Resolving Draft Issues

[NTL1991]

There's a bit of a backstory here... Years ago my 3-family home (1948) had 1-pipe steam with a WM SGO-3 oil boiler and a 75 gallon NG water heater. Everything was separated for individual metering; plumbed the three apartments in PEX with baseboard heating loops, installed boilers and individual 40 gallon NG water heaters, re-piped all the hot fixtures in the house, and everything seemed right in the world.

We started off by replacing the single oil tank with two half-size's, converted the SGO-3 to a WGO-3 for the 1st floor, Installed the LAARS Newport NP-85 oil fired for the 2nd, and a small Superior NG boiler for the 3rd.

We have since replaced the WM oil and Superior NG with matching WM CGA-25 NG boilers, which are much better sized for the small 1000 sq ft apartments. The LAARS oil is still there for the 2nd floor. It will eventually be replaced with a NG boiler too when it's time comes.

The conversion is coming back to bite us, though, in the form of draft issues with the chimney. More fuel-burning appliances, more openings in the chimney, and the house is getting tighter with weatherization. I had an energy audit done with RISE, and all my insulation work is dependent on fixing some low draft issues.

They reported the following draft issues:

The first thing they recommended I do was add a combustion air inlet. I did, in the form of 2 - 7" round galvanized vents and insulated flexible duct to a foot above the floor near the appliances.

I think if I can replace those three water heaters with either indirects or sealed combustion units, I should have sufficient draft for the three remaining boilers. I'm not sure of sizing limitations with those small WM CGA-25 boilers, so I've posted another topic about Small Boilers and Indirect heaters.

Any advice?

 

[Cacher_Chick]

It would seem to me that replacing the water heaters with direct-vent or power-vent models would eliminate several pieces of the puzzle. In some areas the cost of electric is also competitive, making for another possible option.

 

[Sponsor]

 

[Dana]

It would seem to me that replacing the water heaters with direct-vent or power-vent models would eliminate several pieces of the puzzle. In some areas the cost of electric is also competitive, making for another possible option.

There are cheaper/better options:

1: Replacing the water heaters with electric heat pump water heaters would have an even lower operating cost than natural gas, even at high RI type electricity pricing, and would have the beneficial effect of drying out the basement during the sticky summertime weather, and "harvesting" the jacket losses of the boilers during the heating season, lowering the temp in the boiler room a couple of degrees. This might even be cheaper than option #2:

2: Replacing the water heaters with indirect fired water heaters would have somewhat higher hot water delivery performance, and better overall net efficiency on the boilers + hot water compared to separate direct vented (or atmospheric drafted) water heaters.

3: When it comes time to replace the remaining oil burner it might make more sense to replace it with a stainless condensing gas water heater like HTP's Phoenix Light Duty, with a 76KBTU/hr burner and an isolating plate type heat exchanger to run the space heating loop:

That would have even better hot water delivery, and higher net heating+ hot water efficiency for the ultra-small space heating load of a 1000' apartment that has heated apartments both above & below it. This type of system is very simple to design, and the thermal mass of the water in the water heater keeps the modulating (25K-76K BTU) burner from short cycling despite the very low space heating load. To make this work at max efficiency would require calculating the heat load of that apartment and measuring it's radiation, but it's not rocket science.

Even the tiny CGa-25 is overkill for the space heating loads of 1000' apartments, and may not have sufficient thermal mass to avoid excessive cycling unless there is a lot of thermal mass in the radiation. Are the systems using converted steam radiators, or has it all been replaced with low mass fin tube baseboard? If the latter, how many feet of baseboard is in each apartment?

Measuring the burn times and duty cycle of the existing CGa-25s would give some idea as to whether it's "worth it" to retrofit them with heat purging economizers, even without knowing the particulars on the radiation.