Mod/Con and old oil boiler controls

BenjaminMadore

I'm working on a plan to upgrade my parent's system.

I've done some homework, and some work previously on it. I'm considering a Tekmar 265 to run a natural gas Mod/Con and an existing 80's coal-fired boiler converted to oil. I'd like to set up indirect HW as well.

I'd like to hear if anyone has experience using this controller with a Mod/Con as the primary boiler and a cast iron as a secondary.

The house was built in 1789, it's a solid masonry structure with a full basement and two additions. The main structure was originally one room deep and two stories. A third story under a mansard-style roof was added at some point, and has been insulated, heavily on the top. An addition that was rebuilt several times up until the 1970's and 1-1/2 stories is on the back.

The main part of the house was retrofitted with a gravity hot water system sometime after the third story was added. The back addition has a single serial loop. All radiators are cast iron.

I have already re-plumbed the boiler for primary-secondary pumping and am using a Taco zone controller and two round Honeywell thermostats to control it. (The original loop in the addition was plumbed in to the gravity system as if it were a single radiator and had a booster pump on a thermostat.) For the first time I can recall the house heats evenly. I have left tees for the Mod/Con and the indirect already in the system.

The boiler is 180,000 input BTU, but is downfired and has no trouble keeping up. Depending on assumptions about the walls, I'm expecting 60-80,000 BTU loss on a cold day in central PA with the house staying above 66°. Gas wasn't available when the boiler was installed, but it's plumbed in now.

bio_guy

Why in heaven's name do you want to keep the oil boiler going?

SWEI

60k is pretty close to the smallest mod/con from many manufacturers. How are you 'expecting' this heat loss number?

BenjaminMadore

I measured the house, windows, and doors and put the numbers through a couple of heat loss calculators. The house is fairly vertical so there's not a lot of roof to lose heat through. Without modernization and insulation, and with heating to 72° it would be around 110,000 btu.
I'm not targeting 72°, the thermostat is seldom set above 64° and the current boiler is capable of making up any difference on the coldest days of the year. The mod/con would only carry the average heating load. The oil boiler is there, in good condition, and it would cost some money to remove it and the two large tanks.

I practiced the calculations a few times including a friend's house built in 1910 with an original gravity system and no insulation then checked it against the installed radiation and came within a couple of thousand BTU. (It was a beautiful install, radiators were sized to each room by sections.)

The installation instructions for the Tekmar don't reference mixing mod/con and atmospheric boilers.

Zman

http://tekmarcontrols.com/images/_literature/284_d_06.pdf

This one looks like it would do it.

Why do you want both connected and controlled together?
The mod con gas will be far more efficient.
You could just turn one boiler off when not needed.

Don_197

Lochinvar has a control that is designed to do this. Using a standard efficiency boiler when it makes sense to do so (water temps above condensing) and switching to the mod-cons when the heating water setpoints are in the condensing range. From the very beginning, Lochinvar saw the benefit to bidding multiple boiler jobs with mod/con being a PART of the plant and proposal, and standard efficiency being utilized when the water setpoints reached out of the mod/cons efficiency range. Pretty damn smart. Talk to your local Lochinvar rep......as I never bought one or used one. (I would just sell multiple mod/cons lol!)

SWEI

It's a great strategy on larger commercial jobs, especially when the conventional boiler is already installed and paid for. The added cost of the controls to do this on a residential job is more than the incremental cost of a larger mod/con IME.

bio_guy

I saw a study that included this strategy, among others, for retrofitting homes. I believe it was done for Minnesota homes by a MN state agency. Unfortunately, I can not find it right now. It should be applicable to other regions.

clammy

The only thought i could have is sizing to that low a inside temp may be fine for yourself bout what about the next owner.I think i would size it to a 70 inside temp .I also would be sure to install flow chk on the boiler supply and returns it sucks when when thermal migration lets your running boiler keep the other one warm.One smaller note would be to install a good magntic dirt seperator to keep iron oxide and sedimant out of your mod con heat exchanger.Personally i have taken to using hydro seperators when its warranted and now with a built in magnetic dirt seperator it kills 2 birds with one stone .As for your indirect i would have it piped and wired so your mod con is its heat supply, that will definetly cost less to make hot water then your oil and as mentioned early i always install flow chks usually and internal flow chk in the circ pump and a standard flow chk on the return side that keeps your tank from migrating durning the off months (summer) and bleeding out when using outdoor reset and low water temps (100)in the winter it also helps your indirect from over heating ,got to remenber the law of thermal dynanics hi goes to low weather it s pressure or temperture The other side of sizing to a 70 or 72 degree inside temp is now you will have some extra btu s for production of hot water and being from what your existing radition is comprised of i don t think that your miniun fire for your mod is more then the smallest zone .Its one of the things i look at when using mod cons and trouble shooting issues with there operation and short cycling Sorry for the semi rant hope this helps u out ,on the chk valves most mod con p/s diagrams from the manafactures used to show and suggest them but they seem to get not get installed alot and it sometime created issues peace and good luck clammy

SWEI

bio_guy said:

I saw a study that included this strategy, among others, for retrofitting homes.

Again, the incremental cost of going one or two sizes bigger on a residential or light commercial boiler is less than the cost of the controls to manage two boilers. Side note: One or two sizes bigger is almost never necessary in residential applications. It's much more common for us to be wanting a smaller boiler than the smallest one made.