Lower Flow Temperature & Higher Radiator Temperature?

[berty3000]

Hi all,

I have a 2018 Worcester Greenstar i combi boiler. No water tank. 1960s bungalow. Radiators replaced about a year ago.

I've set the flow temperature to 55C in the belief that this is an efficient setting that allows the boiler to run in condensing mode. I also have the TRV valves turned up high on my radiators (4-5) in the belief that this will allow as much heat to be released as possible through the radiators for the given flow temperature.

Is my understanding correct and is this an efficient way to use the heating system? I find that the room temperature is comfortable doing this. Having the radiators set lower means some of them start cooling before the temperature is how I want it in the rooms (not sure how accurate the TRV thermometers are for the rooms as a whole?) Besides which, if gas is being burnt to meet the demand set by the thermostat, which is in a cooler room, might as well make best use of that heat through all the radiators?

Thoughts?

Is there a diagnostic check I can do through my boiler to check what return temperatures are?

Thanks

 

[monkeysballs]

trv's shut the valve when it gets to temp it has no impact on the actual heat output of the radiator that is controlled by the temp of the water and the speed it goes through the radiator

to test your return temps you just need a temperature probe even one for cooking will do

all you are doing is allowing the room to get to maybe 30c before it closes the valve (some smart trv's can partially close to throttle the flow through the rad but your standard ones cant)

but really you are doing it wrong just set the trv's to give you say 21c or just over how ever hot you want the rooms to get (it depends on the manufacturer what number that will be) make sure that the trv in the area with the thermostat in is set to a higher temp that what you want the thermostat to see or just take the trv off in there or it is potential that it will close that one off early then making the boiler work to try and get that room upto temp even though it cant as the trv has closed the flow


but there are much better ways of doing it so you dont even need trv's

 

[berty3000]

trv's shut the valve when it gets to temp it has no impact on the actual heat output of the radiator that is controlled by the temp of the water and the speed it goes through the radiator

to test your return temps you just need a temperature probe even one for cooking will do

all you are doing is allowing the room to get to maybe 30c before it closes the valve (some smart trv's can partially close to throttle the flow through the rad but your standard ones cant)

but really you are doing it wrong just set the trv's to give you say 21c or just over how ever hot you want the rooms to get (it depends on the manufacturer what number that will be) make sure that the trv in the area with the thermostat in is set to a higher temp that what you want the thermostat to see or just take the trv off in there or it is potential that it will close that one off early then making the boiler work to try and get that room upto temp even though it cant as the trv has closed the flow


but there are much better ways of doing it so you dont even need trv's

Thanks for the info.

The radiator in the room with the thermostat doesn't have a TRV, so that radiator is just working to the thermostat's demands and tends to be very hot.

The rooms never get to 30C, more like 20C by the time the heating goes off as I have it come on for two hours, three times a day. I'd prefer them to have as much heat as possible coming out of them whilst the heating is on whereas a couple of them cool off after a while if set lower. No doubt they think the room has reached the stated temperature but I don't think so.

Anyway, does flow temperature of 55C guarantee efficient condensing boiling or there more to it than this?

 

[monkeysballs]

condensing starts at 57c for natural gas iirc and its more economical the further below that you can go BUT the lower you go the longer it will take to get your rooms up to temperature and your rads wont feel hot (but they dont need to)

you just need to put more heat in than your loosing to make a room heat up

currently im running 29c flow to get a 20c house at 10c outside temperaturebut my heating is on from 6am till 9pmand its actually running for most of that time

you can play with your flow temp but unless you actually have data logging of room temps and flow temps etc you may not get too far but it can be fun experimenting

 

[berty3000]

Interesting.

So do you consider it more economical to run very low flow temperature all day long? Is yours the same type of heating system as mine?

In order to figure out whether I'm using the boiler efficiently, what do I need to check - the temperatures going into and out of the radiators and the difference between these? What does the difference need to be?

 

[Johntheo5]

Anyway, does flow temperature of 55C guarantee efficient condensing boiling or there more to it than this?

There is a bit more to it, if you require the max output of that rad at that flow temperature, then it would have to have a very high flowrate, realistically to give a dT of 5C, with flow/return temps of 55C/50C, this rad will then give you 57.1% of a normally rated T50 rad, 50C return means that the boiler will, just about start condensing, if, you only require say 30% output from that rad then you either keep reducing the flowrate through it by reducing the pump speed or installing a simple TRV, you will then have a dT of 30C with a return temp of 25C and almost a fully condensing boiler. Problem is that different rooms require different heat outputs so there are various means of achieving this, smat type roomstats in each room etc or simply TRVs in most rooms, TRVs will give excellent control as long as the boiler is continuously firing and they are not shut completely which they will do if the rad demand is "too" low, they become erratic in operation, for example above, if the rad demand fell to 26.5% then the return temp would be 21C (with a required room temp of 20C)) and the TRV would be just about shut.

 

[monkeysballs]

Interesting.

So do you consider it more economical to run very low flow temperature all day long? Is yours the same type of heating system as mine?

In order to figure out whether I'm using the boiler efficiently, what do I need to check - the temperatures going into and out of the radiators and the difference between these? What does the difference need to be?

For me yes as I work from home so prefer comfort to outright savings but yeah so far I'm saving money over my old boiler just heating a few rooms


All you can do is play and keep taking meter readings to see how much you use and also keep an eye on the outside temp to see if it's colder one day or the other so you can compare usage HDD is useful for this

Today I've used about 22kwh keeping it at about 20cinside

 

[berty3000]

There is a bit more to it, if you require the max output of that rad at that flow temperature, then it would have to have a very high flowrate, realistically to give a dT of 5C, with flow/return temps of 55C/50C, this rad will then give you 57.1% of a normally rated T50 rad, 50C return means that the boiler will, just about start condensing, if, you only require say 30% output from that rad then you either keep reducing the flowrate through it by reducing the pump speed or installing a simple TRV, you will then have a dT of 30C with a return temp of 25C and almost a fully condensing boiler. Problem is that different rooms require different heat outputs so there are various means of achieving this, smat type roomstats in each room etc or simply TRVs in most rooms, TRVs will give excellent control as long as the boiler is continuously firing and they are not shut completely which they will do if the rad demand is "too" low, they become erratic in operation, for example above, if the rad demand fell to 26.5% then the return temp would be 21C (with a required room temp of 20C)) and the TRV would be just about shut.

Interesting, thanks for explaining it.

So if I have the flow temp set to 55C on the boiler and the TRVs set to 4-5 (that's at least 25C), then the return temperature is most likely quite a bit below the 55C input - therefore condensing more or less guaranteed?

My TRVs are all turned up high so presumably I wouldn't have the problem you refer to? Only one radiator doesn't have a TRV and that's the room with the thermostat.

 

[Johntheo5]

Interesting, thanks for explaining it.

So if I have the flow temp set to 55C on the boiler and the TRVs set to 4-5 (that's at least 25C), then the return temperature is most likely quite a bit below the 55C input - therefore condensing more or less guaranteed?

My TRVs are all turned up high so presumably I wouldn't have the problem you refer to? Only one radiator doesn't have a TRV and that's the room with the thermostat.

If you require a room temperature of 25C then in mild conditions the TRV will still be just about in full control at a flow/return temp of 55/26C dT 29C, output 21.8% but in colder conditions that rad will give its max output (46%) with flow/return temps of 55C/50C, dT 5C.

 

[berty3000]

So what is the significance of the dT, does that have a bearing on whether the boiler is in condensing mode?

 

[Johntheo5]

Yes, it does, its the mean rad temperature that determines its output.
A rad with flow/return temps of 60C/40C, dT 20C has a mean rad temperature of 50C and will output 51.5% of a T50 rad (30/50)^1.3
A rad with flow/return temps of 52.5C/47.5C, dT 5C has a mean rad temperature of 50C and will output 51.5% of a T50 rad. (30/50)^1.3
The one with the higher flow temp but the lower return temp gives improved condensing.

 

[berty3000]

Yes, it does, its the mean rad temperature that determines its output.
A rad with flow/return temps of 60C/40C, dT 20C has a mean rad temperature of 50C and will output 51.5% of a T50 rad (30/50)^1.3
A rad with flow/return temps of 52.5C/47.5C, dT 5C has a mean rad temperature of 50C and will output 51.5% of a T50 rad. (30/50)^1.3
The one with the higher flow temp but the lower return temp gives improved condensing.

I'm going to test the radiator temperatures tomorrow at the flow and return pipes. So I want to aim for dT of 20C? If I am well below that, what needs adjusting based on the equipment I have?

So what would be better from a cost point of view?

- flow temp of 50C where return temp is 40C.
- flow temp of 60C where return temp is 40C.

The first has the advantage of burning less gas to begin with but second is running more efficiently with condensing?

 

[Johntheo5]

Its the return temperature that determines the condensing effect, both returns are 40C so condensing effect or benefit, the same.
Obviously the 50C/40C option will use less fuel but at the cost of comfort, you will "save" 21% but loose 21% rad output resulting in a colder room, you would also have to increase the flowrate to achieve this.
If you just reduce the flow temperature to 50C then the return will be 36C (a improvement in condensing effect) the saving will be 36% but room colder again as you now loose 36% rad output.

 

[berty3000]

Its the return temperature that determines the condensing effect, both returns are 40C so condensing effect or benefit, the same.
Obviously the 50C/40C option will use less fuel but at the cost of comfort, you will "save" 21% but loose 21% rad output resulting in a colder room, you would also have to increase the flowrate to achieve this.
If you just reduce the flow temperature to 50C then the return will be 36C (a improvement in condensing effect) the saving will be 36% but room colder again as you now loose 36% rad output.

So how do I improve the condensing effect, is it just playing around with the flow rate? Is that bu adjusting the TRV the other side, or both?

 

[Johntheo5]

The easiest way is to just decrease the temperature setting of the TRV, but as stated above at the expense of a colder room, its not much fun having a return temperature of 27C but a cold room. You could also let the TRV setting as is and set the flow temperature to say 70C and let the TRV act as a on/off control but apart from probably getting room temperature overshoot/undershoot you will still use more gas as the rad demand is higher.