What should a thermostat control?

I was reading another thread and the original poster described what sounds like one circulating pump, with valves that open zones as needed (that is how I understood it).

My existing system has two zones, with the thermostat turning on circulating pumps for it's respective zone, when heat is needed.

To me, my arrangement doesn't exactly give me total control of my heat.
If my system is set to ~20PSI, water should technically be moving through my pipes, regardless of the circulating pumps running or not. This would explain why it's hard for me to bring the temp down (say from 68'F to 62'F at night) but reasonably easy to bring it up when needed.

Is my arrangement proper or does it sound like something slightly out of the ordinary?

 

Did yu post pictures on another thread somewhere?
Typically when there are pumps on each zone, there is also a device called a flow check.
These flow checks stop hot water from gravity flowing into the zones.
Sometimes they clog up and stop working.

Oh and the static pressure of 20 lbs is not what makes the water flow when the pump is off.





Peter

 

Mike, yer the guy with the T T terminals on your boiler jumpered, right? Your boiler is hot at all times?

As Peter said, you would need functioning flow check valves to control the gravity flow of heat into the home.

The vast majority of boilers are NOT jumpered to run as yours is.

Thermostat will either call a relay to run a pump, and this relay would in turn call the boiler to fire,

OR, thermostat would call zone valve to open, which would in turn call the pump to run and the boiler to fire.

 

I will be going picture happy soon as well as doing a full set of P&ID drawings of the existing setup. This will give a good idea of how my many retrofit system is setup and allow for better arrangements when the summer comes and I look at cleaning things up (the pipework).

At a fast glance, my system goes from the boiler, to the expansion tank, and from there to a tee that splits it into the two zones. After the tee, I have a circulation pump (one per zone) that is connected to a control box (haven't looked closely at the box to figure out what is all in it) and the control box has two wires that run up to thermostat for that respective zone. I don't see any check valves or flow check valves between the pump(s) and the rads on the loop.
After all the rads in each zone, the two loops come back together in a tee, then go through what looks to be a check valve (no markings on it indicating flow direction or anything, but it is a ~4" long shinny brase piece). After the check valve, it's back to the boiler.
Hope that all makes sense. As I mentioned, I'll do a proper set of P&IDs for this system so that the as built system can be fully understood.

Honestly, I am not 100% sure what you mean by the T T terminals jumpered on my boiler.
My boiler is hot all the time (temp range is ~160'F low, ~180'F high).
I will need to confirm the what the thermostat controls from the relay box, but I do know it turns the circulating pump on when water is needed.

Now, not having a lot of experience with these systems (going on 2 months of ownership now), I would assume that the better arrangement would be to have the boiler maintain a min temp and when the thermostat turns the heat on, it fires the boiler up to running them and turns on the circulating pumps. Am I totally out to lunch on this concept?

 

The thermostat turns on the pumps. The pump circulate the water through your house and boiler. When the water temp gets below the temp set on your aquastat the aquastat turns on the boiler.

Pressure in the system doesn't cause flow, the pump does (think of car tire at 32psi, the air isn't circulating there either).

 

Um... Car tires and hot water circuits are totally different.

A hot water heat circuit of say ~300ft in length (2" pipe), heated to ~180'F on the input of the circuit, will have more preasure then the exiting cooler water (which is more dense). The high to low preasure will move the water provided there is a check valve preventing the water from going backwards and balancing the preasure (which there is one on my system).

When we moved in December 27th, 2011), the circulation pump that moved the water on the second/third floor was dead (first floor pump was making a lot of noise, but was moving). With an outdoor temp of -25'C that first night, there was still a slight hint of warm water in the upstairs rads.

 

Typically that means the flow check valve is being forced open a little bit.
To stop that, if it matters, flow checks are put on both the hot and cold ends of a one circuit.

For sure if the boiler is making hot water in the Summer, it would be very bad business for it to also be heating the house.


Peter

 

Gravity flow may occur but the 20 psi is not the cause of that.

 

Of course they are! But this is actually a good analogy.

STATIC pressure and DYNAMIC pressure are ALSO totally different!

The STATIC pressure in a system is not what moves the water. The water is moved by the energy input to that water from the PUMP, which produces the DYNAMIC pressure DIFFERENCE that causes the water to move.

And this would be due to 'gravity flow' or 'thermo-siphon flow'. Because hot water is less dense than cold water, it is more buoyant. It will 'float' to the TOP if it is allowed to do so. This difference in density causes a difference in pressure, which will cause flow in the absence of a pump. In the days of old, most heating systems were 'gravity systems'. There was no pump. These systems relied on this difference in density to induce flow in the pipes, and they were OPEN systems, NOT pressurized!

You've heard the term "hot air rises" ? So will hot water.

And this gravity flow has NOTHING to do with the 20 PSI STATIC pressure, only, as you have noted, with the DYNAMIC pressure difference caused by the differing density of the water.

No, not at all. But, the most cost effective arrangement is to have the boiler go completely cold in the absence of a heat call and only turn on the burners and pumps when the t'stat calls for heat.

 

Would that be called a 'cold start' boiler ?
My config originally had the TT on the aquastat fired off of the end switches of the zone valves.
I read that it was better to have a 'warm start' boiler (cast iron). Thats when I jumpered the TT terminals.

Seems maybe we do things oddly up in the north here.

 

DaveC72,
What part of Ontario are you in?

I'm still a little in the dark as to the complete arrangement of my boiler. I was not in the house when the boiler was fired this fall.

 

Yes.

Better in what way? To my thinking, it just doesn't make sense to waste the fuel to keep the boiler warm/hot at all times. That's like leaving your car running in the driveway overnight. Why?

I think I was thinking of Dave's system when I asked about having the T T jumped... sorry that I can't keep track of all these different systems well! So many boilers, so little time.

 

If i recall, the cold start method would cause some type of boilers to leak prematurely ?

@mike - up in the tbay area

 

Going to cold start on a boiler that had been running as warm start for many years SOMETIMES will cause a boiler to 'weep'. Going to warm start from a cold start that had been running that way it's whole life and not weeping makes no sense at all.