Is oversizing a big problem with a modern boiler with a modern aquastat and controller?

roserez

I live in New York and have a 5600 sf. (plus 700 sf. cellar) house currently heated with a Burnham V8 8 section boiler. The block is cracked and I'm leaning toward replacing it with a Buderus G215 series. The Burnham has 239,000 net BTUs. The 6 section Buderis is 223,000 net BTUs and the 7 section is 256,000. The house is 100 years old, stucco, 3 floors plus cellar with a ton of window glass area. Main heat piping is 4" cast iron. We have only owned the house for only 3 years.

The Burnham, depending on the outdoor temperature, might cycle on for 5-15 minutes, off for 5-15, on for 5-15 etc... which I assume means that it isn't working excessively hard. However, I'm nervous about replacing it with the slightly smaller 6 section Buderus because there is about 1000 sf. of the house that is under renovation and has never been heated during the time we've been here. I have no easy way of knowing how this will impact things. I don't yet know what the space will be like in terms of doors, windows, type of heat (it might be radiant floor). If I get the 7 section Buderus at 256,000 BTUs, it will likely be a bit big but is that a such a bad thing in a modern system? I've read a lot about the problems of oversizing a boiler but is this advice outdated? My understanding is that intelligent aquastats and controllers can adapt based on water temperature, outdoor temperature, cycle times, etc. Does this mean that it's now okay to err on the side of big with such a system like the G215-7? I'd hate to find out, after completing the unfinished sections of the house, that I went too small so if there is no harm in bigger, I'd like to play it safe.

Thanks!

- Mike

Jamie Hall

The only proper way to do this is to figure the actual heat loss of the building which you are trying to heat -- which includes the new addition -- and size the boiler based on that. There are a number of heat loss calculators available. Others will comment on their preferences...

roserez

Thanks Jamie. I know that I need to do that if I want to know how many BTUs I need. What I'm trying to determine is if I get too many, is a serious problem for a modern boiler/controller. My future needs will change. Up, as I try to heat more space, down, as I make things more efficient. A CORRECTION: the main piping is 4" cast iron. Thanks!

Robert O'Brien

Over sizing is never good, do the heat loss and don't get bigger than needed. I assume this is a converted gravity system with that size piping? The boiler needs to be protected against cold return temps.

roserez

Thanks Robert. Smaller than needed could be a disaster and I have no way of knowing what the load will be on the unfinished space. I don't know what I'm going to be doing there in term of doors and windows. I might use radiant floor heating. Regardless, the system would certainly be oversized until that space is done. I'd like to find out if today, that's as bad as it was in older systems or if smarter controllers alleviate the problem. Thanks!

Jamie Hall

There are two problems with oversizing for hot water. You will either have to cycle the boiler on and off -- which is less efficient -- or your return temperatures may be too low, which is very bad for the boiler. Size it as closely as you can to what you think you will do in the unfinished space.

The large diameter pipes from your old gravity system make no difference at all to sizing the boiler. It's all about matching heat loss to heat input.

lchmb

an over sized system for a short time is no big deal..unless that time is 20 years.... I would do a heat loss using the worst case windows and doors I would plan to use and go from there...you say you plan to increase insulation but sadly you cant figure two years from now cause you may freeze in between..

I would pay close attention to my boiler piping, whether you use a bypass or some other strategy but 4" pipe holds a lot of cold water...

roserez

Thanks Jamie. The reason I mentioned the size of the pipes is because the high water volume creates a lot of thermal inertia so the water does not return cold to the boiler. My understanding is that one of the selling points of the Buderus is that it is extremely tolerant to rapid temperature swings. Smart controllers don't help the boiler cycling?

lchmb

I am glad I dont have to lug either boiler in or out...

roserez

Ha ha. That's why my plumber told me to call someone else!

Zman

Your system volume and mass will really help with oversizing issues. I would still not grossly oversize the boiler.
At the same time, do be careful of boiler condensation. You will frequently satisfy the t-stat before the boiler comes up to temp. With a non condensing boiler, I would suggest an ESBE valve.
Have you considered a condensing boiler? They are a nice for systems like yours.

KC_Jones

roserez said:

Smart controllers don't help the boiler cycling?

My question to you is how would it do this?

The boiler in question has a fixed output, it's either on or it's off. When it's on it's putting out a fixed BTU. If the thermostat is satisfied the boiler shuts down. If you are talking about a mod/con boiler that is a completely different scenario as those can change their output (within specified parameters).

I would also suggest the possibility that even the "smaller" 6 will be oversized for your application.

As far as heat loss, you need to do it. The differences between different windows in the remodel will make a negligible difference to the numbers as long as you are talking modern windows of some sort. Your comment about not knowing the load in the new space makes no sense, this is the calculations you need to do. Even if you assume worst case scenario (no insulation and the windows are original without storms) you need to run the numbers. There isn't any way around this.

If it was me I would make sure I had a comprehensive plan in place for the entire remodel and then you can do a fairly accurate heat loss calculation and know how to proceed. Honestly I don't quite understand how a remodel was started without a complete plan in place already? Especially at the level you describe. Are you doing the remodel work yourself or is a contractor doing it? Also is the 5600 counting the 1000? Meaning is the whole house 5600 or 6600?

Brewbeer

If you go with a smaller boiler that *might* be slightly undersized, it's only going to be noticeable a couple of time per year, and even then it's not going to be a disaster (e.g, 62F inside instead of 70F for a few hours). Having a slightly undersized boiler and a secondary heat source like a fireplace insert would be a good set-up.