Should circulator pumps stop running when burner kicks on?

woodpecker49

Morning gentlemen,
Have a question here. In a hot water system with hot water coil/ aqua stat (no hot water heater tank) should the circulators pause operation when the burner kicks on?

I thought I had heard this from a guy at some point in the past. 

What I’m noticing is that in the event that someone is in shower and the main zone circulator is on, the furnace will run near constant and the shower hot water will be inconsistent. 

I think I can wire a relay into the “enable “ wire at the aqua stat and have it break open the thermostat control wire until furnace reaches temp. 

Anyone see a reason this wouldn’t work or is there another conventional way to wire it?
Additionally, will this configuration use more oil or save oil? 
My thought process is, the furnace will run less to satisfy its temperature however, the heat delivered to the zone will decrease and cause circulators to run longer to reach room temp. Do these cancel each other out and make no difference to oil consumption?

Thanks in advance,
Have a great day!

Joe
Levittown, NY

Dave H_2

If you are not running out of hot water and the house is warm, I see no difference in fuel used. You need the BTU's for the hot water, you need the BTU's for heating. Same amount of BTU's.

Now if you were running out of hot water in the shower, thats a different story and yes you would want to set it up as priority, shut off the heating zones while the boiler makes DHW.

Ironman

Are you using a tank water heater to heat both domestic and space heating?

HVACNUT

If the boiler has a tankless coil for DHW, then there should be a triple aquastat. In the event the boiler temperature drops below the low setting, power will be removed from C1 until the low reaches the setting plus the differential. 
If you're using a zone panel with multiple zones, then you must use ZR,ZC between the aquastat and zone panel. 

EdTheHeaterMan

Is your boiler still in the original Levittown location under a kitchen countertop? You should be using a boiler that has a tankless coil for heat and hot water. The boiler water goes thru the radiation system and the tankless coil is inside the heating boiler transferring boiler water heat to the potable water inside the tankless coil. This L8124 control should be standard equipment for that boiler and already has the design you suggested incorporated therein. Newer boilers may have an electronic version like Honeywell L7224U, Hydrolevel, or AquaSmart control

they all work similar to this, from the Honeywell L8124 I/O sheet.

OPERATION
High Limit Controller
The high limit opens and turns off the burner when the
water temperature reaches the setpoint (usually 180°). The high limit
automatically resets after the water temperature drops
past the setpoint and through the 10°F (6°C) differential.


Low Limit and Circulator Controllers

On a temperature rise, with the adjustable differential at
the minimum setting of 10°F (6°C), the burner circuit
(R-B) breaks and the circulator circuit (R-W) makes at
the control setpoint (usually 160°). On a temperature drop of 10°F (6°C)
below the setpoint, the R-B circuit makes and the R-W
circuit breaks. See Fig. 12.
At any differential setting greater than 10°F (6°C), the
R-B make temperature and the R-W break temperature
remain the same (control setting minus 10°F [6°C]). The
R-B break and R-W make temperature are the setpoint
temperature plus the difference between the differential
setting and 10°F (6°C).



WARNING
Explosion Hazard.
Can cause serious injury, death or equipment
damage.
Use this product only in systems with a pressure
relief valve.

So this will offer priority to the hot water, it does this by shutting off the circulator if the water temperature in the boiler drops near the minimum required to maintain potable hot water (DHW for the shower). At any time during a shower the boiler water temperature increases above the minimum needed to maintain DHW the circulator will operate to heat the space. The circulator may cycle off and on several times during the DHW usage.

When there is no demand for hot water, the circulator may withdraw boiler water to a temperature to a point near the minimum needed for DHW and the circulator will stop until the boiler water temperature recovers. The circulator will start to heat the space once the water temperature increases above the setpoint selected for minimum DHW needs.

EdTheHeaterMan

Continued:

since the tankless water heater inside the boiler can dispense DHW at any time during the cycle, that tankless water coil may be subject to the minimum less the differential (160° - 10°) like 150° and as high as the high limit during a call for heat which can be 180° plus any over run that may happen from time to time (200° for example).

Installers of this equipment are instructed to add a mixing valve to the DHW coil in order to prevent scalding and provide a constant, even water temperature to the faucets and showers. Like this device:https://www.supplyhouse.com/Resideo-Braukmann-AM101-1LF-3-4-Female-NPT-Lead-Free-Mixing-Valve-70-145F

@woodpecker49, The idea you have is a good one, So good that someone already thought of it over 100 years ago, and that idea has been honed and modified to the electronic controls available today. The mixing valve is often omitted by the installer but that idea is over 100 years old too.

All you may need is a mixing valve

woodpecker49

Thanks for taking the time to comment guys.

The aqua stat is a Honeywell L4081B. 

The system has a manual valve from fresh water to hot water. Nothing automatic. I’ll typically crack it open a little bit during winter months when aquastat is at scalding temps.

The problem occurs when both the circulator for the main heating zone is running and shower is running. The shower water temp swings up and down. My goal is to get shower water temp more consistent. 

Yes boiler is in original Levittown location. in kitchen next to fireplace. 

If the system is in fact set up as you describe with a low temp cutoff for circulators, then it must never really dip low enough to cause any damage however it dips low enough to become a nuisance when showering and frequently needing to fidget with temp.

woodpecker49

Honeywell aquastat

Kickstand55

Whereas your boiler is in a confined space and possibly no practical place to install a mixing valve, you could have one installed under the kitchen sink and in a common hot/cold piping area in the bathroom, quite likely at the head of the tub in a closet.
Check to see if the aquastat settings are High 180/ Low 160/ differential 10 (can be changed if needed). Maybe the tankless coil is coated with minerals, which will need to be cleaned/flushed or replaced. Be sure to put a calcium reducing cartridge at the water main if possible or somewhere that feeds the cold water to the boiler. Keep it changed, these do work.
Let us know what you decide and what worked.