CH design for condensing boiler

[Andy Hall]

Do you know how to get the donkey hind legs out of your freezer for dinner? It must be full of them.

[John Rumm]

You would usually balance for a wider flow/return differential. So if you were to run the boiler at say 60/40 nominal[1] then that would give an average dT to the room of 30. So scale the rad outputs by 30/55.

[1] The concept of nominal becomes rather elusive with a system that has a modulating boiler and and TRVs on nearly every rad. You will probably not often be running in the flat out conditions that match your calcs. The other thing to consider is that with a conventional boiler you can chose your own return temperature by virtue of the selected flow temperature and how you balance the system. With a modern modulating boiler, you can probably specify a maximum flow temperature, but not do much more than that. The boilers closed loop control system will adjust down from that maximum to optimise the return temperature, and the nested control loop elements that are the TRVs will in effect try to lower the flow/return differential as they throttle the water passed through each rad.

[Doctor Drivel]

This is where a heat bank scores very well. The items are separated. The CH circuit is run off the cylinder with a modulating Smart pump (Wilo Smart or Grundfos Alpha). TRVs can be on all rads with any affect on the boiler. The rads can be set to 60-40 temp differential. The boiler (pref non-modulating) heats the cylinder. A weather compensator dictates the temperature of the CH section of the cylinder. This ensures a low return temperature and no cycling as the boiler heats up a mass of water to the exactly the right temperature. If a warm day the water will be warminsh, cold day will be very hot. The boilers is in an ideal hydraulic environment with full flow through at all times and far less activation of the controls prolonging longevity.

[John Rumm]

Which is a lot of extra "stuff" to achieve what the modulating control system of the boiler will do quite well for you in the first place, especially if you choose one with a wide modulation range.

What you describe above has more merit with a oil boiler since they tend to have far more restricted modulation ranges.

[Doctor Drivel]

No. The boiler on heat bank is always on full flow and may be a cheaper simpler boiler.

Nope. The boiler is an ideal hydraulic environment with a heat bank. Easier on the controls and no boiler cycling. The CH circuit can just drip feed 0.5 kW from the beat store at just the right temperature. Zoning is easy and can taken off the cylinder too.

Modulating boilers are poor compromise fix to retrofit boilers into existing systems that were not design for condensing boiler systems.

[Andy Hall]

You still haven't justified the "cheaper, simpler boiler" point.

If you refer back to the original question, it was about potentially changing the radiators to match a condensing boiler that may be installed in the future. Hence this point is not relevant.

[John Rumm]

you are spouting rubbish again...

[Doctor Drivel]

He is engaging in major work. This requires a re-think. He asked something and now clearly has pro advice on different approaches.

[Doctor Drivel]

You don't even understand what I wrote.

[Andy Hall]

[John Rumm]

See, we can agree on something.

[Doctor Drivel]

No. O'Really.

[Doctor Drivel]

We clearly do!!

[Ed Sirett]

I don't think the output is a simple proportion of the "Delta T". The radiation output is very non linear and will be next to nothing at all but the highest temperatures where it becomes a significant minority aspect. The unforced convection is also non-linear. Both of these effects are in the "unhelpful" direction in that 1/2 Delta-T will be much less than 1/2 output.

[Roger Mills]

Thanks Ed. I was actually only asking what Delta-T to use. I can work out the effect on the output using tables such as those which appear on a Stelrad radiator brochure, and am aware that it's not a simple linear relationship.

Based on the answers received, a mean rad temp of 60 degC (70/50) seems a reasonable thing to go for, giving a Delta-T of around 40 rather than 55. Taking the ratio of the factors for 40 and 55, the output of my rads will be reduced to about 0.605/0.898 - i.e. about 67% of their current output (whereas scaling the Delta-T values would erroneously give about 72%.)