Shoulder Season Discomfort

SweatyInToronto

Dear Heating Experts,

Our building is a 4-story walk-up circa 1955, with a partially underground basement and 21 units in total.
located in Toronto Canada.

Heating is very good during the long winter though can be uncomfortably warm during the shoulder seasons.
Baseboard heating, rather long taking up the full length of 1-2 walls. Probably cast iron but not certain. A few typical height radiators in the stairwells.

There are 2 natural gas atmospheric 250MTH Hydrotherm boilers about 15 years old, Cast Iron HEs I'm told.
A Tekmar 261 control with outdoor reset of 4F and WWSD of 58F currently.
WWSD was 65F however I lowered it because of the complaints, and this has helped to a degree.
This is the only control for the boilers for the building, there are no internal thermostatic controls in units.

I've been told that there are two heating loops however there is no bypass or mixing valve to adjust the temperature of the heated water. So it leaves the boilers and enters the heating loop at HW boiler temperature.

My understanding is that for atmospheric boilers the minimum temperature should be 140 degrees to avoid corrosive condensation.

I've informed residents that they can reduce the flow through the radiators using the shutoff valve at the entry point into the radiator. Some do this but it's not easy to adjust and a very rough method.

I've lowered the room temperature setting to around 60F but it doesn't have any noticeable impact compared to when it's 70F, at least when the minimum water temperature is 140F.

I've lowered the WWSD temperature to 58F as mentioned. We wouldn't want units to be this temperature, however the building holds heat well. I suppose the worst case would be temperature stays at 58F for days on end however typically it drops below this at night so the heat kicks in. This has helped. It could be raised if there were too many complaints that units were too cold.

So my main question is, is it safe to lower the minimum water temperature to 120-125 degrees during the shoulder season, or would it result in corrosive condensation even in the warmer and typically wetter weather at this time of year? We'd like to extend the life of the boilers for another 10 years if possible.

Is the ideal solution (short of upgrading to condensing boilers) setting up a bypass and mixing valve to help deal with this time of the year? I'm sure how large and expensive of a job this is. Would this be the preferred approaching your opinions?

Thanks much,

LB

Jamie Hall

Yes, a primary/secondary setup with a mixing valve -- preferably controlled by outdoor reset -- will help a lot. Adding a buffer tank won't hurt, either.

STEVEusaPA

What @Jamie Hall said. Plus...Are the boilers staged, and is it enabled? What about all the other parameters.
Are you the owner? Only because you used the phrase 'I've been told'. By who (or is it whom)?
Hopefully both boilers aren't running during the shoulder season.
But yes, proper near boiler piping with protection, and the proper parameters on the Tekmar, should do the trick.
After that, some TRV's and a delta P pump would be the most comfortable.

EdTheHeaterMan

A re-pipe is in order. Unless the 15-year-old boilers are the type that are designed to condensate flue gas, you need to keep the return water temperatures at or above 130°F during burner operation.

EdTheHeaterMan

I have solved this problem in a bed and breakfast in Cape May NJ with a bypass and a full port 3-way zone valve that will open and close based on the return water temperature. as the water temperature needed to keep the rooms comfortable drops below 140° the valve will short circuit the boiler water to the return when needed to operate the burner with higher temperature return water. I also added non-electric thermostatic radiator valves to many of the radiators so each radiator loop could be regulated by room temperature, not just by a tenant manual adjustment.

This was oil heated and I reduced the 4000-gallon annual usage to below 2000-gallons

the investment of the re-piping and valves were offset by the fuel savings within 2 years savings.

your actual savings may vary.

SweatyInToronto

Thanks much Jamie and Steve.

The two boilers are staged in the sense that one boiler will start first for a period of time. However, t does seem that they both run together quite often when heat needs to be supplied, as I guess they are both trying to bring the water up to minimum temp 2x as fast as one would?

Would you advise just shutting one boiler down completely, or is there a setting on the tekmar for this? And would it make a difference if the minimum water temp is still 140 degrees?

The building is co-owned but I'm the person who deals with the appliances the most as I'm the most technical of the lot. Not super technical just more than the others!

I looked up the variable speed circulation pump and the thermostat controlled radiator valves. Looks interesting!
By the way our circulation pumps seem to run even when the WWSD turns the boilers off completely - not sure if this is normal.

Of the lot, which of the changes do you feel would have the most bank for the buck, if we can't afford to do them all?

No doubt I'll have to get people in to inspect and quote the job but it's good to know some of the possible options, and I do appreciate your insights.

Cheers,

LB

SweatyInToronto

Sounds very promising Ed. Much appreciated. Every system and locale is different but would you be able to guess the approximate cost of these retrofits? There are two boilers in our configuration so I guess that might double the cost, lol.

STEVEusaPA

All those settings are adjustable in the Tekmar. The problem you may be having is that because of the large water content of the system, and the improper near boiler piping and lack of boiler protection, the boiler (or where you sensor is located) isn't seeing a temperature rise fast enough and is signaling the other boiler to fire.
I wouldn't disconnect one boiler unless you take it out of the programming, especially if you boilers are rotating along with staging. You may end up with no heat.
I've spent a tremendous amount of time (my house) with a similar control. I can tell you from a lot of staring/measuring/adjusting/recording/staring, only fixing the near boiler piping as mentioned above, and properly programming of the boiler control is the only way to solve this. The control won't do it by itself.
If you want, post pictures of the near boiler piping, print out a copy of your tekmar manual. Then we can go thru the programming to see if it's correct and/or can be optimized.
Edit: The circulators just might be exercising. Otherwise, something is wrong if this is for heat only, you're in WWSD and a circulator is running.
Here's one way for multiple boilers:

Erin Holohan Haskell

@SweatyInToronto, we've merged your two posts here. Thanks!