How to best measure flow and return from boiler?

[HRP123]

Hi there, I have the following system boiler setup with a low loss header and four different circuits. I've set the boiler flow to different levels between 65 to 70 degrees but the surface temperature readings never seem to reach much above 55 degrees into the LLH - I have tried a clip on thermometer, a wifi surface temperature and IR measurements on tape on the pipe.

What tool can I use that would most accurately give me measurements of the temperature within the pipes, to verify the boiler is actually spitting out 65-70 degrees? I have seen some claw type products - anyone recommend an accurate one?

The radiators show around 50-55 degrees max.

Thanks for your advice

 

[Enthalpy]

I tried the clip on thermometers but, like you, I found that they read low when clipped to pipes .

I use cheap aquarium thermometers with the wire knotted round the pipe and the sensor fixed to the pipe with a small cable tie. Add a bit of heat sink compound or Vaseline and cover with pipe insulation.

 

[snowhead]

The most reliable type are the ones fitted in a pocket in the water flow, as commonly used in Commercial systems.
If you're using clip on / strap on external ones then the probe needs to be fully covered with insulation.

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[John.g]

Don't think the boiler flowtemp or the return temps should change, the secondary (rads etc) flow/return temps will change, see some calcs I did sometime ago, havn't really checked them out yet.

 

[HRP123]

The most reliable type are the ones fitted in a pocket in the water flow, as commonly used in Commercial systems.
If you're using clip on / strap on external ones then the probe needs to be fully covered with insulation.

Thanks , yes I agree ones fitted inside are best but given I cannot easily modify the pipework myself now I'm trying to work out the best option of getting the surface temperature on a resi install. Good tip on the insulation!

 

[HRP123]

Don't think the boiler flowtemp or the return temps should change, the secondary (rads etc) flow/return temps will change, see some calcs I did sometime ago, havn't really checked them out yet.

View attachment 81962

View attachment 81963

View attachment 81964

Thanks for these, it correlates with some of what I see online. I'm trying to by process of elimination first check whether the flow temperature stated in the boiler is actually reaching the LLH to make sure this isn't the issue. Nice calculations!

 

[gmartine]

Just buy a used FGA (uncalibrated) with temperature dif function, save you arsing about.

Don't know if this is up to much...

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[ShaunCorbs]

You have a llh the primary temp will have to be around 10 dc higher than the rad temps so if you want the rads 60 boiler will have to be 70 etc

What boiler do you have system or heat only and kw size ?

 

[HRP123]

You have a llh the primary temp will have to be around 10 dc higher than the rad temps so if you want the rads 60 boiler will have to be 70 etc

My question, also posted on a separate thread, is whether if just the UF circuit is on (which blends down to 45 degrees and returns lets say 30-35 degrees back to the LLH) - this distorts the temp inside the LLH going back to the boiler. So basically the boiler is burning at 65 degrees full capacity, even if just a ssmall part of the UF is on (and nothing else). Thanks

 

[John.g]

I got a present a few years ago of a Proster Model: PST095 Dual Channel Thermocouple Thermometer, it gives very accurate quick response readings on the few occasions I've used it. I just stick the probes to the pipes with a bit of sticky tape, no doubt clamps will be even better.

 

[ShaunCorbs]

My question, also posted on a separate thread, is whether if just the UF circuit is on (which blends down to 45 degrees and returns lets say 30-35 degrees back to the LLH) - this distorts the temp inside the LLH going back to the boiler. So basically the boiler is burning at 65 degrees full capacity, even if just a ssmall part of the UF is on (and nothing else). Thanks

Yes you will find it cycles a lot eg short heat up time and longer off time etc

 

[John.g]

My question, also posted on a separate thread, is whether if just the UF circuit is on (which blends down to 45 degrees and returns lets say 30-35 degrees back to the LLH) - this distorts the temp inside the LLH going back to the boiler. So basically the boiler is burning at 65 degrees full capacity, even if just a ssmall part of the UF is on (and nothing else). Thanks

If you don't have HW priority and require a boiler flowtemp of 65C for cylinder heating etc and assuming (10kw) UFH flow/return temps of 40C/32C then the boiler will run very efficiently because the return temp will be the same as the UFH return temp as long as the primary and secondary flowrates are the same, see below, problem is the boiler dT is now 33C which will cause most boilers to trip, you then have increase the primary flow by 1.39LPM to reduce the boiler dT to a acceptable 25C and a boiler return temp of 40C, still very efficient. I showed the LLH in the form below as you can see the whole picture better, a temperature controlled by pass would work exactly the same and possibly give tighter control.

 

[HRP123]

If you don't have HW priority and require a boiler flowtemp of 65C for cylinder heating etc and assuming (10kw) UFH flow/return temps of 40C/32C then the boiler will run very efficiently because the return temp will be the same as the UFH return temp as long as the primary and secondary flowrates are the same, see below, problem is the boiler dT is now 33C which will cause most boilers to trip, you then have increase the primary flow by 1.39LPM to reduce the boiler dT to a acceptable 25C and a boiler return temp of 40C, still very efficient. I showed the LLH in the form below as you can see the whole picture better, a temperature controlled by pass would work exactly the same and possibly give tighter control.

View attachment 81971


View attachment 81972

Thanks again for this, I appreciate the time.

I think the boiler is actually running efficiently and condensing and it rarely cycles when I have observed it, usually (I only have surface measurements) the flow and return are about 20 degrees different.

The issue is more that when only a small portion of the UFH is calling for heat, the boiler seems to run at full capacity during that time. So while the heat demand should be lets say 9kw, the boiler is burning 30kw. I feel like the LLH is allowing a large flow to go to the large underfloor loop and although just a couple of room zones are open - this flow is causing the boiler to work harder than it needs to. So my feeling is the flow and low return temp are working together to make the boiler efficient but work unneccesarily hard if that makes sense?

 

[John.g]

Doesn't make much sense to me, if you have a 9kw heating demand and a 30kw non modulating or a oil fired boiler then the boiler will cycle with a 30% run time but the average output is still 9kw. A modulating gas boiler will run continuously at 9kw and will only cycle if the heating demand is < the boiler minimum output which may be around 5 to 6kw. To prove it take a snapshot of your gas meter at the exact start and finish of a 3 minute period and the boiler output can easily be calculated from that.

 

[HRP123]

Doesn't make much sense to me, if you have a 9kw heating demand and a 30kw non modulating or a oil fired boiler then the boiler will cycle with a 30% run time but the average output is still 9kw. A modulating gas boiler will run continuously at 9kw and will only cycle if the heating demand is < the boiler minimum output which may be around 5 to 6kw. To prove it take a snapshot of your gas meter at the exact start and finish of a 3 minute period and the boiler output can easily be calculated from that.

Thanks! So this is what confuses me. I have a minute by minute snapshot of what the boiler is doing from a Ring camera positioned on it. For example when I woke up this morning at 5am, my gas consumption between 12am to 5am was 65kw since 12am (half hour reads). It was at 200kw by around 4pm.

During this time I can see

12am to 2am only the underfloor heating was calling for heat (it could only be 2 zones as the others were set back)
2am to 4am nothing on
4am to 5am hot water and radiators came on

The boiler currently range rated to around 30kw. I will do more accurate tests over the next few days, but definitely it seems the boiler is working hard than it should be to fulfill the heat demand.

If the return temperature will be delta 20 due to the LLH working - so the flow temperature doesn't decrease. Can a boiler modulate down gas usage without the flow temperature decreasing?

 

[John.g]

A gas meter reads in m3, to convert to kwh, x the difference in readings by 11.0, to get the consumption for one hour, x by 60/time period in minutes, this is the consumption in kwh or rhe boiler output in kw.
Boilers control their output by maintaining the target set point temperature to its setting, if its 65C it will burn just enough fuel to maintain that setpoint temperature and will only stop firing if the heat demand is less than the boiler minimum output, in which case the actual flow temperature will start rising and the burner will cut out at the target flowtemp + 5C, in the above case 65+5 = 70C. The burner will then refire when the the flowtemp falls by 5C below the target flow temp and after the anticycle time has elapsed.
How many loops are in the 2 zones above?.

 

[HRP123]

A gas meter reads in m3, to convert to kwh, x the difference in readings by 11.0, to get the consumption for one hour, x by 60/time period in minutes, this is the consumption in kwh or rhe boiler output in kw.
Boilers control their output by maintaining the target set point temperature to its setting, if its 65C it will burn just enough fuel to maintain that setpoint temperature and will only stop firing if the heat demand is less than the boiler minimum output, in which case the actual flow temperature will start rising and the burner will cut out at the target flowtemp + 5C, in the above case 65+5 = 70C. The burner will then refire when the the flowtemp falls by 5C below the target flow temp and after the anticycle time has elapsed.
How many loops are in the 2 zones above?.

Thanks for explaining...yes I saw the calcs on my energy bill for converting the readings to kwh - however even easier, I have a home hub that displays gas consumption every half hour in kwh.

Appreciate the explanation for modulation. So in theory the boiler could show a flow temp of 65 degrees (the set temperature) but be consuming less than the maximum amount of gas it can burn (so you don't know it is modulating down - I guess maybe just from the noise levels but mine is in the garage). So the burner has the capacity to increase or reduce the flow of gas while burning, it is not just on and off then? This is a 10 year old WB 40cdi.

So the two UF loops which would have fired are an area of 30 + 13sqm = 43sqm. I know the 30sqm is at 100mm centres (we installed during the extension) but not sure about the 13sqm (installed about 15+ years ago - so lets assume maybe 200mm centres?). Not quite sure how to work this out but looking online, lets assume 7 metres of pipe for each sqm then around 300 metres of pipe is what I am guessing is within this area. How would I take this number to convert into what the boiler should be burning?

Thanks for your help!!

 

[ShaunCorbs]

Flow rates on the manifold we can convert into a kw figure

 

[John.g]

Thanks for explaining...yes I saw the calcs on my energy bill for converting the readings to kwh - however even easier, I have a home hub that displays gas consumption every half hour in kwh.

Appreciate the explanation for modulation. So in theory the boiler could show a flow temp of 65 degrees (the set temperature) but be consuming less than the maximum amount of gas it can burn (so you don't know it is modulating down - I guess maybe just from the noise levels but mine is in the garage). So the burner has the capacity to increase or reduce the flow of gas while burning, it is not just on and off then? This is a 10 year old WB 40cdi.

Yes, your boiler will modulate but maybe "only" down to 8kw or so.

So the two UF loops which would have fired are an area of 30 + 13sqm = 43sqm. I know the 30sqm is at 100mm centres (we installed during the extension) but not sure about the 13sqm (installed about 15+ years ago - so lets assume maybe 200mm centres?). Not quite sure how to work this out but looking online, lets assume 7 metres of pipe for each sqm then around 300 metres of pipe is what I am guessing is within this area. How would I take this number to convert into what the boiler should be burning?

Thanks for your help!!

Not familiar with UFH but have seen figures ranging from 75W to 90W/M2 so assuming the lower number then 43M2 might require, 43x75/1000, 3.2kw or 3.8kw using the higher number. IF so, and assuming no other heat demand then would imagine that your boiler should have been cycling fairly regularly.
Those flow loops should/may have flow meters showing the flowrates in litres/min, can you check and post the readings, also the manifold flow/return temps may be displayed.

 

[HRP123]

Yes, your boiler will modulate but maybe "only" down to 8kw or so.

Not familiar with UFH but have seen figures ranging from 75W to 90W/M2 so assuming the lower number then 43M2 might require, 43x75/1000, 3.2kw or 3.8kw using the higher number. IF so, and assuming no other heat demand then would imagine that your boiler should have been cycling fairly regularly.
Those flow loops should/may have flow meters showing the flowrates in litres/min, can you check and post the readings, also the manifold flow/return temps may be displayed.

Yes, one of my manifolds has flow meters but the older one does not have them. Either way what I have observed is the boiler doesn't cycle when only a couple of loops are calling for heat right now. Let me try and get some pictures and take some temp readings from the manifold (Just ordered some pipe clamp type flow/return thermometers). Thanks