Explanation of thermostat vs control temp

[Dylan T]

Hi all - I've been trying to get my head around better ways to run my heating in the rise of heating bills lately (like everyone) but this week I took my little newborn home so now am realising more than ever i want to sort it!

I've been reading various articles about boiler modulation, including various little DIY microcontroller projects for this but one overarching thing I can't get my head around is flow temperature, and I was hoping someone could help me get my head around it.

Basically, I can't work out why changing the flow temp is different in terms of efficiency than changing the thermostat. For example, if my room thermostat is set to 20deg then if my control temp is set to the max, then the rads might come on hard and hit the temp quickly but then hover around the 20 degrees, whereas if the control temp is lower then that process would just take longer to happen. In my head the only difference here is time as the thermostat is the one determining final output khw which is uiltimately the smae in both situations.

I must be wrong in my above assumption, but can someone explain how? I can't find anywhere a simple explanation of this - am I allowed to blame new baby brain?

Thanks!

 

[Friday round]

I think of a boiler being an engine. When you drive, do you hit the pedal and accelerate all the way to the speed limit and then lift off? No, you accelerate gradually setting your foot on the pedal where it’ll maintain desired speed. Flow is the power you put into the pedal and the thermostat is the speed you want to travel at. If you do as you’ve assumed, then your rads will get to temp faster but will use up more gas getting to temp and likely overshoot, also wasting gas, not to mention additional wear and tear.

 

[JonathanM]

I've been looking at this myself the past few weeks. I don't think there is one simple explanation. I would say there are technical, physiological and psychological factors to why running cooler might save money. Here are some thoughts for starters whilst you wait for the pros to answer.

The easiest is the technical stuff. If you can get your boiler condensing more, and cycling less, you might save some money. For it to condense at all, I think the water has to return to the boiler at 54C or less. And the lower it comes back, the more it condenses. So, the lower you set the flow, the more it will condense. Reducing cycling is more difficult to get my head round.

How old is your boiler? Is it a combi? Make and model? How many radiators?

If you run your boiler cooler, it won't overshoot the room thermostat temperature as much, and you should find you can put up with a cooler temperature, as it will be more steady, rather than hot and cold. Also, it just feels nicer, because you have gentle heat, rather than a hot blast. I find that makes it easier it to reduce the temperature.

I've found the most useful thing is having an accurate, fast responding thermometer which reads in 0.1C increments, so you get a proper idea of what's happening to your room temperature.

 

[Harry Bloomfield]

For a condensing boiler to be able to work at it's most efficient, then the Return water temperature needs to be as low as possible, which means flow temperature also needs to be kept low, preferably below 55C.

Ideally, your boiler needs to know not only that there is a demand for heating, but how big that demand might be. Without that feedback, your boiler will simply run flat out, until your thermostat clicks and says 'enough'. If the boiler knows how big the demand is/ how much difference there is between the present room temperatures and the desired room temperatures - it can run the boiler much more gently, and is much better able to modulate the output to suite the demand for heat. That also means better economy, and less overshoot of room temperatures, plus steadier room temperatures.

 

[JonathanM]

Something I forgot is modulation actually saves money. When a boiler is modulating, particularly at 30% and under, it is considerably more efficient than when running flat out.

EDIT: So, if your boiler is running cool, modulating at 25%, and also condensing, you should be able to save 10%+ on it running flat out at a higher flow temperature.

Graph from https://www.heatgeek.com/what-is-boiler-modulation/

 

[JonathanM]

In my head the only difference here is time as the thermostat is the one determining final output khw which is uiltimately the smae in both situations.

I've realised I may have rambled on a bit above. The very succinct answer to this specific question, is that the boiler will run more efficiently, so it will output the same final KWh, but use less gas to do it.

 

[amcluesent]

My understanding is that combustion of natural gas creates hot steam which why you sometimes see steam clouds from flues. That's wasted energy

A condensing boiler passes these flue gases over a heat exchanger which contains the circulating cooler water coming back from the radiators. So long as that returning water is cooler than the dew point, then the steam will condense and pass residual heat energy to the water. That means you need to burn less gas to re-heat the water before it's pumped back to the rads. The cooler the returning water is, the more energy is given up by condensing steam and the more efficient the boiler.

The water that condenses around the heat exchanger is trapped and drains out of the boiler to waste.

 

[ScottishGasMan]

Something I forgot is modulation actually saves money. When a boiler is modulating, particularly at 30% and under, it is considerably more efficient than when running flat out.

EDIT: So, if your boiler is running cool, modulating at 25%, and also condensing, you should be able to save 10%+ on it running flat out at a higher flow temperature.

Graph from https://www.heatgeek.com/what-is-boiler-modulation/

View attachment 282590

I wouldn't read too much into that graph, there are inaccuracies there or at least omissions. For example, standard efficient boilers are generally much less efficient when modulating (depending on fan control)

And current condensing boilers to the best of my knowledge are not considerably more efficient when they modulate, there will be a slight gain due to the slower flue gas speed allowing more time for condensing to take place, but I've a feeling that graph will be taking into account the lower flow temperature that will be associated with the outside temperature. Flow and return temperature is what will make the most significant difference.

 

[ScottishGasMan]

So think of it this way. You put a pot of water on a stove and heat it up. Youre constantly putting energy in, and the pot of water is constantly heating up. Once the water reaches 100°C it stops heating up. Leave it on the stove, still putting the same energy into the pot of water but the water will never go over 100°C.

When the water hits 100°C, the heat that's being put into the water is no longer increasing its temperature, it is now breaking the bonds that hold the water molecules together as a liquid and allowing them to turn to a gas. To boil (evaporate) the whole pot of water, you need to put an "amount" of heat into the pot, the equivalent to what it would take to heat the water to over 500°C if it stayed a liquid.

This is "Latent Heat". Additional heat that needs to be added to turn the liquid into a gas. And it works both ways. So when water vapour condenses it gives this heat back off. (One reason steam burns are so nasty, apart from very hot steam hitting your skin, the water condensing on your skin is giving off extra heat)

When you burn Methane (Biggest content of Mains Gas) You produce carbon dioxide and water vapour (amongst other things) If you have a boiler running at high temperature, these flue gases including the water vapour leave the flue/chimney and the water will condense giving up its latent heat in the atmosphere.

If you run a condensing boiler correctly (how they were designed to run) then the system water temperature is low, which means the flue gas temperature is low, which means the water vapour condenses inside the boiler giving its latent heat over to the system water before it leaves.

This is the difference between Gross and Net Efficiency of boilers, and why when quoting Net Figures you can have over 100% efficiency on a boiler (which you cant)

The difference in available energy from Methane combustion with and without this additional latent heat of condensing water vapour is 10% so simply running condensing boilers cooler will produce more heat per unit of gas burned than it would if you ran it at higher temperature.

 

[JonathanM]

I wouldn't read too much into that graph, there are inaccuracies there or at least omissions. For example, standard efficient boilers are generally much less efficient when modulating (depending on fan control)

I used the graph because I think it originally came from Viessmann, and I thought it would be reliable. Another graph I found shows a 3-4% efficiency increase with modulation at various return temperatures, which would presumably rule out the cause being lower flow and return temperature. I'm just wanting to understand this stuff!

 

[JonathanM]

I've never understood why there are two different x-axis on that graph! Load and outside temperature. I did wonder whether the temperature applied only to the on/off boiler, because I couldn't see how an/off boiler could have a variable load - or maybe it could?

 

[ScottishGasMan]

I've never understood why there are two different x-axis on that graph! Load and outside temperature. I did wonder whether the temperature applied only to the on/off boiler, because I couldn't see how an/off boiler could have a variable load - or maybe it could?

Well on heating the load will reduce fairly proportionally to the outside temperature.

i.e.: at 0°C outside my heatloss could be 10kW, so at 10°C my heatloss would then be 5kW, at 15°C outside id need 2.5kW to keep inside at 20°C

 

[ScottishGasMan]

so if I fit a boiler thats 30kW with a minimum modulation of 5kW, it could heat the house without cycling until the outside temp was 10°C, then at 11, 12, 13°C outside and so on, the boiler would have to cycle as it would be putting more heat in that is required

 

[JonathanM]

so if I fit a boiler thats 30kW with a minimum modulation of 5kW, it could heat the house without cycling until the outside temp was 10°C, then at 11, 12, 13°C outside and so on, the boiler would have to cycle as it would be putting more heat in that is required

That makes sense for me now with the modulating boiler. But how can an on/off boiler have a variable load. Isn't it either 100% or zero?

EDIT: are they factoring in the cycling and counting that as load? So if an on/off boiler is only firing 50% of the time, that is 50% load?

 

[ScottishGasMan]

Same terminologies for different things.

An on/off boiler, could be reffering to one which has no ability to modulate its heat output. They havent made these for 15+ years (domestic gas boilers I'm talking about) so most boilers will be able to modulate their heat output (Basically turn the flame from big to small)

On/off controls, generally refers to your typical clock/programmer and room thermostat arrangement, where it either sends a signal saying have the heating on, or dont have the heating on. The boiler will run with a fixed target flow temperature, but will still modulate the flame size depending on how near or far it is from that flow temperature.

Modulating Controls, will communicate with boiler, and tell it to change the target flow tempearture, so if I want 20°C in the room, and its currently 15°C in the room, it knows it has to lift the temperature of the house by 5°C, so it uses a high flow temperature initially. As the room gets warmer and its say, 19°C the controls tell the boiler to use a lower flow temperature with a view to keeping the room at 20, without turning on and off, but rather lowering the flow temperature to only put in the amount of heat required, and if that can be done at low flow temperature, the boiler will condense more and be more efficient.

If the boilers minimum modulation is higher than what is needed, it will inevibetly keep increasing the flow temperature as it cant adjust the power any lower, and will cut off to cycle and allow the system water to cool a bit, before coming back on, other wise it would get too hot.