Why range rate a modulating boiler?

[mrbg07546]

Yes . I have a system boiler. With hwp. You can individually range rate the hot water too. But I left that as is.

 

[mrbg07546]

Reason I left hot water is because the manual of cylinder advises what is best output for cylinder size. I had a 260L cylinder and I left it at 22kw. I think my engineer would have set this. That’s one thing he set correctly. I did notice with vaillant. It’s best to leave pump on auto and range rate the boiler. The pump will work depending on what you range rated it at.

 

[JonathanM]

Interesting. I have a large 5 bed and range rated my 27kw vaillant system boiler down to 6kw. Seems to be fine. I might even try a bit lower

I had recently been wondering, myself, how range rating to such a low level, with a large property, would work in practice. So, how do you run the boiler? Presumably, it must be on for long periods at a stretch at the moment. Do have weather compensation or OpenTherm. Do you ever feel the house needs a boost first thing in the morning.

Yes . I have a system boiler. With hwp. You can individually range rate the hot water too. But I left that as is.

I was thinking you must have.

 

[Johntheo5]

I think Vaillant depend more than some other makes on range rating to avoid overshooting the flowtemp SP by 5C and tripping the burner, from what I have interpreted from some vaillant owners is the the burner doesn't seem to start to modulate until the flow temperature reaches and exceeds the SP temperature even allowing for the 60 sec delay, some posted that even with the boiler running normally that if they increase the SP by 4 or 5C that the temperature will exceed the SP by 3 or 4C and in some cases even reach 5C and trip the burner. Any burner control IMO should have a derivative term in which the burner controls start modulating before the SP is reached and not just as it exceeds it.

 

[mrbg07546]

I had recently been wondering, myself, how range rating to such a low level, with a large property, would work in practice. So, how do you run the boiler? Presumably, it must be on for long periods at a stretch at the moment. Do have weather compensation or OpenTherm. Do you ever feel the house needs a boost first thing in the morning.



I was thinking you must have.

I did a test by range rating down to 5kw it was a little slow to heat up for my liking so I left it at 6kw.

Remember the 1.5kw for radiator only apples to very leaky homes, running a flow temp of 75. With smallish radiators. I found if you have set your trv well(never typically turn them off but a low set back, for rooms your rarely use) then I use a low heating curve which relates to a low target flow tempreture, and my pipe work is new and radiators won’t really need a target flow of over 60 for even the coldest of days you only need 0.5kw per radiator. Heating up slow gas it’s advantages too. More time at lower temperatures encouraging high effeincy condensing. Also you have to factor in , bigger rads, better insulation = lower flow temps and better condensing.

 

[mrbg07546]

I think Vaillant depend more than some other makes on range rating to avoid overshooting the flowtemp SP by 5C and tripping the burner, from what I have interpreted from some vaillant owners is the the burner doesn't seem to start to modulate until the flow temperature reaches and exceeds the SP temperature even allowing for the 60 sec delay, some posted that even with the boiler running normally that if they increase the SP by 4 or 5C that the temperature will exceed the SP by 3 or 4C and in some cases even reach 5C and trip the burner. Any burner control IMO should have a derivative term in which the burner controls start modulating before the SP is reached and not just as it exceeds it.

I tested this. When it nears your set room tempreture it decreases target flow temp (and I presume this is the point it’s modulating). I do think work out your max target flow is important. Most advice is to set your thermostat to 24 on a cold day and test. I disagree. To save money you set to a realistic value . Eg 21 and then test. Also a big factor can be how long your willing to wait for your set tempreture to be met. If your at home al day it can be lower still. As mentioned if your get flow tempreture can be low, it’s worth range rating low . Eg 0.5kw a radiator

 

[Johntheo5]

I would prefer to set the range rating to the max output I calculate based on my requirements from say -5C and my rad output, if I reckon that a 37% rad output will be sufficient to heat my house from -5C to 22C then assuming total rad installation of 25kw, I would range rate the boiler to , 25*37%, 9.25kw, say 10kw and set the flow temperature to 52C (to achieve the required 37% rad output) if outside temperature compensation is installed then set it to curve 1 and that particular curve will pretty closely match the rad outputs to the heat requirements as the ambient rises, ie at 7C it will reduce the flow temperature to 42C, 6.7kw rad output.

 

[vulcancontinental]

I would prefer to set the range rating to the max output I calculate based on my requirements from say -5C and my rad output, if I reckon that a 37% rad output will be sufficient to heat my house from -5C to 22C then assuming total rad installation of 25kw, I would range rate the boiler to , 25*37%, 9.25kw, say 10kw and set the flow temperature to 52C (to achieve the required 37% rad output) if outside temperature compensation is installed then set it to curve 1 and that particular curve will pretty closely match the rad outputs to the heat requirements as the ambient rises, ie at 7C it will reduce the flow temperature to 42C, 6.7kw rad output.

View attachment 286042

If the radiators are sized accurately, and most are not, they're oversized so of less importance, the the design load for that room would only be met with a 75⁰C flow and a 65⁰C return as that's how the rads are certified. If you need X watts that's what you'll need, cooler water with a closer ∆T will not give you the output.

 

[Johntheo5]

Oversizing definitely helps to get lower return temps = more condensing but even if they were sized perfectly as 50deg rads there are still quite substantial savings to be made at ambient temps from around 7C upwards by temperature compensation or even manuall adjusting the flow SP.
If the rads are designed for a room temp of 20C from a low of -5C then they are 50 deg rads,
rad output flowtemp/returntemp/dT/flowrate. 1kw/75C/65C/10C/1.43LPM
if ambient is 0C then 80% rad output required. 0.8kw/66.2C/58.15C/8.1C/1.43LPM
if ambient is 7C then 52% rad output required. 0.52kw/52.9C/47.7C/5.2C/1.43LPM
if ambient is 10C then 40% rad output required. 0.40kw/46.7C/42.7C/4.0C/1.43LPM

Set manually or, in this case, curve 2, above.

 

[vulcancontinental]

Oversizing definitely helps to get lower return temps = more condensing but even if they were sized perfectly as 50deg rads there are still quite substantial savings to be made at ambient temps from around 7C upwards by temperature compensation or even manuall adjusting the flow SP.
If the rads are designed for a room temp of 20C from a low of -5C then they are 50 deg rads,
rad output flowtemp/returntemp/dT/flowrate. 1kw/75C/65C/10C/1.43LPM
if ambient is 0C then 80% rad output required. 0.8kw/66.2C/58.15C/8.1C/1.43LPM
if ambient is 7C then 52% rad output required. 0.52kw/52.9C/47.7C/5.2C/1.43LPM
if ambient is 10C then 40% rad output required. 0.40kw/46.7C/42.7C/4.0C/1.43LPM

Set manually or, in this case, curve 2, above.

Always interesting to see someone else's approach, nicely put.

I find however most installed radiators are very often oversized and a curve of 1.4 re even 1.2 can be used. I'm currently experimenting with a curve of 1.0 (different brand of boiler graph) but it's only 9 degrees today so time will tell and I have one room particularly difficult to get heat to circulate so probably will need to adjust up again.

 

[Johntheo5]

Your rads temp of 49C + 9c ambient = rad output of 35% would possibly suggest that your rads are oversized by a factor of ~ 1.26.
((20-9)/(20--5) / 35 X 100.

 

[vulcancontinental]

most are oversized by more than that but the main one in what we call the snug is in an exposed bay with a lot of single glazing and a ceiling lower than the rest of the room so the air doesn't circulate well. The room sensor is also therefore the whole property revolves around an imperfect room.

 

[mrbg07546]

Always interesting to see someone else's approach, nicely put.

I find however most installed radiators are very often oversized and a curve of 1.4 re even 1.2 can be used. I'm currently experimenting with a curve of 1.0 (different brand of boiler graph) but it's only 9 degrees today so time will tell and I have one room particularly difficult to get heat to circulate so probably will need to adjust up again.

I’m actually using 0.8 in a 1960 home. At the moment it’s working ok. It’s always hitting tempreture. I find a low heating curv works better if your at home a lot, and hence you have a low set back set back tempreture. A low heating curb struggles heating quickly enough when you have been away and heating.

At my 0.8 At 10 degrees does 36-38 and 6 degrees is doing 40.

 

[JonathanM]

I’m actually using 0.8 in a 1960 home. At the moment it’s working ok. It’s always hitting tempreture. I find a low heating curv works better if your at home a lot, and hence you have a low set back set back tempreture. A low heating curb struggles heating quickly enough when you have been away and heating.

At my 0.8 At 10 degrees does 36-38 and 6 degrees is doing 40.

If you ever feel like you need a quicker temperature boost, what can you do? Do you simply increase the heating curve?

 

[Johntheo5]

The curve above, 40C at 6C follows Curve 1 on the Vaillant? one above in post 22 which means that it looks for 53C at -5C ambient, this in turn means the rads are rated at 50% of their design output, so if you had say 20kw of installed radiators then you might range rate the boiler to 10kw, so if you need a quicker temperature boost by chaging to a higher curve then you also have to range rate the boiler to a higher output, no big deal I suppose as long as you remember it.