Replacement Boiler Advice

Eric Peterson_3

I'm in the market for a replacement boiler.
My house is a three-zone (valves) hot-water arrangement with a mixture of baseboard and free-standing radiators (all cast iron). It's a converted gravity system (with some additional non-gravity zones added). To accomodate higher efficiency boilers I'll be replacing the atmospheric venting with PVC venting, but out the rear of the house rather than up the chimney chase.

I've been looking at the Lochinvar Knight boilers. Any reason that these would not be a good fit for my application? I've also considered the Munchkin and Revolution boilers.

Wall thoughts?

Thanks,
Eric Peterson

Steamhead (in transit)

First do a heat-loss calculation

then you'll know how small your boiler needs to be. Only then do you start considering which boiler to use.

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Brad White_104

To add to Steamhead's advice

(spot-on as usual- start with a heat loss before all else)...

I assume you are looking at a simple boiler replacement as opposed to re-piping and re-radiating your house. If that is the case, I would recommend a constant-circulation system with TRV's and outdoor reset. You are fortunate you are all cast iron, that is IMHO, the best way to fly if you have radiators.


Mod-Cons are ideal, just go with the one best supported by your installer of choice.

Eric Peterson_3

heat-loss

Well of course a heat-loss calculation is needed to size the boiler - I did one a few years ago and will have to look it up, believe it was in the range of 140,000 BTUs (output).

The Knight boiler has some attractive features, I was just looking for feedback / experience per its suitability in my type of situation.

I do use TRVs but only in one zone. At present my house is pretty well balanced so yes I am just planning to replace the boiler. Repiping and balancing were done a few years ago.

Mike T., Swampeast MO

I'd suggest reviewing that heat loss. Unless home is very large and/or very poorly insulated and/or in an extremely cold climate, 140 mbh is an awful lot of condensing/modulating boiler power... By all means resist the drive to over-compensate when conducting the loss calculation and to oversize the boiler when selecting.

I'm going to guess that the TRVs (since only in one zone) are there because there was an overheating problem in that zone. The zone with the standing radiators?

After conducting your room-by-room heat loss, compare the loss in each room to the output of the radiation. I'd HIGHLY suggesting using output at 160° as an absolute maximum. If you find at least one zone of good size that has a consistently higher- or lower-than average amount of radiation, consider a two-temp system with mixing valve. If any one zone or room is radically undersized compared to others, considering adding some radiation.

Don't forget that the efficiency of any condensing/modulating boiler will be determined by the zone with the highest temperature requirement. If you can keep required supply temp below 140° in all but the coldest weather, you can expect extraordinarily efficient operation. Such temperatures are certainly not uncommon in a system such as yours provided that the added iron baseboard was sized quite generously.

Eric Peterson_3

TRVs

> I'd suggest reviewing that heat loss. Unless

> home is very large and/or very poorly insulated

> and/or in an extremely cold climate, 140 mbh is

> an awful lot of condensing/modulating boiler

> power... By all means resist the drive to

> over-compensate when conducting the loss

> calculation and to oversize the boiler when

> selecting.

>

> I'm going to guess that the TRVs

> (since only in one zone) are there because there

> was an overheating problem in that zone. The

> zone with the standing radiators?

>

> After

> conducting your room-by-room heat loss, compare

> the loss in each room to the output of the

> radiation. I'd HIGHLY suggesting using output at

> 160° as an absolute maximum. If you find at

> least one zone of good size that has a

> consistently higher- or lower-than average amount

> of radiation, consider a two-temp system with

> mixing valve. If any one zone or room is

> radically undersized compared to others,

> considering adding some radiation.

>

> Don't

> forget that the efficiency of any

> condensing/modulating boiler will be determined

> by the zone with the _I_highest_/I_ temperature

> requirement. If you can keep required supply

> temp below 140° in all but the coldest weather,

> you can expect extraordinarily efficient

> operation. Such temperatures are certainly not

> uncommon in a system such as yours _I_provided

> that the added iron baseboard was sized quite

> generously_/I_.

Eric Peterson_3

return water temperature

Heat loss is from one computer ago - will have to search for it or recompute.
What I did not calculate was the heating capability of the radiators in each room. The free-standing ones are certainly oversized for their rooms, and the baseboard I added was generous (except for one bedroom in the attic).
To date I have used setback thermostats. This has the effect of having the system shut off for hours, perhaps all night, only to then work hard in raising the temperature four to six degrees when the setback ends. This does cause the water temperature to hit the high limit of 160° output, the return temp is about 140° which seems to be the upper limit of return water temperature for efficient operation of the Knight boiler.
So now I am thinking that if I simply give up the setback, or reduce it, I may actually save money on fuel.
Of course there is the extra cost of a modulating boiler to consider. Also I doubt they will last more then 10-15 years. However they are light, easy to install, and the 100' allowed for venting (at least for the Knight) would make my installation viable.

Brad White_105

Setback versus Reset

Just to be clear on the two different concepts which you may -or may not- be confusing, Eric, but also for any readers new to this:

"Setback" is the reducing of a space's/zone's temperature at night and/or when unoccupied. This of course requires a temperature boost when and before occupancy again occurs. You save whenever the system does not run at all of course, and save when it maintains a lower temperature. You do spend some but not all of your savings on the "morning warm-up", so you probably come out ahead. Many variables play into this strategy and how effective it works for you at saving fuel.


"Reset", or better termed, "Outdoor Reset" is the automatic adjustment of the heating supply water temperature indexed to the outside air temperature. This is a very precise way to heat a home once set up properly and this is where the most money is saved.

Some system owners forego setback entirely in favor of very precise and deep/agressive reset schedules. This is typically with modulating condensing boilers. Just wanted to delineate the terms, that's all.

Eric Peterson_3

setback

Thanks for the clarification. Setback was indeed what I meant.

The term "outdoor reset" has always seemed to me to a very badly named concept, since reset usually implies a restart of a system from a set of known initial conditions. Like when you reset your computer.

"outdoor reset", though, to my understanding, is the continuous monitoring of the temperature outdoors and the corresponding adjustment of the heating system to those conditions. It only makes sense if tou think of your system as continually resetting.

Brad White_9

I can see that, Eric

The term "reset" has been used in the HVAC controls lexicon well before my time. Resetting of chilled water supply temperature based on return water temperature and resetting of discharge air temperature based on space return temperatures are other common applications.

When automatic reset was first introduced, I am guessing in the 1920's when pneumatics became a standard, it was also at a time when every building had a resident engineer who would constantly tweak and adjust setpoints for the comfort of his occupants. Indeed, continual resetting is exactly what happens.

That job involved anticipation, knowing how long it took his building to warm up after a cold night and at what temperature it was, when to start the process in light of that and when to stop the process to prevent overheating. Perhaps fuel was cheap but he also had to shovel the coal, shake the grate and rake out the clinkers, so he had a vested interest in minimizing his labors.

(I say "he" for to be realistic and with few exceptions, it was a "he". Unsung woman building engineers, forgive me.)

The notion of doing this all automatically must have had tremendous appeal to everyone -except the building engineer I imagine.