Can I Use The Same Installer For Venting?

[njb52]

We will be replacing our existing over sized boiler with a new higher efficiency boiler and want to reuse out existing chimney flue to vent the exhaust. Since the new boiler will produce lower exhaust temps, it was recommended that we use a 4" liner for the existing flue with 4" B-vent between the boiler and chimney. Can a heating contractor/plumber do the new liner and associated ductwork install, or would I need a chimney sweep to do that work and get separate pricing? It would be nice to have one trade involved so that if any future problems occur, the original installer can rectify them.
Thanks in advance.

 

[Dana]

Most boiler installers in MA are capable of installing flue liners. Those that aren't are probably out of business by now!

What boiler are you installing? (or do you not know yet?) Have you done a Manual-J or a fuel use/heating degree-day load calculation for sizing the boiler?

FWIW: Some of the small fire-tube mod-cons are priced right in there with small cast iron. If you can get a MassSave rebate (and I think you can), it starts to look like a no-brainer to go the condensing boiler route.

[edited to add] Never mind, I found the prior thread where we came up with a ~36KBTU/hr heat load, etc.

 

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[njb52]

Thanks Dana for such a quick reply to this thread. For some reason I did not get any email notification for your response to my other thread in March, which I just read and appreciate. Thanks also for the info on making sure that the flue work is spelled out in the quote, so I can be aware of any attempt t o raising the quote, since you said it's a requirement under MA code. I appreciate your opinion on a system bypass vs a boiler bypass as well.

I will now be measuring exterior wall and fenestration areas to help a potential contractor do a calculation quicker and more accurately.

 

[Dana]

If you have a fuel use load calc that indicates a heat load in the 36,000 BTUhr range there's really little point to doing a Manual-J as well.

Even the smallest mod-cons out there will cover that load, very few cast iron boilers that wouldn't either. Any 50K (input or output) boiler covers your design temp load with margin. With your load and radiation any one of the following would be a better choice than any cast iron beast, and may even be cheaper to install:

Boiler Model Minimum input Maximum input

NTI Trinity TX51 7100 BTU/hr 57,000 BTU/hr

Navien NHB80 8000 BTU/hr 80,000 BTU/hr

HTP UFT-80W 8000 BTU/hr 80,000 BTU/hr

Lochinvar CDN040 9000 BTU/hr 40,000 BTU/hr

Despite the stubby radiation of your six micro- zones the overlap of calls for heat would be high with any of the above if you dialed them in. With a total of 185' of baseboard and a 36,000 BTU/hr load you would be able to run in mid-90s combustion efficiency even under outside design conditions. If any two zones are calling for heat there is sufficient baseboard to balance with the minimum output of the above boilers.

With a non-modulating cast iron boiler (even if right-sized for the whole house load) you would be short cycling the thing on zone calls unless you combined zones unless you combine some zones. If you insist on cast iron about the best you'll do for right-sizing is the W-M CGi 2.5. But the 42,000 BTU/hr of 190F output needs a minimum of 70' of baseboard running to not cycle at all, and at least 50' to be able to keep it from short cycling. With a bigger boiler than that the cycling problem grows. None of your zones are sufficient on their own:

From your other thread you have:

• Zone 1 - Downstairs office one, bath & hall (640 sq. ft), baseboard 24 lineal feet

• Zone 2. - Downstairs office two (504 sq. ft), baseboard 38 lineal feet

• Zone 3 – Upstairs Master BR & Bath, baseboard 25 lineal feet

• Zone 4 - Upstairs LR, DR, Main Bath, & Kitchen baseboard 42 lineal feet

• Zone 5 - Upstairs bedrooms 2 & 3, baseboard 28 lineal feet

• Zone 6 – Future expansion to unheated loft/attic, projected baseboard 28 lineal feet

 

[njb52]

Thanks, Dana, for the info and the specific unit recommendati0nds. I'll let then use the fuel consumption for pricing and then have the contractor that we select do a more accurate calc.

BTW, am I correct in assuming that the new metal flue liner needs to be insulated in order to keep the exhaust warm enough to fully vent up the chimney?

 

[Dana]

Gas appliances flue liners don't need to be insulated in order to vent. The down-sizing of diameter guarantees sufficient stack velocity. If it's an exterior chimney there may be a measurable difference in the amount of flue condensation if the liner is insulated, but if you right-size the boiler the duty cycle will be high enough that it's not really a problem.

A Manual-J is rarely more accurate than at fuel use derived load calculation, since the latter is a measurement, and the former has too many unknowns. Running the load calc at both base 60 and base 65F and over an entire winter (late December to late March, with no shoulder season fuel use included) will reliably bracket the range. A fuel use calculation also already includes the distribution losses, etc. If there is a large variance between the contractor's load calculation and a fuel-use derived calculation, trust the fuel use calc. Unfortunately most HVAC contractors don't do these calculations well, though some do. If you want an accurate Manual-J you'll have to hire a professional engineer or HERS rater or some other energy-nerd outfit who are only selling the accuracy of their numbers, not HVAC equipment.

 

[njb52]

Thanks for the clarification on the flue liner not having to be insulated. I don’t know if I mentioned before that I will also be using the domestic hot water existing 4 inch exhaust connected to the same new flue as well. Does that appreciably effect the new flue liner sizing or is it fine just staying at 4 inches?

I greatly appreciate the insight into the fuel use calc vs the contractor probably not doing a manual J calc. I will give him the historical fuel data that you used and compare his calc to the one that you graciously did for me to see if he is close or way off.

 

[Dana]

You'll be better off from an efficiency and capacity point of view using the boiler to heat the domestic hot water than sticking with a standalone atmospheric drafted unit.

With a forced draft boiler or forced draft HW heater you may or may not not be able to share the flue- it depends.

If both are atmospheric drafted you can share the flue if all code protocols for that are followed. It's unlikely that upsizing from 4" would be required if the boiler is right-sized to your load. If the top of the chimney is even 20' above the top of the hot water heater a 4" liner is good for something like 80,000-90,000 BTU/hr of boiler input. If taller, the capacity is higher.

The installer is legally on the hook for getting the venting right, but it's not a bad idea to figure it out the sanity checking ahead of time. It's your house and health on the line, after all.

 

[njb52]

Thanks, Dana - I'll be sure to pick their brain when they come to check out the scope of work for their pricing.