Dual Boiler Hybrid Series Design

Hello,

I have on order a Leisure Line 220K boiler for my home. I've been doing lots of reading on this site for how other people hook up their boilers. I am certainly not a plumbing expert by any means, but I have some thougts and observations.

I understand there are various multi-boiler designs. For the most part, people are going with Series, Parallel, and Primary Secondary loop designs (people are mistakenly calling this parallel) when integrating dual boilers in their homes. Series makes sense for keeping both boilers warm, but it may have some inefficiencies as far as a second boiler being warm all the time. It also provides a single point of failure. Parallel seems to be the most efficient, in my opinion, however you could potentially introduce leaks or corrosion as you have a cold boiler. Primary Secondary is a neat, high tech design, however I think it is overkill and inefficient for a home design. Especially if you have multiple 80 Watt circulators running when you only need one in a residential heating system. I'm also thinking in terms of being able to run your system on UPS or generator when you have no power. If you have a call for heat on 4 zones, 5 circulators running, that's 400 watts of power. It also has a cold boiler. These are my observations, and I welcome anyone to counter my thoughts.

I currently have 8 zones in my house. One zone is for a Boiler Mate, one zone (although it is deactive) is for the basement, three zones for the first floor, two zones for the second, and one zone in the attic. The house is 3300 SQ Feet. There is already a recent Burham oil burner in the house and I want to add a Lesuire Line 220K as a second boiler.

My plan is to go with a Hybrid Series design that I came up with. It is a series design which will use the LL 220K to warm the Oil Boiler, however it will utilize 4 3-way valves to allow me to isolate each boiler. It will have a single circulator that sits on the return side. Expansion tank and air purge will sit on the supply side. Please see the below drafts I came up with on my current and dual boiler design.


Current Design




Proposed Design



Obviously these designs are missing some valves and I missed the expansion tank.. I would consider a parallel system, but is it a real concern about a cold boiler leaking (mine doesn't today when it is cold) and is there any damage that could occur to the boiler in terms of corrosion from it sitting for years full of water but cold? Does anyone have any thoughts or comments?

Miner Greg

A boiler sitting full of water should suffer no damage unless it happens to sweat from extreme ambient temperature changes. This is a totally different problem from piping design.

I am not sure what you mean by a cold boiler leaking. Boiler should be able to sit cold without leaking. If they do, then they have some sort of problem.

I would pipe the boilers parallel with an aquastat rigged up to switch control from the coal boiler to the oil boiler in case of an outfire. Manual switch over if fine also, I wired in selector switches in my system.

Thanks a lot for the response. Like what you recommend, I was originally planning on going with a parallel design using an aquastat to fire up my oil boiler only when the coal boiler was cool. However my dealer and other users posting on this site have expressed concerns with an idle boiler sitting cold over a long period of time. I assume the primary concern was leakage (most likely through the tankless gasket or any valves - although my boiler doesn't leak when it's cold) and any potential deterioration from it sitting cold full of water. My basement is sealed, dry, and stays warm by itself. If the general consensus is that a cold standby boiler doesn't deteriorate (I honestly can't see why it would), I'd much rather stay with a parallel type of design. I will more than likely use the coal boiler all year long and will only use the oil boiler as backup. I want the design to be efficient and want to do it properly from the start.

For a dry sealed basement like mine, does anyone else feel there is any need to keep my standby boiler warm? If not, I will scrap the Hybrid Series design and just go with a less complicated parallel design.

Miner Greg

A series setup is simple and quick - but not the most efficient as you have some [ to a lot ] of standby loss on the so called cold Oil Boiler - depending on lots of things including but not limited to having a mechanical flue damper with proof of draft - that will sort of limit the parasitic loos up the stack - then there is the condition of the OB jacket insulation and the near boiler piping loss - OH ME

Don't get too comfortable with that OB sitting there full of [new] water -- once you get the system filled - all the O2 will need to be mechanically or chemically scrubbed out - if not it will drill where ever it can

Something you wrote in your first post haunts me " Primary Secondary is a neat, high tech design, however I think it is overkill and inefficient for a home design. Especially if you have multiple 80 Watt circulators running when you only need one in a residential heating system."

one -- if you "think" that - why are you asking if the OB will rot - this is not a statement expected from a casual reader

two -- P/S piping is very efficient for many reasons. Hands off fail over in my biggest, but you then also have the ability to limit the amount of energy moving out to radiation based on degree day load - This saving in fuel far exceeds the cost of one extra pump operation per season

but

it depends

Let me get back to you on that

I agree and understood that there would be heat loss from the standby boiler in a series system. My primary concern which prompted this thread was preventing corrosion of my standby boiler in a dual boiler design.

I know there are other factors such as hard water, but is the largest contributor of corrosion in a boiler the oxygen in the water from when adding fresh water to the system? Is the temperature irrelevant (other than the heating process which helps deoxygenate the water in a closed system)?

On one of your comments:

Don't get too comfortable with that OB sitting there full of [new] water -- once you get the system filled - all the O2 will need to be mechanically or chemically scrubbed out - if not it will drill where ever it can

Are you recommending just cycling the water in the offline boiler once in a while until all of the water in the entire system becomes mostly deoxygenated as new boilers would have been filled with fresh water? Or are you recommending something else?

What ever your comfortable with - only making the point of new water drama

unless your strapped for cash i would hook it up primary secondary, hands off back up, either boiler could be serviced while leaving the other online, the list goes on and on, the piping gets pricey because of all the t-s and a couple extra pumps, wire etc. but when your done you wont regret it. plan it out and do it right the first time and you wont be dissiapointed.

I would pipe it as a primary secondary system. If you are concerned about the primary loop pump operating costs, don't use one but use a passive hydraulic separator. Check out the separator products from Caleffi North America. There are a lot of advantages to be able to change boiler sources with a circulator pump on/off switch. If you do a primary secondary system put your coal boiler circulator down low on the return side.