Boiler only fires up when indirect water heater pump is on

haunt · 12:05AM

I have a Burnham oil fired boiler with an indirect water tank. I also have a solid fuel boiler in tandem with the oil boiler. A circulation pump just keeps the hot water flowing between the two when the wood boiler is hot. This summer I switched the Nest thermostat from Heat to Cool. I have been keeping a small wood/coal fire going as my wife thinks that endless hot water must always be flowing. A few mornings when the wood fire died out, I noticed that the oil boiler had a low temperature, like 95 degrees. It did not fire up when it fell below the low limit. I figured a bad aquastat sensor (Honeywell L7224A,C). BUT if I turn up the thermostat on the indirect water heater, once the contacts click shut and the circulator starts, the oil boiler fires up. If I turn the water heater thermostat down, once the contacts break the boiler turns off again. Very confusing. What do you think?

kcopp · 12:07AM

Do you still have hot water?

Prob not an issue

haunt · 12:10AM

Yes. when the indirect water heater temp. falls below the setting and the pump turns on the oil boiler fires up. It runs as usual until the boiler high limit is reached then shuts off normally.

kcopp · 12:26AM

Then no issue. Dont be concerned.

haunt · 12:44AM

The boiler would always fire up in the summer for the only reason that the boiler temp fell below the low limit. Why would it not be doing that now? In winter, the boiler would fire up when the wood/coal boiler couldn't keep up and the low limit was reached. That's my real concern. That it won't fire up then and I don't want to figure it out when its 20°.

kcopp · 12:46AM

You will have plenty of time. If it making hot water it will make heat.

109A_5 · 1:08AM

Since you have an indirect water tank you don't need to maintain the boiler water temperature to have hot water. The Honeywell L7224A,C may be working as configured.

You are expecting this.

But it may be configured for this.

GroundUp · 1:25AM

What is the problem? You're not heating the space right now, so of course it's not going to fire unless the indirect calls.

EBEBRATT-Ed · 1:44AM

Maybe the op thinks the oil boiler should maintain boiler temp when the coal fire is out. I think that is what @109A_5 is getting at.

@haunt

The boiler should run for heat when the thermostat calls for heat or when the indirect thermostat calls for DHW

haunt · 3:00AM

https://forum.heatinghelp.com/discussion/comment/1899824#Comment_1899824

Thank you, You are exactly right. I thought that the oil boiler would always maintain temp between the low and high limit. Am I wrong?

haunt · 3:05AM

https://forum.heatinghelp.com/discussion/comment/1899823#Comment_1899823

Thanks. I just never noticed that the boiler temp was dropping so low and got worried about it.

haunt · 3:09AM

@109A_5

Thanks for all of that. I don't understand most of that but I guess that it made me realize that you and

kcopp are correct and it's probably working as designed.

109A_5 · 5:56AM

" my wife thinks that endless hot water must always be flowing "

Another way of thinking about it, apparently there is no thermostatic control of the solid fuel boiler's fuel so you have to keep it hot enough to maintain DHW (Domestic Hot Water) usage and not use any Oil.

If you let the solid fuel boiler go out you would have no DHW, but you have the Oil as a backup, so you do have adequate DHW.

Since you have an indirect water tank you don't need to maintain the boiler's water temperature to have hot water. So you don't want to burn Oil just to keep the Boiler's water temperature at or above above a minimum temperature. There is no need to, and that would waste Oil.

As an example; If you went on vacation and the solid fuel boiler just went out, you would not want to have the Oil boiler maintaining a minimum temperature. Rather the Oil boiler would just fire enough to maintain the indirect water tank's temperature. With no DHW usage (your are on vacation) the DHW demand would be minimal. So minimal Oil usage.

haunt · 10:33AM

https://forum.heatinghelp.com/discussion/comment/1899831#Comment_1899831

Thank you for taking the time to explain that.

haunt · 10:35AM

Thank you ALL for sharing your knowledge.

So when I switch back to the Heat mode, then the boiler sould kick on when the low limit is reached and maintain a minumum temperature?

GroundUp · 2:42PM

A cold start boiler only runs when there is a call for heat- they don't "maintain temperature" unless there is an active call for heat from the thermostat or indirect. If you switch to heat mode right now and set the stat to 65*, the boiler still won't fire unless the air temp falls below 65* (for example).

EdTheHeaterMan · 3:11PM

https://forum.heatinghelp.com/discussion/comment/1899833#Comment_1899833

You have your L7224 configured as a cold start boiler. that means the burner only operates when something needs heat (something = rooms in winter or water tank all year)

So if nothing is calling for heat, the burner does not operate. That is all you need. There is no low limit needed to maintain a minimum water temperature in either the summer or the winter.

For example, on December 14 of any given winter, the outdoor temperature reaches 75°F at 2:30 in the afternoon. No rooms need heat, and since everyone is at school or work, there is no need for hot water, so the indirect water heater does not call for heat. At that exact time, the boiler temperature may drop as low as 64°F, or whatever the ambient temperature in the basement happens to be. No low limit is needed.

Setting the thermostat to HEAT does not automatically require a low limit on the boiler. The boiler is allowed to cool to the boiler room temperature because there is no need for heat.

This is actually an energy-saving feature of a cold-start boiler. If no heat is needed, the burner does not operate just to maintain a minimum boiler temperature.

So when you say, "then the boiler should kick on when the low limit is reached," that is not correct. On a cold-start boiler, that would simply be a waste of fuel.

EdTheHeaterMan · 3:26PM

To make it a little more clear, let's use a more typical December day. One or more heating zones may call for heat at different intervals. Zone 1 calls while Zone 2 is off. Zone 1 becomes satisfied, and the boiler temperature starts to drop from 180°F to 148°F when Zone 2 happens to call for heat. At that point, the burner operates to heat Zone 2, and the boiler temperature rises again until Zone 2 is satisfied.

The boiler temperature may then drop to 138°F when you decide to take a shower. The indirect DHW tank calls for heat, and the boiler temperature increases back to 180°F during your shower. When your shower is finished, the DHW tank reaches its set point, the burner shuts off, and the boiler temperature drops to 157°F when Zone 1 calls for heat again... and so on.

In this scenario, the boiler temperature always seems to stay above the so-called "low limit" temperature. However, that is not happening because of a low-limit setting on either the Nest or the L7224. It is simply a coincidence in the timing of the different calls for heat that keeps the boiler temperature above the boiler room's ambient temperature, which might be around 64°F.

Does that make sense?

To make that happen set your L7224 low limit temperature to OFF. That will make you system a "Cold Start" boiler and result in the lowest operating cost.

Boiler only fires up when indirect water heater pump is on

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