Chimney vs. Power Vent

RonWHC

operate w/ higher stack temperatures than gas. Those higher temperatures, & lower water vapor content in oil flue gasses, won't wash out the tile lined chimney. Lower temps, & more water vapor, in gas flue gasses will. And. As efficiencies go up, & stack temperatures down, all chimneys will require lining some day.

Under certain conditions, in very cold weather, chimneys (particularly oversized ones) can form ice dams at the exit point. Another reason for a properly sized & installed flue liner.

stacie b_2

Hi, I'm a homeowner considering a gas conversion from oil-fired steam. I was told that our chimney could be lined, or a power vent could be installed by cutting a hole in the basement wall. What are the pros and cons? Thanks, Nelson

ed wallace

chimney vs power vent

if you must really switch from oil to gas go direct vent vs a power vent you didnt say if it is steam, hot wate or warm air ned some more info

stacie b_2

Hi Ed,
Yes, we have steam. It is an asbestos wrapped, oil-fired antique and the oil tank is questionable also. When you say "direct vent", do you mean through the chimney? What are some of the drawbacks of the power vent?
Thanks,
Nelson

ed wallace

direct vs power vent

direct is normally thru either stainless steel or pvc out the side of the house the blower would be built into the heating unit power vent is a blower unit mounted on the side of the house coud be difficult to service

lee_7

go with a unit that is a direct vent unit. External power venters are nothing but a pain to service and repair. Also parts are sometimes scarce in winter. I have a Crown direct vent hot water system and I love it.

A.J.

stay with the direct vent

We seem to have more problems with the external draft units.

Nelson_4

Guess I'm still a bit confused

Thanks for all the advice! The gas company gave me two choices for a steam boiler... 1) Power vent 2) Natural draft/Chimney vented. So, are "natural draft" and "direct vent" the same thing?

Bruce Stevens_2

Your first question was worded wrong

Three Types of venting

Natural Draft ie chimney

Direct Vented goes directly from the boiler/furnace etc to outside via PVC or SS pipe throught wall or up chimney chase etc, may or may not have an exaust fan in the furance or boiler depending on model.

Power Vented ie throught the wall with an exhaust fan that is a seperate unit from your boiler/furnace

Nelson_4

Thanks for the clarification

Since I don't have a direct vent option, sounds like I would be better off getting the chimney lined instead of dealing with any power vent issues. Also seems to me that there could be an issue with fumes if it's vented from the basement, but maybe not?

RonWHC

Power vented

gas boilers, like the Burnham PV, have the power vent attached at the boiler. They can be vented thru the wall, or into a properly lined chimney. Atmospheric gas steam boilers are limited in efficiency.

Their are steam boilers that use a power gas burner, much like an oil burner. The best venting option for one of those is into a properly lined chimney.

Check some web sites. Burnham, H. B Smith, Peerless, etc. There is a wealth of information for your consideration.

J.C.A._3

Cost to own....

How long do you plan to stay in the home?

For a longterm solution, I think lining the chimney is a better value...as long as you use stainless steel and NOT aluminum, and have the chimney checked to make sure it is safe and sound.

For a short term solution, I'd probably pick the power venter. The cost is less than a chimney liner, (Did I mention that it should be STAINLESS?)and they really aren't the service nightmare that some would lead you to believe. Really folks...read the books, follow the instructions and these are pretty easy things to make work well, but not for as long as a chimney would last.

There's something to think about. Chris

Nelson_4

Yes, I am looking for a better long term solution, so I would get the stainless steel liner. Follow-up question...Why doesn't the oil-fired boiler require a liner?

Bruce Stevens_2

In many cases

oil requires them too actually more times than is actually practiced

Jim Davis_3

Power vent provides more consistant opeating efficiency on a steam boiler and less off cycle losses. It helps control the air to the burner whereas with the chimney every time the draft changes the fuel/air mixture changes.

ed wallace

p0wer vent vs direct bvent

I have to ask how is gas company sizing the boiler ?have they measured the connectd load?

Bob Harper

The chimney must meet the class of service regardless of fuel type. This is determined by a Level II inspection. If the chimney is unstable then it cannot be relined until that portion repaired.

The exhaust point is determined by codes and your nose. You must meet the clearances to openings into the building, public walkways, etc. and be above any possible snow drifts. The State of Mass recently upgraded their code to require a min. 4ft above grade due to a carbon monoxide fatality from a blocked vent. With a chimney, it must be 3ft above the roof; 2ft above any point within 1- ft. horiz.

A chimney w/ liner works off negative vent pressure. As long as there is a draft, it will evacuate the gases. A power vented boiler with the blower at the appliance works off positive pressure, which forces the gases out through hopefully tight pipes. The external power venter works off negative pressure so if any vent pipes leak, they will entrain room air rather than leaking fumes. Notice they all are dependent upon tight joints and things working according to plan. Nothing is guaranteed. Whomever plans and installs your vent must take all factors into consideration. Can you meet the clearances off the vent connector to combustibles? Termination clearances? Adequate makeup air? A power venter gobbles up large volumes of air. How tall is your chimney and how strong the draft? If you have a weak natural draft, you may lean towards the power venter as long as you can provide adequate makeup air.

The external power vent relies on sensing adequate negative vent pressure to close a contact and allow the ignition sequence to continue. If anything impairs this sequence, it should fail to light off. With a chimney, you are relying on natural draft. Should that fail, the exhaust can spill our the vent or boiler. There are some safety controls you can install to increase your protection but with either unit, nothing is guaranteed.

A metal liner heats up very quickly so I don't see the fuel/ air mix being a problem. There is a barometric draft regulator to insure it doesn't overdraft and it reaches the min. draft within a few moments of firing. The power venter has to be set and can over=draft as well and not everyone is familiar with that simple procedure or forget it.

If yuo have a gas fired water heater, it should be common vented with the boiler into a liner. If you power vent the boiler, the orphanned water heater will need its own smaller separate liner. Power venting with a gas water heater in the Combustion Appliance Zone is asking for CO spillage. See Worst Case Depressurization Test below. You may also consider installing an indirect hot water tank instead.

Once it is all done, you should have a complete combustion analysis, tweak the unit within mfrs. specs., perform a Worst Case Depressurization test and verify no CO spillage. Then install a low level CO monitor--not a UL listed CO alarm.

HTH,
Bob

stacie b_2

Thanks for the thorough and informative response, Bob. We are in the Boston area so we would require the 4 feet to meet the code, if we decided on a power vent. It does not seem feasible to use a power vent because of the location on the house. Anyway, I had a chimney cleaning today. The bad news is the clay flue (currently used by the oil-fired boiler) is crumbling. So I need a liner regardless of whether we convert to gas or stay with oil. It is a 30 foot chimney, 7" square flue. The chimney cleaner suggested using a tool to break up and remove the flue before installing the liner; another chimney person suggested leaving it in there. Any opinion?

J.C.A._3

I would ....

1st, get the spec's for the intended new equipment, and follow the directions that the manufacturer recommends.Venting of appliances is now the biggest portion of most manuals... Then I would find the right installer (NOT necessarily a chimney sweep or check guy...but a liner guy) and see what he/they think.

Maybe the equipment can fit inside the existing liner...maybe it will need to be a bit bigger, but the right person and the manufacturer are the ones who should decide that.

This is nothing to be taking chances with. The wrong job could void any warranty that the manufacturer offers. Pretty big deal in MY book. Choose carefully! Chris

Bob Harper

Nelson I would check their credentials carefully. Make sure their insurance includes completed operations and is at least 1 million per. Workers comp. for any employees, etc. Pull permit and make sure contractor holds all licenses required.

Most appliances require liners smaller than your typical heater flue. Oversized high mass chimneys and liners rob heat from the stack, which hinders draft and can cause condensation, which erodes the flue lining. Some mfrs. get specific on the liner sizing while others refer to the codes. NFPA 31 for oil requires the chimney connector be the same size as the appliance collar right up to the chimney, where it can then be downsized. They provide tables in the back, which, though not part of the actual code, are there as guidelines to assist you in sizing.

The issue of removing flue tiles is debatable. If they intend on installing a pumped in liner using the bladder or vibrating bell systems, you must remove the tile. If you intend on using vermiculite based insulation, you may or may not remove the tiles. Still others simply drop the liner, seal both ends to trap the dead air and create a "Dewar's Flask" effect or Thermos bottle. Check with your jurisdiction to see what they require for the liner and contractors credentials & permits.

If the chimney is unstable, it must be rebuilt. Liners are not a repair for unstable chimneys. If it is relatively intact but could use a little structural support, the pumped liners do strengthen them. However, being thermal mass liners, they take a long time to heat up but once warm, hold a stable draft. If your appliance(s) fire with relatively long standby times, the metal liner will heat up much quicker thus affording a faster draft with less chance of spillage. Pumped liners are great for long term continuous burning such as wood stoves and Rumford fireplaces.

When planning the installation, you want the water pipes out of the way of the flue pipes so you can get a nice vertical rise before turning as directly as possible to the chimney breeching. Make sure they place the test hole for combustion gases at least 1.5-2 duct diameters away from the appliance collar and any elbows or tees so when they perform a combustion analysis, they get an accurate sample.

HTH,
Bob

Phil_17

chimney

Had the same problem with the old boiler running so cool that the condensate ate out the mortar in an unlined chimney. Went with a modcon boiler that sends the vented gases up the PVC pipe that we put in chimney and gets its air from a PVC pipe that comes down the chimney. The use of SS to line the chimney would have cost me almost what the new boiler cost (>$16/ft + install). The pros are confusing you by too many suggestions. Look at the new hi-eff boilers and the simple ways to vent them.

[Deleted User]

buzzg.

You got a mod/con steam boiler?

Brewbeer

Is this a "free" boiler that the gas company is giving you because you are converting from oil?

Nelson_4

They discount the Burnham Independence, but you have to pay for service from the street, tank removal, and installation.

Nelson_4

Bob,
Thanks again for another informative and interesting response! I am getting an estimate on the chimney tomorrow and will ask him about his insurance, references and permits. Everything relating to the liner is pending my decision on Gas vs. Oil.