Oil boiler conversion - Low mass or high mass gas boiler?

Macko

We have recently removed the existing 100+ year old oil system in our home, including the removal of a truly behemoth cast iron boiler. Home is about 4,000 sq ft above grade with cast iron piping and radiators.

I've spoken to two different heating specialist that have recommended very different systems.

The first option is a Viessmann Vitocrossal 300 Floor Stand Boiler. Being told a high mass boiler is necessary given the cast iron system and the amount of debris/build-up/etc. in the system that will be needed to be filtered out/managed. Did not recommend a low mass boiler for the same reason - suggested it would get clogged/broken in several years. Currently piping for outflow/return is 4" which will need to be reduced down to 2".

The second option is a Navien NCB 240/110 low mass boiler. Also suggesting we remove the two hot water tanks and move to a combi system. Did not seem concerned when I mentioned the potential debris from an old cast iron system and noted that water could be flushed through the system before re-starting it which should eliminate this issue. Piping would be reduced down to 1" .

Option 1 is more than twice the up front cost of Option 2, but would pay off in the long run if Option 2 does wear down after several years from the debris.

Any thoughts on the right approach here?

Many thanks.

mattmia2

Either can work if sized and installed correctly. I would put a magnetic dirt separator on the return before either boiler. I prefer an indirect water heater over a combi boiler because an indirect always has hot water, a combi either has a period of cold water or needs to have a keep warm function. The Viessman is a higher end product than Navien. Both will probably not last as long as a cast iron boiler but the Viessman is probably likely to last longer but it has little to do with the mass of the boiler and being connected to a converted gravity system.

Jamie Hall

Before you do anything... has anyone actually calculated the heat loss of the building to size the new boiler? I know it sounds obvious -- but simply replacing the old boiler with a similarly sized new one may not be correct. Without a heat loss survey, you just don't know.

Second, it sounds as though what you may have is a gravity hot water system. These can be converted to a pumped system -- but it takes more than just swapping out boilers. Have either of your contractors accounted for that?

Hot_water_fan

Viessmann makes low mass boilers too if you want a more apples to apples comparison. The high mass is irrelevant to the fluid cleanliness - you can filter it or even entirely separate the boiler from the system using a heat exchange if so desired. High mass helps with short cycling, but if the system is as large as you say, you’re probably able to avoid that. 

 Figure out the heat loss first - how much oil did you use last year ? 

Steamhead

@Macko , I'd look into a modern, three-pass cast-iron boiler such as the Solaia. These can be fired with either oil or gas burners, and should not need dirt separators. Go here:

http://boyertownfurnace.com/residential/solaia-boilers

Where are you located?

Macko

Thanks for the responses.

Jamie - We have not done a heat loss survey. With the oil boiler removed, there is currently no heat source, so not sure we could even do that now? It's an old building so I'm sure the envelope is not very efficient. Also, it is a pumped system.

Hot_Water_fan - it was communicated to me that the main reason for the high mass boiler was that it could more efficiently manage the debris in the system (possibly due to the additional water capacity / pipe size? - I'll follow-up on this). I'll also ask about the separate heat exchange. The high mass system is a lot more expensive, so lots of room to modify the low mass boiler system. We recently moved into the house, so unfortunately I don't have data on the oil use

Also attaching some before and after pictures in case it's helpful for additional comments.

Steamhead

Wow. That old boiler was probably 40% efficient. Whatever you choose will do a lot better.

Jamie Hall

While you can estimate heat loss from fuel usage, it is more accurate -- if you are careful with your inputs -- to use a program which takes the structure -- dimensions and construction -- as inputs and gives you the heat loss. Slant/Fin makes a very good one here: https://slantfin.com/slantfin-heat-loss-calculator/

It's easy enough to use that you can do it yourself.

EBEBRATT-Ed

@Macko

First of all, you need the right contractor. This is not a run of the mill conversion, so the contractor is most important. If you take the lowest price, you are sure to be unhappy.

The first thing is a heat loss as @Jamie Hall mentioned why would you want a contractor to "guess" at the correct size boiler?

Any dirt/rust in the system can be managed with the correct install regardless of the boiler used. I would steer away from Navian.

I prefer conventional boilers are my preference, but a condensing boiler makes sense in your application.

It would have been better to tighten up the building envelope first, insulation, windows and doors but it s what it is.