How to guarantee maximum condensing?

[Ltnman]

I am about to replace a thermal store that was set at 60C with a condensing combination boiler.

Seeing that the radiators could never go above 60C, which was never an issue in my well insulated house, if I set the output temperature of the boiler to 60C would I be guaranteeing permanent condensing as the return temperature would always be below 57C?

In the boiler manual it states the boiler should be set to maximum as it implies that the TRV’s should set the room temperature but surely this means that the boiler would be less efficient as the chances are the returning water would be above 57C so the boiler would not condense.

 

[ChrisR]

All depends on the boiler

 

[BigBurner]

Some combis have separate temperature control for DHW and CH. Then you can set the CH to 60C. Having a combi with an integral outside weather compensator would guarantee condensing operation for most of the combi run time and lower your gas bills.

 

[Ltnman]

Sorry it is a Worcester Greenstar which does have separate controls for water and heat

 

[simond]

You cannot guarantee permanent condensing.

If you had bought a boiler with weather comp this would take you a bit further as previously mentioned, but the current Greenstar range does not support it.

If you want to save energy, turn down the heat in the rooms you are not using, on a hour by hour basis. Ideally invest in a home BMS like Honeywell Hometronic.

 

[Ltnman]

This is all new to me so forgive my ignorance. Is it true that condensing only occurs when the returning water is below 57C? If that is the case why would a boiler that was set for 60C not return water below 57C so the boiler would condense? A boiler producing CH water at 60C seems fine to me seeing that the old thermal store was set to 60C so should I have the boiler at a higher temperature for peak efficiency?

What has the outside temperature got to do with condensing?

 

[BigBurner]

This is all new to me so forgive my ignorance. Is it true that condensing only occurs when the returning water is below 57C? If that is the case why would a boiler that was set for 60C not return water below 57C so the boiler would condense? A boiler producing CH water at 60C seems fine to me seeing that the old thermal store was set to 60C so should I have the boiler at a higher temperature for peak efficiency?

What has the outside temperature got to do with condensing?

It is 54C. Most of the time a heating system is on part load. That is not on full. If on full as designed, which is when the outside temperature is below -1C, the boiler is on 100% of the time. An outside temperature above -1C outside means the boiler is oversized, so will cycle or need the burner modulated down. This also means the radiators are too big as they are designed for -1C outside and a set radiator temperature, which is 60C in your case, 80C normally. This means the radiator temperature can be lowered for the majority of operational run time, which means higher condensing efficiency. If -1C outside you may need to manually raise the radiator temperature. A weather compensator does all this automatically. Weather compensators and condensing boilers are an ideal match.

 

[Ltnman]

I’m glad I am not a heating engineer as the whole subject is too complicated for my feeble brain and I really appreciate everyone’s time to educate this simpleton

So to recap, when it is bloody cold (below -1C), due to the heat loss in the house the boiler is going all the time if the right sized radiators have been fitted. When it is say + 5C the boiler switches on and off or modulates (what ever that means), as the room thermostat reaches temperature.

Now here is the bit I still don’t understand. If it is off then it is burning no gas and I assume if it is modulating it is burning less gas so why it that a bad thing in relation to condensing when it is burning gas even though it is doing it efficiently? Surely using no gas means a lower gas bill?

So without a weather compensator, running the radiators at 60C is better than running the radiators at 80C in terms of efficiency as there is more chance of the returning water being below 53C so it will condense but to maintain efficiency on cold days I should turn the boiler up a notch or two.

 

[BigBurner]

I’m glad I am not a heating engineer as the whole subject is too complicated for my feeble brain and I really appreciate everyone’s time to educate this simpleton

So to recap, when it is b****y cold (below -1C), due to the heat loss in the house the boiler is going all the time if the right sized radiators have been fitted. When it is say + 5C the boiler switches on and off or modulates (what ever that means), as the room thermostat reaches temperature.

Heating systems are designed for -1C outside and the boiler and radiators sized to suit this temperature. When sized right the boiler will be on 100%. Modulating mean raising or lower the burner, like on a gas hob when you tune it up and down with your hand. Older, and simpler, boilers. had set gas rate burners, that were either off or on. Modulation automatically lowers and raises the gas rate of the burner.

Modern boilers tend to modulate and are most makers try to go for a one size fit all (in most cases).

Now here is the bit I still don’t understand. If it is off then it is burning no gas and I assume if it is modulating it is burning less gas so why it that a bad thing in relation to condensing when it is burning gas even though it is doing it efficiently? Surely using no gas means a lower gas bill?

Modern boilers, depending on the boilers design, lower the boiler temperature to enhance condensing.

So without a weather compensator, running the radiators at 60C is better than running the radiators at 80C in terms of efficiency as there is more chance of the returning water being below 53C so it will condense but to maintain efficiency on cold days I should turn the boiler up a notch or two.

A weather compensator raises and lowers the radiator temperatures depending on the outside weather temperature. At 20C outside it will give 20C radiator temperatures, which is off. At -1C outside it will give 80C, at 10C outside it will give 50C. It is easy to draw a graph from this and calculate the approx radiator temperatures. . The return temperature could be 10 to 20C below this flow (radiator) temperature as the radiators give off heat. It is easy to see the advantages of compensation with condensing boilers. The "curve", or graph line, of the weather compensation can be usually set. The max temperature of the rad could be 60C (you have fitted larger rads) with 0C outside, so the graph will adjust to suit down to 20C rad temperatures when 20C outside.

Many weather compensators sense the inside temperature, say set to 21C, and adjust the rad temperatures down again to suit when 21C is neared. When 21C is reached the boiler is then off. The idea is to keep the boiler operating as low as possible matching input to output and enhances condensing efficiency.

Many boilers come with weather compensators as standard, or an option. You should always go for them as they really do work and drop gas bills and keep the house at a nice stable temperature. Unfortunately most plumbers don't understand them and few outside weather sensors are fitted. Most put on crude room stats. A waste of a good boiler when they do this. Best get men who specialise in heating to give advice.

 

[Ltnman]

Thanks Big Burner for taking the time to write this up. I'm getting there

 

[Agile]

At -1C outside it will give 80C

If the system is correctly designed, when its -1° C outside, the maximum boiler flow temperature will be only 70° C allowing it to still be well into condensing mode.

Just to add more confusion, Dave ( gas 4 you ) says its now -2° C reference instead. But he has not confirmed that and I have seen no reference to any change elsewhere.

Tony

 

[baxpoti]

A flow temp of 63°C is the optimum setting for maximum condensing mode.

 

[BigBurner]

At -1C outside it will give 80C

If the system is correctly designed, when its -1° C outside, the maximum boiler flow temperature will be only 70° C allowing it to still be well into condensing mode.

If a house is correctly sized for -1C outside and 80C radiator temperatures then having 70C radiator temperatures will mean the house will be underheated. Incidental heat, like people, may compensate for the lower radiator temperature at -1C outside

It depends on what is meant by correctly designed. A regular none condensing boiler is designed for 80C radiator temperatures and 70C return temperature, to prevent condensation.

For maximum condensing efficiency, radiators should be designed for 60C flow and 40C return. That may mean they are larger. But as I have emphasised, as most of the time the system is on part load the radiators are too large for 90 to 95% of running time, so over sizing them too much may be unnecessary extra expense. Sizing the radiators to 70C at -1C outside temperature will bring great efficiencies when weather compensation is used and the correct slope is selected.

Having rads at 60c to 40C is fine in places like Scandinavia and northern Germany, as they stay below freezing all day for weeks on end.

Just to add more confusion, Dave ( gas 4 you ) says its now -2° C reference instead. But he has not confirmed that and I have seen no reference to any change elsewhere.
Tony

Many specify to -3C outside. In northern Scotland they do. In a sheltered English city district -1C is fine.

When replacing a boiler and fitting a condensing boiler, the existing rads will be sized to 80C flow temperatures (rads temperatures). Most of the time these will be oversized for 80C flow temperatures then by adding double glazing and extra insulation they will be oversized again. A weather compensator will take great advantage of this and enhance condensing efficiencies by lowering radiator temperatures.

 

[BigBurner]

A flow temp of 63°C is the optimum setting for maximum condensing mode.

That is not enough. It depends on what the temperature drop is across the radiators. If 10C then on when -1C outside 53C should be at the return. But when the house is up to temperature and on part load (the most common operation) the differential may narrow to 2, 3, 4 or 5C, giving a return temperature above condensing point.

 

[D_Hailsham]

A flow temp of 63°C is the optimum setting for maximum condensing mode.

How do you know that? Can you prove it?