How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site. Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Rick Your Own Question
Rick, HVAC Supervisor
Category: HVAC
Satisfied Customers: 21792
Experience:  40+ yrs. experience as a licensed oil & gas technician.
Type Your HVAC Question Here...
Rick is online now
A new question is answered every 9 seconds

My problem is excessive short cycling of the oil burner resulting

Resolved Question:

My problem is excessive short cycling of the oil burner resulting from interfacing a single circulator, 5 zone valve hydronic heating system through a Taco ZVC 405 controller with an MH L8124A aquastat. The system burner will frequently run for less than 30 seconds before the upper limit or the thermostat causes the burner to shut off. Aquastat settings have not been sucessful in eliminating this. I would like to separate the thermostat demands from the boiler temp. control. Can I effectively do this by wiring the circulator to a free set of contacts in the Taco controller which close when any end valve contact closes in the Taco 570 zone valves? The TT terminals of the aquastat would be disconnected from the Taco ZVC and jumpered. Boiler temp. control would be by High Limit settings.
Submitted: 7 years ago.
Category: HVAC
Expert:  Rick replied 7 years ago.

The re-wiring you propose is not necessary. When there is a call for heat the circulator will run provided the boiler temp is above the low limit. The low limit maintains boiler temp for domestic hot water (DHW). The high limit runs independent of the circulator maintaining the boiler temp between the set point and the differential setting. Most hydronic systems were designed at a boiler temp of 180 to 200. So your high limit setting sounds too low. Try a 15 or 20 degree differential. Your low limit should be at least 140 to 150. If the burner continues to short cycle you may need a new operating control or you may have a bad connection.
The circulator and burner functions are already separate.
Customer: replied 7 years ago.

I have changed limit settings without solving my problem. If I can separate T-Stat/circulator circuit from the boiler temp control circuit I believe I can eliminate short cycling caused by T-stat/upper lim interaction. Then boiler control will be based on boiler temp limits only. I would live with variation of temp around the Hi Lim set point. The furnace appears to be oversized for the house at 100,000btu-h. Max burner on time last season on a 50 HDD day was 5 hrs/day. So recovery time to handle T-stat demand should not be a problem.


Raymond Hudson

Expert:  Rick replied 7 years ago.
The circulator circuit and boiler temp circuit are separate now. When there is a call for heat the circulator comes on and runs until the stat is satisfied, provided the boiler temp is above the low limit setting. The high limit is activated on a call for heat (entirely separate circuit from the circulator) and will cycle the burner on and off between the high limit setting and it's differential.
What you propose won't change anything. You may have a faulty R8124.
Customer: replied 7 years ago.

However, now the burner runs until either 1. the t-stat is satisfied, or 2. the hi lim cuts it off, or 3. all t-stats are satisfied and the temp drops below the hi lim diff. With five t-stats on line the probability of the temp hitting the upper lim before any t-stat is satisfied and another comes on line, etc. is much greater. With the t-stat out of the boiler circuit and T-T jumpered the burner on-off cycle will be determined by the constant factor of high lim setting and the differential. Without any circulation the time to raise the boiler temp from low to high is 6 minutes - enough time to get the stack temp up and minimize condensation. A 5 to 30 second cycle time is not long enough to do that.

My objective is to maximize efficiency within the system limits (minimum condensation and corrosion).

Thanks, XXXXX

Expert:  Rick replied 7 years ago.
If you want to maximize system efficiency you should install a device like the Beckett Heat Manager

I don't know how to word it any differently. What you propose will not change the system dynamics. The functionality you hope to achieve is already how the operating control works.
Rick and other HVAC Specialists are ready to help you