Hydronic system question

[Rockycmt]

I have a HW hydronic heating system. Very standard monoflow loop. I am in the north east. What I have noticed over the years is that the furnace fights hard to get up to temp when the call comes for heat. So when the house drops in temp and the radiators go ice cold... the call comes and it races to get up to temp. I feel this is not very efficient. I would like the radiators to stay luke warm and when the call is made to step it up. Is there a zone controller that will achieve this?

I guess if the house is way over its setting then it should turn down completely. I just feel the house would maintain nicely with radiators at a constant 80-90 deg rather than push to 140 every 2 hrs. And it takes that long for the house to get the heat. I do use a Nest Thermometer. But not a fan of the eco mode during the day. The fight to get back up to temp is not worth the drop. If this was hot air I can see it. Cause you get that heat right away.

Thoughts. Feel free to tell me I am just overthinking this.

 

[Dana]

I have a HW hydronic heating system. Very standard monoflow loop. I am in the north east. What I have noticed over the years is that the furnace fights hard to get up to temp when the call comes for heat.

A couple of things...

It's a boiler, not a furnace- the terms aren't interchangable.

Does the boiler have a model number?

There is no such thing as a boiler that "...fights hard to get up to temp...", it either brings it up to temp or it doesn't.

If you have all high volume radiators (= a lot of thermal mass in the radiation) it can take a lot of time for the system to get up to temperature. That's a function of the burner size, the total amount of thermal mass in system, and the rate at which heat is being emitted from the radiators as their temperature rises. When they're at room temperature or colder the radiators don't emit any heat, but as they rise even a few degrees more of the burner's heat starts radiating into the room, and that rate increases as the difference between room temp and radiator temp increases.

So when the house drops in temp and the radiators go ice cold... the call comes and it races to get up to temp. I feel this is not very efficient. I would like the radiators to stay luke warm and when the call is made to step it up. Is there a zone controller that will achieve this?

A standard mono-flow system is usually operated as a single zone, ergo there probably is no "zone controller" on the system. Maintaining a radiator temperature isn't the function of a zone controller in any event.

I guess if the house is way over its setting then it should turn down completely. I just feel the house would maintain nicely with radiators at a constant 80-90 deg rather than push to 140 every 2 hrs. And it takes that long for the house to get the heat. I do use a Nest Thermometer. But not a fan of the eco mode during the day. The fight to get back up to temp is not worth the drop. If this was hot air I can see it. Cause you get that heat right away.

Thoughts. Feel free to tell me I am just overthinking this.

If it takes 140F radiators to satisfy the thermostat keeping them at 80-90F isn't likely to keep the rooms at the correct temperature. What you're looking for is probably some variation of an "outdoor reset" control, either on a mixing valve, or the boiler. An outdoor reset control strategy adjusts the system's water temperature up or down in response to the outdoor temperature, and has to be programmed/adjusted to where the water temp is just barely high enough that the radiators are emitting the same amount of heat as is being lost from the room via conduction/convection/infiltration, which is dependent on a number of factors (sun, wind, indoor power use, etc) but primarily outdoor temperature. When an outdoor reset is programmed perfectly, the thermostat (almost) never gets satisfied, but the room temperatures never drop.

Outdoor reset is a standard feature on all modulating condensing boilers, but there are retrofit outdoor reset controls that can be added to cast iron boilers too.

From a raw thermal efficiency point of view outdoor reset doesn't help non-condensing systems very much, but from a comfort point of view it's a good strategy, as long as you keep temperature setbacks small. With high thermal mass systems deep setbacks are often problematic- the recovery times take too long, and/or the temperatures overshoot the thermostat which (reading between the lines) seems to be what's going on.

It's still possible to use a setback strategy, but you have to factor in the thermal mass delay and start the recovery times early (apparently 1.5- 2 hours early, based on your description.) Using features on the Nest that automatically sets temperature back are never going to work correctly on a high-mass system, but with any programmable thermostat (and probably the Nest, though I've never seriously looked at it) you should be able to set the beginning of the recovery ramp an hour or two earlier than you need/want the house to be fully up to temp.

Many high-mass systems have boilers that are ludicrously oversized for the actual design heat load, intentionally oversized to deliver a more rapid response, but that oversizing is bad for efficiency and bad for comfort, since the overall duty cycle is low, standby losses are high, and the room temperatures overshoot. For yuks, take a look at the BTUin/out numbers on the boiler, and use the boiler to measure your actual design heat load using this methodology. If you ever need/want to replace the boiler a right-sized modulating condensing boiler under outdoor reset control will improve efficiency by a lot (sometimes cutting the heating bill in half, if the boiler is 5x-6x-7x oversized for the design load.)

 

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[Sylvan]

Great answer Dana

 

[Dana]

Great answer Dana

What- you actually read the whole thing?

 

[Rockycmt]

Dana,
You make a lot of good points. I am not an HVAC professional and I do not come across to be.

My monoflow system has 3 zones. 2 heating zones (One first floor and one second floor) and one indirect Hot Water tank. All thermostats are wired direct to the zone control valves. I have seen this very standard in my area.

The issue I am thinking is my boiler needs to work well over an hr to bring my temp from 65(When I am away at work) to 70 (when I get home). This is due to the thermal mass of my radiator units. I wonder if it makes more sense to just let it maintain 70 then to let it do the swing every day. My Smart thermostat is recommending it lower the temp when we are not home for 6 hrs. I may do a study for one week to burn all consistent at 70 vs smart. The therm logs usage. Unfortunately we have had very warm weather this past fall and winter. I would need to compare two similar weeks.

 

[Dana]

Dana,
You make a lot of good points. I am not an HVAC professional and I do not come across to be.

My monoflow system has 3 zones. 2 heating zones (One first floor and one second floor) and one indirect Hot Water tank. All thermostats are wired direct to the zone control valves. I have seen this very standard in my area.

Zoning the space heating by floor, and zoning the indirect as the "priority" zone (suppressing space heating calls whenever the indirect is calling for heat), is pretty much the standard approach everywhere, whether mono-flow or simple piping, whether high thermal mass radiators or low-mass baseboard convectors.

The issue I am thinking is my boiler needs to work well over an hr to bring my temp from 65(When I am away at work) to 70 (when I get home). This is due to the thermal mass of my radiator units.

That's right- it's all about the thermal mass, a combination of what's in the radiators, but also the thermal mass of the house and it's contents.

I wonder if it makes more sense to just let it maintain 70 then to let it do the swing every day. My Smart thermostat is recommending it lower the temp when we are not home for 6 hrs. I may do a study for one week to burn all consistent at 70 vs smart.

Only if you have a condensing boiler operating under a finely tuned outdoor reset control would a "set and forget" thermostat setting use less fuel. For on/off non-modulating non-condensing boilers, there will be a net savings by allowing the house to cool to some lower temperature. At the lower indoor temperature the difference between indoor and outdoor temp becomes smaller, which means less heat is flowing out the walls & windows.

Since you know the delay is about an hour, start the recovery ramp an hour before you get home (or get up), and you'll have the best of all worlds- saving fuel due to the lower heat loss while the house is cooler, but always have it up to temp when you're at home (or awake). Smart thermostats that operate with IR occupancy/vacancy sensors aren't likely to get it right unless your schedule is VERY regular AND the algorithms are capable of adapting to the thermal mass lag.

The therm logs usage. Unfortunately we have had very warm weather this past fall and winter. I would need to compare two similar weeks.

I'm not sure what means "The therm logs usage." in 'merican dialect(?). You have some sort of data logger on the gas meter?

Correlating heating fuel use to total daily heating degree-day data (use base 65F data) over a reasonably long periods from a nearby weather station would allow you to still get good comparisons even if the weather differed between those periods. As long as there was a persistent heat load during each period (no days where it broke 65F for several hours) the therms per degree-day ratio would fall within a fairly tight range if operating the system the same way. As a general rule of thumb, if you're setting the temperatures back by 5F for 10-15 hours in a day it will deliver about a 7-8% net fuel savings (1.5% per degree F). But there are big error bars around that number due to differences in how well insulated & air tight the house is, the oversizing factor of the boiler, the amount of solar gains, the total thermal mass in the house, etc.

There's no cheating the basic physics- a cooler house will always lose less heat per hour than a warmer house, and a cooler idling boiler will lose less heat to the boiler room than a boiler that is maintaining a higher average temp while cycling on/off just to maintain the house temp. But with a condensing boiler, properly adjusted, the savings of a much higher combustion efficiency by being able to operate at a lower boiler temp will usually have a greater magnitude than those other effects, and will burn less fuel overall.

 

[Rockycmt]

Zoning the space heating by floor, and zoning the indirect as the "priority" zone (suppressing space heating calls whenever the indirect is calling for heat), is pretty much the standard approach everywhere, whether mono-flow or simple piping, whether high thermal mass radiators or low-mass baseboard convectors.



That's right- it's all about the thermal mass, a combination of what's in the radiators, but also the thermal mass of the house and it's contents.



Only if you have a condensing boiler operating under a finely tuned outdoor reset control would a "set and forget" thermostat setting use less fuel. For on/off non-modulating non-condensing boilers, there will be a net savings by allowing the house to cool to some lower temperature. At the lower indoor temperature the difference between indoor and outdoor temp becomes smaller, which means less heat is flowing out the walls & windows.

Since you know the delay is about an hour, start the recovery ramp an hour before you get home (or get up), and you'll have the best of all worlds- saving fuel due to the lower heat loss while the house is cooler, but always have it up to temp when you're at home (or awake). Smart thermostats that operate with IR occupancy/vacancy sensors aren't likely to get it right unless your schedule is VERY regular AND the algorithms are capable of adapting to the thermal mass lag.



I'm not sure what means "The therm logs usage." in 'merican dialect(?). You have some sort of data logger on the gas meter?

Correlating heating fuel use to total daily heating degree-day data (use base 65F data) over a reasonably long periods from a nearby weather station would allow you to still get good comparisons even if the weather differed between those periods. As long as there was a persistent heat load during each period (no days where it broke 65F for several hours) the therms per degree-day ratio would fall within a fairly tight range if operating the system the same way. As a general rule of thumb, if you're setting the temperatures back by 5F for 10-15 hours in a day it will deliver about a 7-8% net fuel savings (1.5% per degree F). But there are big error bars around that number due to differences in how well insulated & air tight the house is, the oversizing factor of the boiler, the amount of solar gains, the total thermal mass in the house, etc.

There's no cheating the basic physics- a cooler house will always lose less heat per hour than a warmer house, and a cooler idling boiler will lose less heat to the boiler room than a boiler that is maintaining a higher average temp while cycling on/off just to maintain the house temp. But with a condensing boiler, properly adjusted, the savings of a much higher combustion efficiency by being able to operate at a lower boiler temp will usually have a greater magnitude than those other effects, and will burn less fuel overall.

What I mean is my thermostat logs "on" time. So I can see how many hrs the circulatory runs over the two periods of like outside temp. This will tell me roughly which is more economical for my home.

 

[Dana]

The thermostat has no way to determine the state of the burner, only the duration and times of the call for heat. With a likely ridiculously oversized boiler the burner may be cycling on/off during those long recovery ramps. The only way to know for sure what the burner's duty cycle is (rather than the circulator's) is to measure fuel use, not the duration times of the call for heat.

 

[Jadnashua]

The Nest can learn your recovery response and adjust itself but you need to tell it the type of system you have. It also knows the outside temp, at a least for your zip code, not your exact location from its internet connection.

 

[Dana]

The Nest can learn your recovery response and adjust itself but you need to tell it the type of system you have. It also knows the outside temp, at a least for your zip code, not your exact location from its internet connection.

That's only useful if you have a very regular schedule that it can anticipate- it can't read your mind. But remotely bumping up the thermostat at the roughly correct ramp time ahead of your expected to arrival time should work even on totally random schedules.

Many WiFi thermostats (and probably the Nest) have geotracking capabilities using a cell phone app that will adjust the thermostat when you get within some user programmed distance from home, but with hour long ramp times that strategy isn't likely to be useful here.