Temp differentials

[Andygasman2010]

I'm just after some clarification on what it actually means. I know you should get about 11C on a conventional boiler and about 20C on a condenser but if it's low, say 6C, what would that indicate?

I had that today on a micron/s-plan/sealed system/with bypass, complaint was int overheat which I didn't actually experience myself.

I know if temp diff is too high it means there's too little flow around the boiler so if it's too low does that just mean flow rate is too high? Could it mean the boiler/pump is oversized?

Thanks!

 

[ianniann]

It doesn't necessarily indicate anything. It just means that water is flowing through the radiators at a speed that only allows the temperature to drop 6C before it flows out the other side. It doesn't sound like a symptom of an intermittent overheat, just a poorly configured system.

The 11C you see quoted for boilers is just plucked out of the air. It can be higher or lower without anything being wrong. The 20C you see quoted for condensing boilers is again just plucked out of the air but it is a figure that the boiler designers like you to aim for. The only really important number is the return temperature on the flow, which you should certainly try to keep below 60C and really as low as practical. A large temperature drop allows you to get a low return temperature while still getting enough heat out of standard sized radiators. Particularly when some TRVs close and all the water is feeding only one or two radiators (on a system that doesn't modulate the pump down automatically or low enough) then the temperature drop will be quite small and there's nothing you can do about it.

Do all the radiators have the same small temperature drop? Drop the pump speed. If only some of them then balance the radiators. If you're stuck with it because of a fixed rate pump then it isn't the end of the world. You can drop the configured flow temperature to keep the return temperature down for efficiency, if the home owner is comfortable with that. Then you may find the delta T is very low. For example, you're never going to get a 20C drop on a 40C flow.

 

[Andygasman2010]

Thanks for that, I suspected they didn't have to be spot on. I usually get a good idea of how a system is working by just feeling the flow/returns.

The only really important number is the return temperature on the flow

I might be being stupid here but does that bit make sense? Did you just mean the return temp?

Thanks

 

[DP]

11 degree C is often quoted for cast iron boilers as larger differential could lead to cracked heat exchanger (have seen this take place). 20 degrees is often quoted for copper, SS or aliminium HE (especially the steamers) which need return to be 55 or less (as has already been suggested)

 

[ianniann]

The only really important number is the return temperature on the flow


I might be being stupid here but does that bit make sense? Did you just mean the return temp?

Yes, the temperature of the water returning to the boiler. For a condensing boiler, you can wring an extra few percent of efficiency by having this low. It can be near impossible to get a 20C drop with existing radiators, but there's no point running a pump faster than you need to when the boiler is designed to work well with a big temperature drop.

 

[D_Hailsham]

The 11C you see quoted for boilers is just plucked out of the air.

Not correct. It goes back to the days when Fahrenheit was the normal temperature scale and boilers were designed to work with a differential of 20°F. A quick bit of maths shows that a differential of 20°F is equivalent to a differential of 11.11111°C.

The 20C you see quoted for condensing boilers is again just plucked out of the air

It's connected to radiator specs and dew point. Modern rads are tested with a flow temp of 75°C and return of 65°C; a 10°C differential. Boilers do not condense until the return temperature is below the dew point (about 55-57°C). So, if the boiler runs with a flow temp of 75°C and return of 55°C, the differential is 20°C.

The problem then is that the radiator only produces about 85% of the stated output, so they have to be oversized by about 20%.

 

[ianniann]

There's still nothing magic about those numbers. A condensing boiler can work just as efficiently with a 10C drop or a 25C drop. I suspect half the condensing boilers in the country are running with less than a 10C drop, at least for a good chunk of the time.

The dewpoint temperature is also not a magic on-off point where you suddenly get an extra 10% efficiency. Above the dewpoint, you are stuck without condensing at about 88% efficiency. Below the dewpoint and you get a more or less gradual increase in efficiency as the return temperature gets lower and more heat is extracted from the exhaust gas. At very low 20C-30C return temperatures the efficiency will be around 96%. One degree above the dewpoint and one degree below it you are getting essentially the same efficiency. All this is easier to show with a picture, I'll run one up if people need to see it.

 

[D_Hailsham]

There's still nothing magic about those numbers.

Agreed. I was just pointing out that there was an historic and logical basis for them. Presumably boiler designers carried out experiments and found that the old cast iron versions worked best with a 20F differential; and this figure has stuck, but converted to 11C.

A condensing boiler can work just as efficiently with a 10C drop or a 25C drop. I suspect half the condensing boilers in the country are running with less than a 10C drop, at least for a good chunk of the time.

I agree that the actual drop is not significant. It will, in any case vary according to the instantaneous heat requirement. It all depends on what temperatures they are running at and what type of control system they have. I also suspect that there a many condensing boilers which seldom condense.

All this is easier to show with a picture, I'll run one up if people need to see it.

That would be helpful.

 

[ianniann]

OK, I think I have an image uploaded now.

This is a rough sketch of the efficiency of a condensing boiler at different input water temperatures. The flat portion is more or less fixed at 88% for just about every gas boiler out there. The exact dew point temperature where condensing kicks in can vary slightly but in practice is always at 55C-60C. The shape of the curved portion varies with the details of the heat exchanger sizing, the overall power, input and output temperatures, etc, but the basic shape remains the same. It might rise a bit more or less steeply near the dew point temperature, but it will always tend towards the 96%-98% level at very low input temperatures.
View media item 21640

 

[Slugbabydotcom]

but there's no point running a pump faster than you need to when the boiler is designed to work well with a big temperature drop.

While that is largely true it does not apply to combi's which need maximum flow for the HW function. Most new combis if not all now have the pump speed function removed.

 

[ianniann]

While that is largely true it does not apply to combi's which need maximum flow for the HW function. Most new combis if not all now have the pump speed function removed.

I don't get this at all. Many new combis have controllable pump speeds, even quite sophisticated pump maps since they often have automatically modulating pumps to cope with variable flows when TRVs turn on and off. Maybe some have no pump speed control, not certainly not all.

And what does it have to do with the hot water? How many combis run the hot water through the circulating pump? Am I missing a trick here?