How to size panel rads to allow boiler condense

[tellme_why]

Hi,

Getting ready to change my Potterton50e to a new condensing boiler and want to make sure it will actually condense - particularly when supplying water to rads.

My Rads are sized to operate at high temperature supply. Last winter I set the boiler at around 70C; apart a couple of weeks when outdoor min temp reached -6C, they did the job to get me to 20C indoor rather fast (about 1.5hours to get from 12C indoor to 20C indoor in the mornings of January 23).

Taking as example the Lounge rads, they deliver 6000 BTU with at Delta T= 50C (i.e. 75C water in from boiler, 65C water out of rad, average in the rad = 70C rad minus ambient of 20C, the Delta Temp is 50C).
Now if I reduce the boiler supply water to say 55C, then Rad power dramatically reduces to 3000BTU - operating at Delta T=30C (i.e. 55C water in from boiler, 45C water out of rad, average in the rad = 50C rad minus ambient of 20C, the Delta Temp is 30C).

Basically I have to double the size of all 12 rads in the property to operate in condensing mode. This is going to cost at least 1.5 grand; is it worth it just to gain 10% more efficiency in the boiler because of condensing mode?

I cannot fit underfloor heating because the ceiling is very low and it will cost a fortune - house was built in the sixites, so cannot improve insulation really.

What people do? Am I overkilling it thinking of changing the rads???

 

[Swwils]

You won't need to buy all new rads, you do the calcs then move your current largest rads to your current smallest rooms, buying only new rads where needed.

You should really go for much flow flow temps - but are you limited by flow?

 

[tellme_why]

You won't need to buy all new rads, you do the calcs then move your current largest rads to your current smallest rooms, buying only new rads where needed.

You should really go for much flow flow temps - but are you limited by flow?

.. not limited by flow I guess.. I have a 3 speed pump so can play with the settings. Taking as an example the lounge - I used stelrad webapp to calculate the heat loss from scratch - forget the old rads I have right now.. the heat loss is 13300BTU.

I selected "DT=50" and the webapp suggests 2 x double panel rads 600 × 1200 mm.

I change to "DT=30" and the webapp suggests 2 x double panel rads 700 × 2000 mm! Which is lots bigger.

Do people size the rads to DT=30 after installing a condensing boiler??? DT=50 meets the BS EN 442 standard.

 

[flameport]

My Rads are sized to operate at high temperature supply.

Were they ever sized properly to the heating requirements, or was it a case of someone slinging them in because they looked approximately correct for things they had installed before?
The fact they work just means they are somewhere between correct or oversized.

You need heat loss calculations for the whole property first - without those, it's just guessing.
Also forget about BTUs - they have been obsolete for decades. It's W or kW.

Do people size the rads to DT=30 after installing a condensing boiler???

They certainly should, and should have been for the last 20 years.

house was built in the sixites, so cannot improve insulation really.

Age of the property is unrelated to insulation, and there are always options for additional insulation.
Some of those may be expensive and disruptive, but then so is spending £1000+ on new radiators.

 

[desktop987456]

they did the job to get me to 20C indoor rather fast (about 1.5hours to get from 12C indoor to 20C indoor in the mornings of January 23).

So in a slightly less colder climate (outdoor 5DegC) if a system is designed with delta T of 8deg C (20-12), what would the flow temp be considering the ambient temp needs to go up by only 8DegC? I am asking because all systems are designed with a design temp of 20DegC, some climates are milder so what would be the design temp for milder climates?

 

[Swwils]

The design temp is a little bit lower.

 

[desktop987456]

If my design temp is 12deg C and I have calculated heatloss based on it for MWT50DegC, I just subtract 12DegC and my system would be 38DegC, Correct?

 

[D_Hailsham]

Condensing boilers are designed to work with a flow/return temperature difference of 20C. The return temperature needs to be below 55C for condensing to occur (the lower the temperature, the greater the condensing). The current "official" recommendation is to set the flow temp of a condensing boiler to 65C, which gives a return of 45C. Assuming a room temperature of 20C, with these temperatures the output of a radiator is about 60% compared to 75/65/20.






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[desktop987456]

Thanks, but practically speaking, is the 20DegC different between flow and return from boiler achievable?

 

[Swwils]

Condensing boilers are designed to work with a flow/return temperature difference of 20C.

This is not true.

To expand:

Condensing boilers are not specifically designed to work with a flow/return temperature difference of 20C. Rather, the key aspect of their operation is that they are designed to condense the water vapor in the exhaust gases to recover latent heat, which significantly improves their efficiency compared to non-condensing boilers. The condensation process happens when the temperature of the flue gas drops below its dew point (usually around 55 degrees Celsius), which can be influenced by the return water temperature, not necessarily the difference in temperatures.

However, maintaining a certain temperature differential (like 20C) between the flow and return is supposedly important for efficient system operation - this is a a popular misunderstanding within the industry that a DT of 20°c is a specific commissioning target for gas boiler systems. A delta t of 20°c is not something to precisely aim for when commissioning, it is simply something to base rough from for legacy reasons. If the Delta T is too low, the boiler will 'cycle' on and off more frequently, which can reduce efficiency and increase wear on the boiler's components. If the Delta T is too high, it could indicate poor circulation in the system, which can also lead to problems.

A common value like Delta T 20°C doesn't necessarily stem from the design of the boiler itself but rather from the overall system design and how it's operated. Here's why:

1. System Balance: The 20°C Delta T is a typical figure that provides a balance between efficient operation and comfort. A larger Delta T could mean the system is not transferring heat into the building as effectively as it could (resulting in lower indoor temperatures), whereas a smaller Delta T could mean the boiler is cycling on and off more often (reducing its efficiency and increasing wear on the boiler components). In reality balancing should be achieved with accurate room temperatures. Balancing does NOT increase condensing at the boiler.
2. Pump Energy: A higher Delta T allows for lower flow rates for the same amount of energy transfer, reducing the energy needed to pump the water around the system.
3. Compatibility with Radiators: Many radiators are rated/advertised to operate at a Delta T of 20°C (a flow temperature of 80°C and a return temperature of 60°C). Thus, maintaining a Delta T of 20°C makes it easier to calculate radiator sizes and to ensure they can provide the required heat output.
4. Historical Legacy: This is also somewhat a legacy from when boilers were less efficient and when pumping energy was a lesser concern. It has now been widely adopted as a "standard" approach.

In modern condensing boilers and heat networks, a larger Delta T (e.g., 30°C or even more) is often preferred because it reduces flow rates, which saves pumping energy and helps the boiler condense more effectively, increasing its efficiency.

Heat pumps are most efficient when the temperature difference between the source and the heat pump's output is minimized. This is why heat pumps are often used with underfloor heating, which requires a lower flow temperature than radiators. The exact Delta T will depend on the specific heat pump and system design, but a Delta T of around 5-10°C is not uncommon for heat pumps.

 

[tellme_why]

hi, what you do to maintain DT 20C? Do you slow down the circulation pump? What happens when TRVs close?

 

[Swwils]

The return temperature is the important aspect not really the dT.

To target a dT you need to weigh up the operation of the boiler and the circulation pump.

1. Adjusting the circulation pump speed: Some modern pumps are equipped with a variable speed drive that can be adjusted to change the flow rate. By slowing down the circulation pump, you increase the time the water spends in the heating circuit, which allows it to cool down more before returning to the boiler. This helps to maintain the desired Delta T.
2. Balancing the radiators: Radiator balancing is another method to help maintain the desired Delta T. This involves adjusting the valves on each radiator to ensure that heat is distributed evenly across the system. This will also ensure that the returning water is at the right temperature. But to be honest you are usually better off by balancing to accurate desired room temperatures.

When Thermostatic Radiator Valves (TRVs) close, it reduces the volume of the system that the water can flow through. This can mean the flow rate through the boiler increases, which can reduce the Delta T if the pump speed is not adjusted accordingly. Some modern boilers and pumps have controls that can adjust the firing rate of the boiler and/or the pump speed to help maintain the desired Delta T, even when the load changes due to TRVs opening or closing.

Remember, the key point about condensing boilers is that they are most efficient when the return water temperature is below the dew point of the flue gases, allowing them to condense and recover latent heat. While maintaining a specific Delta T can help to achieve this, the actual return temperature is what really matters.

If you're having difficulty maintaining a consistent Delta T or achieving condensing operation, you may want to consider getting a heating engineer to look at your system, with the aim to have the system running as efficiently as possible.

 

[desktop987456]

.. not limited by flow I guess.. I have a 3 speed pump so can play with the settings. Taking as an example the lounge - I used stelrad webapp to calculate the heat loss from scratch - forget the old rads I have right now.. the heat loss is 13300BTU.

I selected "DT=50" and the webapp suggests 2 x double panel rads 600 × 1200 mm.

I am also in the middle of changing to a condensing boiler for winters, didn't mean to hijack your post but thought I'd take the opportunity to get the answers I am looking for. Swwils has explained the philosophy very well so I am grateful to both of you (tellme_why for initiating the post).

Like you I know my rads output and how they performed in winters and at what flow rates. The online calculation tools (see attached excel file) are a bit skewed (upwards) and it does calculate heatloss on the optimistic side of things. I did my own heatloss calculations in Excel from scratch and it came around 9-10kw which I know based on the fabric of the property and air circulation (including leakages) at home is correct. I used online tools just to get the confidence on my heatloss calculations.

I have oversized radiators at my place so for me it is a just a matter of putting theory into practice (with condensing boiler and lower flow rates). In my design case I have taken figures to work at DT35 and if it didn't I know it will work at DT40 anyway (since it worked at these temps last winters). My main driver for this change is to save on gas bills, that is why I am going to decommission existing boiler (efficiency of 65%).

Like Swwils said one need to go with as low flow rates as possible so I am going to follow this approach based on the resources I have which are very limited! Thanks

 

[Swwils]

You can aim for low flow rates but it's critical you still can actually deliver the needed energy to the emitters to offset the heat loss - which can mean new, larger pipes aswell.

 

[desktop987456]

You can aim for low flow rates but it's critical you still can actually deliver the needed energy to the emitters to offset the heat loss - which can mean new, larger pipes aswell.

Are there resources available online to calculate pipe sizes (based on distances) to cater for required flow rates to run at different DTs.