Boiler Setup for 5 Zone Baseboard

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I'm looking to replace my 40 yr. old Peerless oil fired boiler. My main interest is improving efficiency of the system.

Current boiler is 130,000 BTU rated, and cycles frequently based on heat calls from each of the 5 zones. Slant-Fin calc says my heat loss is ~60,000 BTU @ 10F (Maryland) so i'm probably 2X oversized on the boiler and also based on that calc i have 2-3X the required baseboard length in every room except 1 that is a little short (vaulted ceiling, 2 sets of older french doors to exterior and adjacent to un-conditioned garage). I currently have no outdoor reset or any fancy controllers.

I did replace the two original 1/4HP circulators with Taco 007e's which are amazing but thats somewhat beside the point.

DHW is handled separately although my existing system has a tankless coil. I don't need DHW from the boiler with the new system.

After a bunch of research around the web it seems like the setup I have is very inefficient, especially in terms of the zones. Ideally (I think) I'd like to see a system where the boiler keeps a hot water buffer tank at a temperature controlled by an outdoor reset and the zones draw from the tank rather than the boiler. Then the boiler only focuses on keeping the tank at the set point. That seems like a reasonable way to increase the system mass sufficiently to reduce the short cycling from randomly timed zone calls.

It seems like that is an atypical setup. Does the forum have any thoughts or guidance on how to get a better boiler setup in my case?


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Slantfin's calculator ALWAYS overshoots reality, often by 35-50% (sometimes even more).

Since you have a heating history on this house, the first step to getting a handle on the heat load requirements is to run a calculation of fuel use against heating degree day data from a local weatherstation, using the old boiler as a measuring instrument. Use ONLY wintertime fuel use, and ignore the domestic hot water error-it's cancelling out the solar-gain error. Winter is when both of those error factors are lowest relative to the actual heating energy use.

Then calculate the approximate average water temp needed to meet that load with the existing baseboard. Use the Slantfin 30 chart as a guide to performance across temperature, which is pretty typical for residential baseboard..

With an oil boiler there isn't much efficiency to be gained going to ever-lower water temperatures the way there is with condensing gas/propane. If it turns out the water temp requirements are going to be under 140F (pretty likely) you might consider using a "reverse indirect" for the buffer tank, unless you're already using an ultra-efficient heat pump water heater or something(?).


Keep the tank temp fixed using dumb aquastat controls, which un-complicates the boiler/buffer control setup, not outdoor reset. If for comfort reasons you prefer outdoor reset, use a thermostatic mixing valve with outdoor reset to to control the supply temperature to the baseboard. The entering water temp at the boiler has to stay above 130F at all times even if you have a stainless flue liner and a cold-start boiler (some require 140F) to prevent destructive condensation on the heat exchanger plates, and that becomes harder to pull off with good control with a buffer tank that might drop to 100F under outdoor reset control.

With a buffer tank set to a fixed temp the tank is the boiler's only "zone", and with a sufficient high/low differential temperature on the buffer the burn times can be maximized, and total number of burns minimized. A boiler with a heat purging "smart" control does even better when it has the thermal mass of the buffer to work with.

Be aware that below 120F the output of low-rise fin-tube convectors like baseboard is very non-linear. Even though there are charts that indicate an output at 110F or lower, take those with a huge grain of salt- dust kittens, pet hair and drafts can play havoc with the calculated vs. actual heat emittance when water temps are that low.

BTW: There are very few locations in MD where the 99% outside design temp is +10F. Some tools will select the 99.6th percentile temperature bin, which is usually a handful of degrees cooler than the 99% temp, but for an insulated house with double-panes (or storm windows) there is no point to designing to the 99.6% temperature. No matter what it's highly likely that even the smallest oil boiler is going
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