Is there a benefit if range rating down

[Gary27]

I have range rated my boiler down, mainly to cut down the noise when it rushes upto flow temperature. (I have weather comp) I was wondering if there is a benefit from range rating as low as you can so long as it eventually reaches temperature and therefore allowing the boiler to condense for longer, my logic being that when it is cold outside the flow temp target can exceed 60+ plus degrees and I know I can lower the curve but that will perminantly lower the flow target temp and what I though was to minimise the time at the hight temperature, and therefore increase condensing.

Any thoughts, or has anybody tried this.
Gary

 

[D_Hailsham]

If you have a combi boiler, the boiler's CH output will probably be much greater than you need. If the CH output is not reduced the return temperature will rise very quickly, so the boiler will cut out. This could result in the boiler cycling so much that the house never reaches temperature.

If you want to know your CH requirement use the Whole House Boiler Size Calculator. If you have a combi boiler, set the Domestic Hot Water Allowance to 0. The boiler output should be set to just above the calculated figure.

The weather comp and the modulating burner should take care of everything.

Don't forget that the output of a radiator varies with the water temperature. So, if the water temperature too low, the rads may not produce enough heat to get the house up to temperature, no matter how long you give it.

 

[DP]

If the CH output is not reduced the return temperature will rise very quickly, so the boiler will cut out

DH, do you not mean the flow temperature?

 

[bhm1712]

the temp is normally measured on the return

 

[Gary27]

The boiler is not a combi. Part of the problem is the boiler not being able to modulate low enough and the flow temp exceeds the target and the boiler shuts down. If I reduce the output it takes a lot longer to reach the flow temp so therefore does not shut down, and as it is at a lower temp for longer then I assume it condenses better.

It's a valliant 418.
Thanks

 

[DP]

the temp is normally measured on the return

BHM, Is the sensor that controls a gas valve (directly or indirectly) not fitted to the outlet from the heat exchanger?

 

[Agile]

Gary, the effect of reducing the power only has an effect when the WC is calling for maximum heat.

So the only effect its going to have is when its very cold outside and the boiler's maximum set heat is required to produce the calculated flow temperature.

So its going to have little effect except when a flow temp above 70 C is required or if its causing the boiler to cycle off.

The actual efficiency is likely to be little different unless your setting prevents on/off cycling which is not very efficient.

Tony

 

[D_Hailsham]

If the CH output is not reduced the return temperature will rise very quickly, so the boiler will cut out

DH, do you not mean the flow temperature?

It's a chicken and egg situation.

If you have a 20kW boiler feeding 10kW of rads, the temperature drop over the system will not be as much as expected and the return temperature will be higher. The flow temp will therefore increase quickly above the set value. The boiler will modulate down very quickly in an effort to maintain the correct flow temperature and then cut out.

Gary27

Your boiler modulates down to 5kW, which is relatively low compared to some boilers.

Why did you have an 18kW boiler installed?

Have you used the online calculator (I gave the link earlier) to find what size boiler you should have?

What is the total output of your radiators? Use the Stelrad Elite Catalogue to find out.

What actual flow and return temperatures are you getting? Parameters d.40 and d.41 will tell you.

 

[Gary27]

If the CH output is not reduced the return temperature will rise very quickly, so the boiler will cut out

Gary27

Your boiler modulates down to 5kW, which is relatively low compared to some boilers.

Why did you have an 18kW boiler installed?

Have you used the online calculator (I gave the link earlier) to find what size boiler you should have?

What is the total output of your radiators? Use the Stelrad Elite Catalogue to find out.

What actual flow and return temperatures are you getting? Parameters d.40 and d.41 will tell you.

As usual I research after the event, the plumber did explain that unlike a lot of things oversizing with boilers is not a good thing. I have checked on various sites and the size for my property seems to come out around 10-12kW.

My difrential between d.40 and d.41 is normaly about 10c. The house is a 15 year old 4 bedroomed modern detached. I have insulated the loft to over 300mm and it is pretty well draught proofed and insulated. I suppose I have to except the boiler will not modulate low enough for what ever reason (radiators just to small? the 10c drop is not enough) an easy fix is to raise the curve and have hotter flow temp but I want to make the most of the condensing and have a low return temp.


Have you any suggestions?
Thanks

 

[David937]

Measure your radiators and check the outputs of them in the Stelrad Elite or similar, catalogue. Add them all up and set d.0 perhaps just a little higher than the value you get. I have about 17kw of radiators and d.0 set at 19kw. This works fine, I did try exactly 17kw but it did take quite long to heat up.

 

[DP]

If you have a 20kW boiler feeding 10kW of rads, the temperature drop over the system will not be as much as expected and the return temperature will be higher. The flow temp will therefore increase quickly above the set value. The boiler will modulate down very quickly in an effort to maintain the correct flow temperature and then cut out.

Regret I fail to understand the concept.

A 20kw boiler will chuck heat at the radiator but if the temperature was to rise beyond the setpoint dictated by the sensor (most often located on the flow pipe, seldom on the return)/ thermostat, flame will downsize till the controls are satisfied, else the burner will extinguish and pump keeps running to 'cool' things down.

Wether the boiler is rangerated at 20kw or 10kw, pump is not going to run any faster, therefore radiators may heat a tad quicker as desired set point is reached quicker. BUT 'hot' water still needs to fill the radiator to achieve the temperature differential across the radiator.

I see this argument as two drivers sitting in two cars, one a Citreon 2CV and other a Jaguar E type V12. E type will be cruising at 70 (with plenty available on tap to go twice that speed) but 'modulation' has taken place for it to now cruise at 70. Meanwhile 2CV, is pedal to metal and it will eventally reach 70 at a leisurely pace when the Jaguar driver is sitting comforably at home sipping a Martini.

Also fail to see why the return temperature will be higher when running at higher (not necessary) power. It is the lockshields and settings that dictate what the return temperature is going to be, not the rangerateing (which often the boiler controls will exercise anyway unless the boiler happens to be an old cast iron with V4600C1029 type of gas valve).

Conceed the point the modulation will take effect sooner if heat output and load are mismatched.

 

[Gary27]

Concerning the 10c drop from d.40 to d.41 I have just checked the drop across the radiators and on the whole it is about 10c some a bit more some a bit less. Is there a way to increase the drop across the rad? For example increase or decrease pump speed.

Thanks

 

[D_Hailsham]

I have checked on various sites and the size for my property seems to come out around 10-12kW.

But, presumably, not the site I suggested.

My differential between d.40 and d.41 is normally about 10c.

Your boiler is designed to have a 20C differential. If you are running with an 10C differential you will run into problems due to the high resistance of the heat exchanger.

However you need to install "oversized" rads if you want to run at a 20C differential. This is because the output of a radiator is about 15% less when running at a 20C differential. You therefore need to "oversize" by about 20%. So you would need a 1200W rad running a 20C differential to provide the same heat as a 1000W rad running at 10C differential.

Use the link I gave to the Stelrad data to find the output of your rads. Then post the results

The house is a 15 year old 4 bedroomed modern detached. I have insulated the loft to over 300mm and it is pretty well draught proofed and insulated.

Has the insulation been improved since the rads were installed? If so, you may find that the rads are now sufficiently oversized to allow you to run with a 20C differential.

an easy fix is to raise the curve and have hotter flow temp but I want to make the most of the condensing and have a low return temp.

Condensing starts when the return temperature is about 55C and increases as the return temperature reduces. So, if you set the boiler to have a flow temp of 75C and return temp of 55C in the coldest weather, the boiler will condense all the time. The weather compensation will reduce both flow and return temps in warmer weather, so increasing the condensation.

 

[Gary27]

I have checked on various sites and the size for my property seems to come out around 10-12kW.

But, presumably, not the site I suggested.

My differential between d.40 and d.41 is normally about 10c.
Your boiler is designed to have a 20C differential. If you are running with an 10C differential you will run into problems due to the high resistance of the heat exchanger.

However you need to install "oversized" rads if you want to run at a 20C differential. This is because the output of a radiator is about 15% less when running at a 20C differential. You therefore need to "oversize" by about 20%. So you would need a 1200W rad running a 20C differential to provide the same heat as a 1000W rad running at 10C differential.

Use the link I gave to the Stelrad data to find the output of your rads. Then post the results

I have had a go at calculating the output from the site you suggested. I believe I have 1x2100w Lounge, 6x700w Bedrooms/Hall/kitchen, 4x300 small rooms of the house.

I used the delta 40.

Total = 7420

To answer your question about improvments in insulation, yes the house has had cavity wall added, new better UPVC DG windows, and 300+mm insulation in the loft, and as much draught proofing as I can. What I can confirm is that I can run the boiler at 7kW and above freezing the house will reach temp although it does take some time.

Does this help

 

[D_Hailsham]

Regret I fail to understand the concept.

So do I, now I think about it more.

Concede the point the modulation will take effect sooner if heat output and load are mismatched.

That was what I was trying to say, but not very clearly.

If there is a serious mismatch between boiler and load, the boiler modulates down very quickly, cuts out and pump overruns. The net effect is that the average water temperature is low, so the rads never reach working temperature.