuel savings? Thank you for your input and any other information that you could give on my burner I would apprecitate it.

Re: Can someone help me on the correct btu on my new oil burner?

What are the results of your heat loss calculations???

I thought you were going to do a HLC and get back to me.

Unless you stated your square footage incorrectly, the Crown TWZ is too big.

You can contact your heating person and ask them to do a combustion analysis of your burner flame to make sure the air/fuel mixture is not too rich; also ask them if they can de-rate the burner nozzle to 3/4 gph instead of the 1 gph one you have on there now----both measures will save a lot of oil over the heating season.

The smaller nozzle will effectively reduce the boiler output to ~80k btu/hr, allow it to run longer at a lower flame, rather than short-cycling.

The settings you mentioned on the triple aquastat are correct.

How many gallons are you burning per season???

The BEST thing you can do ASAP is to have an insulation co. blow in cellulose insulation into every exterior wall; combine this with improved windows or 2nd hand storm windows right away & you'll see a 30% drop in your fuel consumption this winter.

You need R19 in the walls and R40 in the attic.

It won't hurt the boiler if you leave it off for 2 or 3 weeks in the summer on vacation; but no longer than that.

You won't save much fuel (boilers don't burn that much fuel in the summer), and boilers tend to rust and corrode if they don't stay hot, especially the type of boiler you have, which is pin-type, cast iron wet base.

The cast iron sections are more likely to leak if you leave it off all summer.

You've got a good boiler there, get moving on the insulation and storm windows.

Tommyg's old house
Re: Can someone help me on the correct btu on my new oil burner?

Hello my friend. You were right. I should be at 60,000btu. I called Crown and had a service tech that was very informative tell me this. He said that I could go as low as .65 on the nozzle. According to the specs on the furnace it's at .85. I was told that after tell years the components would start to go. It should'nt be going on a short cycle like it is according to the rep. He told me that it should have a longer cycle when it turns on. According to him it's not that big of a deal, I stated that the furnace since November 17th fillup to the present time just under 1/2 tank. I need to have the nozzle de-rated like you said to the .65 and the technician who does this needs to have it slated on a a 45 degree angle and it has to have a b-nozzle and must use cumbustion instruments to re-state it. Does this sound right to you? What should this cost me? I don't have a service contract with an oil company to do this. I also have a problem with my water softener. It has a slow leak up top. I have a four foot crawl space so the work needs to be done on you knees to fix it. Problem is, everything is very tight. Could you help me with this? I know I havent been very informative about this particuliar project but I will if you can ask me what more you need to know. Thanks.

Re: Can someone help me on the correct btu on my new oil burner?

That sounds like a good plan.

There are plenty of service personnel that will work on your boiler without a contract.

consult the Yellow Pages under "Oil Burner Service & Repair".

The standard fee is $50, but call several of them and make it a point to ask them what they will charge to put in a smaller nozzle, and use a combustion analyzer to adjust the flame, and adjust the oil pump pressure.

If the boiler hasn't been cleaned in a while, it may be a good idea to ask them to replace the oil filter, the pump strainer and vacuum out the heat exchanger.

Most techs don't mind if you watch while they do their work, & this is an easy way to learn how to do this work yourself next time; the parts are readily available and the work is not that complicated.

Try to get the lowest firm quote you can get over the phone; some techs don't bother carrying a CA on their trucks because it's an expensive piece of equipment (over $1k) and needs periodic calibration.

Other than that, it's only a 1/2 hour's job.

I know very little about water sofeners; if you could post this as a separate issue, I'm sure some of the other posters will be able to help.

Tommyg's old house
Re: Can someone help me on the correct btu on my new oil burner?

Hello Jack. I talked to a plumber who told me that de-rating the nozzle to .85 to .65 that I could do myself and that it's already slanted on that particuliar 45 degree angle already. He also told me that using the combustion analyzer wasnt necessary. Is this true? I hurt my back and kind of cut down on the work that i'm doing in the house but I'm going to get the windows done this spring. Any particuliar brand you have in mind? I was told by one salesman that the Marvin Windows are priced the same as Pella Windows but are a better window. They are aluminum on the outside compared to the Pella vinyl. Please let me know. I live in north west NJ and it's very cold during the winter so I want to get very good windows. what specs would your opinion be on such windows, u-factor,etc.. How much do you think a Bay Window installed would cost? I'm leaning toward a Marvin Window. Looking forward to you reply,thanks again,Tom.

Re: Can someone help me on the correct btu on my new oil burner?

hello i had the rapid cycling it was because my high limit was set to 180 i turned mine down to 155 and very happy with the results. personaly i saved almost a 1/4 tank a month i was filling monthly to get it tuned to find results. it's cheap to try won't break your back either
i'm glad to here about the nozzle switch i'll try that too

Tommyg's old house
Re: Can someone help me on the correct btu on my new oil burner?

Hello Pomer. This question was actually referred to "Jack THe Shack". I was told not to lower the temperature gage because it's the proper setting for the unit that shoudlnt be adjusted. I've actually only gone through 309 gallons of oil this winter since my first fill-up on November 17th of last year. My unit is nearly twice the size that it should be and it short cycles when it should be going a little longer. I could de-rate the nozzle for more efficiency than what I already have. I just wanted Jacks opinion on my last posting. Thank you for your reply though.

Re: Can someone help me on the correct btu on my new oil burner?

you talked about short cycling on your furnace just saying what corrected mine sorry for your inconvienence

Tommyg's old house
Re: Can someone help me on the correct btu on my new oil burner?

Thank you for your opinion. I was told that because the furnace is twice the size that it should be it short cycles instead of cycling a little longer. By short cycling it's not as energy efficient because the blast of the oil comes out quicker than at a slower more efficient pace. I appreciate your reply as I do everyone that is one this site. It's a great site to get differing opinions on.


Sponsored Stories

TV Listings

Find TV listings for This Old House and Ask This Old House in your area.