Balancing for condensing boilers and what pump speed

[sxturbo]

Hi all, me again,

Have sorted out my heating issues, was an issue with the stainless hoses that were fitted many moons ago.

I will be draining and flushing the system next weekend of the fernox f5, and installing fernox inhibitor for their filter.

When I do this for the last time hopefully, I will go around and balance the radiators again, as some of them are hot in places and cool in others, however I am now pretty certain this is a balancing issue.

Although the main culprit which has been all along is still the coolest and is the 4th rad on 15mm pipe.

The boiler I have is a ferroli domicondense 26 he, there seems to be vastly conflicting information all over the internet to what I balance the rads to (not specific to the ferroli but condensing boilers in general). I can't seem to get a 20 degree difference between the flow and return on any individual rad, but I understand condensing boilers are best when the return is 55 degrees or less, do I set the temperature of the flow to be 65 degrees as it leaves the boiler and set the rads at the 10-12 degree difference and hit the magical 55 degree return for the condenser?

Lastly when I moved in the pump was always set to 3 now I have the heating working and have removed the majority of the restrictions I have turned the pump down to 2, this was also to aid the temp difference, however again I have read conflicting information, but most seem to say the pump.on a combination should always be on speed setting 3?

Hopefully you guys will be able to provide me with some definitive answers.

Thanks in advance

 

[Terry lambert]

The pump should be on speed three, delta T is decided at the boiler and is designed to be 20DT across your system.

Cheap valves have little authority and so small adjustments can give big differences in flow, better quality valves make balancing a lot easier

There is a post on how to balance your system in the F&q above

 

[Terry lambert]

The cooler your return the more condensing and the more efficient your boiler will be

If your rads are running at a mean temp of 55 they may or may not heat the room, depends how they were sized

 

[John D v2.0]

but I understand condensing boilers are best when the return is 55 degrees or less, do I set the temperature of the flow to be 65 degrees as it leaves the boiler and set the rads at the 10-12 degree difference and hit the magical 55 degree return for the condenser?

Condensing boilers are better the cooler the return, weather compensation and other smart controls will always choose the best water temperature for your system.
Regarding 55c or other similar magic figures, that is the often quoted point where much of the water will condense. In a powerful Combi you will struggle to get 20c difference on rads without most of the water bypassing, and you'd get more heat output with less temp difference anyway (higher average temp on the rad)
I just set the flow to between 50 to 60 based on the weather forecast and leave it on all day.

 

[sxturbo]

The pump should be on speed three, delta T is decided at the boiler and is designed to be 20DT across your system.

Cheap valves have little authority and so small adjustments can give big differences in flow, better quality valves make balancing a lot easier

There is a post on how to balance your system in the F&q above

Thanks i will up the speed of the pump back to 3, i must admit since fitting the magnet filter and filling with fernox f5 the boiler is so quiet, you can hardly hear it on. i seem to have the cheapest of cheap valves fitted as they are virtually either on or off lol

The cooler your return the more condensing and the more efficient your boiler will be

If your rads are running at a mean temp of 55 they may or may not heat the room, depends how they were sized

I did wonder this and thought that if it is going to take longer to warm the house due to the temperature being set lower it would negate the benefits of condensing as it will be on for longer.

i'll leave the heat setting at 75 degrees and just set the rads to the standard 11-12 degree drop

 

[D_Hailsham]

The pump should be on speed three, delta T is decided at the boiler and is designed to be 20DT across your system.

All Ferroli say is that Delta T must not be greater than 20C.

I don't see how the boiler can decide dT, considering there is only one sensor, which measures the flow temperature. I would have thought a second sensor, measuring the return temperature, would be required. All they can do is modulate the boiler down if the flow temperature rises above the set point.

Why does the pump have to be on speed 3?

 

[Terry lambert]

I can be flippant in my posts sometimes, ill try not to be in future knowing you’ve got my back

The comment about delta T being decided at the boiler probably wa#nt much help to the OP but my head was elsewhere.

The delta T is decided where the heat is inputted as per mass flow rate would have been a better comment but probs should have left it out here

Speed 3 on a combi because you want as narrow a delta T as possible on hot water production

 

[Terry lambert]

What’s you background @D_Hailsham ?

 

[Terry lambert]

To the OP you should be looking for a delta at of around 20 with your boiler, your boilers minimum output is around 10KW which I’ll be a struggle to make efficient on what I’m guessing is quite a small system

 

[sxturbo]

To the OP you should be looking for a delta at of around 20 with your boiler, your boilers minimum output is around 10KW which I’ll be a struggle to make efficient on what I’m guessing is quite a small system

Thanks Terry,

I think the system is average size wise, 8 radiators in total, 4 are rather large, the remaining 3 are medium plus one towel rail, however the actual pipe work for the house is of poor design, and has been modified from a back boiler, with extensions happening in on the house and then modified again to a combination system before I bought the house.

At the moment we have delta 50 through the system.

I can't see how I can get it down to 20, in the manual it states a maximum temperature difference of 20 degrees between flow and return, not a delta of 20.

I apologise for all the questions and not knowing 100% on what I'm on about, however I have had a Number of trades in who have been about as much use as a chocolate fire guard and I have ended up re-doing or doing what they were paid to do. Recently have had loads of plumbers/heating engineers who have only managed to have made things worse (granted most of them were homeserve engineers so may not have been top of the trade who have )

 

[Terry lambert]

Sorry, delta T is temperature difference so you are looking for say a flow of 70 and return of 50

 

[sxturbo]

Sorry, delta T is temperature difference so you are looking for say a flow of 70 and return of 50

Ok that makes more sense I googled delta t, and read the stelrad page about it I must have misunderstood it slightly.

Will see what I can do at the weekend with it.

 

[endecotp]

if it is going to take longer to warm the house due to the temperature being set lower it would negate the benefits of condensing as it will be on for longer.

No; setting the temperature lower (hopefully) means the boiler uses gas at a slower rate, so the total gas usage will be lower even though it's on for longer.

The difficulty is that there is a limit to how low that rate can go (Terry says 10 KW). Below that things get inefficient for other reasons.

 

[sxturbo]

No; setting the temperature lower (hopefully) means the boiler uses gas at a slower rate, so the total gas usage will be lower even though it's on for longer.

The difficulty is that there is a limit to how low that rate can go (Terry says 10 KW). Below that things get inefficient for other reasons.

Ok thanks.

 

[D_Hailsham]

At the moment we have delta 50 through the system.

I can't see how I can get it down to 20, in the manual it states a maximum temperature difference of 20 degrees between flow and return, not a delta of 20.

Boiler and radiator manufacturers' use delta T to mean different things.

To the boiler manufacturer it is the difference between flow and return temperature. So a Flow of 75C and Return of 55C gives a 20C Delta T (75-55)

To the radiator manufacturer it is the difference between Mean Water Temperature (MWT) and Room temperature. A radiator with flow of 75 and return of 55 has a MWT of (75+55)/2 =65C. If the room temperature is 20C the radiator Delta T is 45C.

As the heat output of a radiator depends on Flow, Return and Room temperatures, all rads have to be tested to a common standard, BS EN442. This specifies Flow = 75C, Return = 65C (MWT=70C) and Room =20C. Hence the rad Delta T of 70-20=50C.

If you want to run at a 20C boiler delta T, you have to take this into consideration when selecting your radiators as the output will be less than that quoted by the manufacturer. There is normally a table in the radiator literature showing the correction factor for different (radiator) delta Ts.