Boiler abruptly shuts off when heat stops calling

Tom01

Thanks in advance for your time. I have extensively searched online for an answer to this and only found one thread somewhere that someone described the exact same issue but no one had a solution.

This is the thread: https://www.doityourself.com/forum/boilers-home-heating-steam-hot-water-systems/573525-odd-very-short-cycle.html

Equipment -
Boiler: New Weil Mclain Gold 100k BTU w/ tankless coil
Hydrostat: Honeywell 3250
Hydronic baseboard heat split into four zones in 1800 sq ft home. Provides sufficient heat and hot water and works great.

What I have done -
- Jumper set to Z
- Tried all settings on Economy mode
- Changed to various Hi Lo temps

Current Settings -
Hi - 190
Lo - 160

The issue I have involves the built in subtractive differential of 10. So for example, if none of the thermostats are calling for heat, my boiler will not fire until it reaches my Lo - 10 (160 - 10 = 150 at current settings). This is fine, however when calling for heat, the boiler will fire when it gets down to the low temp of 160. I noticed that intermittently, if the conditions are right, the boiler will not be firing but the thermostat is calling for heat while the temp is above 160. The circulator is on as expected. Once the boiler goes below 160 it begins to fire. It can run anywhere between 1 and 15 seconds and if the thermostat stops calling the boiler promptly shuts off. The boiler will not turn on again until either the temp gets down to 150 or if a zone begins to call again. Otherwise it stays there not running somewhere between 151 and 159 degrees.

This is by chance and happens at the end of a heating cycle. The thermostat may have been calling for 15 minutes prior and the boiler may have run fully to the Hi limit a couple of times during that cycle but the thermostat stopped calling when the boiler was beginning to fire one more time causing it to shut off.

Maybe it is supposed to do this, but I don't like the idea of everything starting up for such a short amount of time and then cutting off. That is not efficient and is pointless. I would rather the boiler just continue to run and heat itself up. Seems this quick on and off thing is a way to wear out the burner more quickly.


I am used to our old 1983 Peerless that didn't do this, so please let me know what you think and if there is some setting that can be adjusted to stop this from happening.

DZoro

What nozzle is in the boiler? Sounds like it is oversized for you.

Tom01

Thanks for the reply. It still has the 1gph nozzle in it that came with the burner. No insulation at all on second floor but hope to change it when renovating in a year or two. Very drafty. Old boiler was a 182k BTU monster with a 1.75 gph nozzle lol.

Living in NY.

HVACNUT

@Tom01
Remove the jumper and put the switch to Z on the Hydrostat.
The jumper is keeping the temp down. Get rid of it.
Without knowing your location, I'd start with 165 high, 145 low.
Economy OFF because you have a tankless coil. And there should be a thermostatic mixing valve for the coil.

And with 4 zones, the wiring to the zone relay(s) needs to be wired ZR,ZC, NOT TT to XX.

KC_Jones

The oversized boiler is not helping your cycling issues either. Too much in = hitting the limit quicker.

Tom01

Thank you guys. I went to my parents' for Thanksgiving last night and tested on their boiler. Same exact setup except they have the 126k BTU version. It behaved the same way as mine. I didn't open up their controller to check the internal wiring though.

I will move the wires to ZR & ZC and see how it behaves then. I will see how the boiler does during this winter (yesterday was cold!) and I will consider a bit smaller of a nozzle and adjusting the intake accordingly. House is so drafty and old and I was concerned about getting enough DHW and heating the house sufficiently in the winter. Yes, I have a thermostatic mixing valve for the coil and have consistent temps in the shower. No ups and downs. I will try to lower the LL as well.

Appreciate your input!

HVACNUT

> @Tom01 said:
> Thank you guys. I went to my parents' for Thanksgiving last night and tested on their boiler. Same exact setup except they have the 126k BTU version. It behaved the same way as mine. I didn't open up their controller to check the internal wiring though.
>
> I will move the wires to ZR & ZC and see how it behaves then. I will see how the boiler does during this winter (yesterday was cold!) and I will consider a bit smaller of a nozzle and adjusting the intake accordingly. House is so drafty and old and I was concerned about getting enough DHW and heating the house sufficiently in the winter. Yes, I have a thermostatic mixing valve for the coil and have consistent temps in the shower. No ups and downs. I will try to lower the LL as well.
>
> Appreciate your input!

But you didnt say "I'll remove the jumper." It needs to come out to achieve temps over 140°.

Do you have the WM Ultra or WTGO?
If the 3 section WTGO, depending on the burner, at .95 GPH input, a 1.00 gallon nozzle might be too large because most burners want 140 psi or better on the WTGO.
Actual firing rate is probably 1.20 GPH or higher.
What model boiler and burner are they?

STEVEusaPA

Which WM model number? Looking for a WTGO or GO followed by 3 (hopefully not 4).
I wouldn't recommend downfiring the boiler without a full combustion test. Those boilers, even perfectly clean seem to not draft very well, which means (technically) higher efficiency and lower stack temperatures. Lower to the point of condensing.
Went to a new customer last week, GO-4 with an .85X80B nozzles, -.01/-.02 draft, 290° net stack temp-not good. Put the 1.35 nozzle in, better, net around 420°.
It's going to short cycle either way (this one is a 4 section in a 1200 sq ft house, 2 zones).
You could try a bigger differential, but until you get the right size boiler, based on your heat loss, it's going to short cycle.

KC_Jones

> @STEVEusaPA said:
> Which WM model number? Looking for a WTGO or GO followed by 3 (hopefully not 4).
> I wouldn't recommend downfiring the boiler without a full combustion test. Those boilers, even perfectly clean seem to not draft very well, which means (technically) higher efficiency and lower stack temperatures. Lower to the point of condensing.
> Went to a new customer last week, GO-4 with an .85X80B nozzles, -.01/-.02 draft, 290° net stack temp-not good. Put the 1.35 nozzle in, better, net around 420°.
> It's going to short cycle either way (this one is a 4 section in a 1200 sq ft house, 2 zones).
> You could try a bigger differential, but until you get the right size boiler, based on your heat loss, it's going to short cycle.

I believe this boiler is brand new, so they will be living with this for a long time. Boggles my mind that these contractors refuse to do their job.

STEVEusaPA

Yeah @KC_Jones, missed 'new'. I'd be making some noise about replacing it. I just hate to see someone severely downfire it, and it rots out due to condensation.

HVACNUT

> @KC_Jones said:
>
> I believe this boiler is brand new, so they will be living with this for a long time. Boggles my mind that these contractors refuse to do their job.
>
>
And it's almost 2019 and they put in a tankless coil. Maybe it was cost, but...