Plume from heating boiler

With a condensing boiler the manufactures default setting may have been a 75C flow temperature and with a resulting 60 to 65C return temperature from the central heating radiator system. This would mean that the boiler wasn't working in its condensing mode. Reducing the flow down to

In general, and for most of the central heating season, most people could live with a 60C flow through their radiators.

The recommendation was carefully worded for the CH heating side of combi boilers where no domestic hot water tank is involved.

There was/is some concern that with a hot water tank being heated by the boiler reducing the flow temperature to the heating coil in the tank wouldn't get the water hot enough (>60C) to kill legionella. The risk is possibly overstated as in an occupied premises the water in the hot water tank is always being replenished with water from the mains dosed with Chlorine. An electric immersion heater could be switched on perhaps once a week for a very short time to boost the boiler heated water to above 60C. Some boilers may also have a inbuilt heating cycle to occasionally heat water in a tank to above 60C.

Last winter Martin Lewis of Money Saving Expert was tirelessly advocating the benefits of lowering the boiler stat to save money in the fuel crisis. Later he said although it would save money, a low boiler temperature below 60 Celsius may not heat some properties enough and you would have to determine your priority.

I was careful to compare the plume from two heating systems in two neighbouring houses. This would mean there are not effects from different air temperature, humidity, wind, etc. Also I have observed the two plumes for many days on different occassions.

However, as you point out, the two heating systems may also be at different points in their daily usage which would create their own variance.

I'm interested because I tend to regulate the heating by varying the boiler temperature daily, which most people wouldn't do. There are TRVs and an portable thermostat but their settings don't reflect room comfort as much as I would like.

That's the problem with a single general recommendation. If designed properly* most old radiator systems were designed for a flow temperature higher than now recommended and for a return temperature perhaps 11C lower. If you lower the feed temperature to an radiator then you get less heat from it. If you have improved the insulation or changed single glazing for double glazing etc. then those old radiators operating at a lower output could still provide enough heat.

You can replace an old gas boiler with a modern condensing boiler and when run at the elevated temperatures you would get the same performance as with the old boiler, and probably with some efficiency benefits due to better technology. To get a better efficiency other components in the system may/will have to be changed or more spent on controlling the boiler.

IIRC it was Grant Shapps:

Yes, I tend to switch the flow temp up if it's colder. No weather compensation here - I had it at my last place but even at its most extreme setting it was still too cold.

Yes, it does seem to cycle more frequently at a lower setting - not measured, just casually observed.

Thanks, yes, I'll take a more critical look!

Mine was, because I designed and installed it myself. Over the years I have made changes (added a conservatory with underfloor heating, increased the boiler size to allow for the extra demand, given each room its own timer/stat and motorised valve). I am in the process (started a few years ago) of replacing now 30 year-old radiators with new, larger radiators to allow for lower temperature supply from the boiler.

Thank you, that is the best description I have seen. In fact, it is the only description that I have seen.

Well, duh!

That recommendation is too simplistic. It relies on the boiler being able to perform at a lower output to be effective, ie it needs to be able to turn the burners down (modulate) to the required level.

I tried this with my 2006 Baxi boiler and it ended up short cycling, so as soon as the radiators were warm enough it would turn off but then shortly afterwards would have to turn back on again. Short cycling is (as I understand it) bad for boiler life, on/off/on/off in short order.

The problem arises because my boiler will only turn down to about 1/3 (Full power 30kW, lowest power 11kW). A more modern boiler would modulate to about 6kW and thus run at a nice "simmer" constantly which is good for the boiler.

The efficiency gain can be seen at:

You may be reading too much into what you can "see"...

With a traditional boiler there would be a significant quantity of water contained in the combustion gases from a normal boiler, but since they would leave the boiler at several hundred deg C, they would be quite well dispersed by the time they have cooled enough to form a visible plume.

A condensing boiler will condense the bulk of the water out of the combustion gases before they even get to the flue (and in the process recover a significant amount of heat). The flue gas temperature will also be much cooler. So any remaining water in the combustion gas will condense to visible water droplets sooner, and closer to the flue than with a conventional one.

So the condenser gets a big step in efficiency from its basic design. However there is a relationship between the return water temperature in the main HX, and that efficiency. It improves further as it gets lower, and shows a slight step change in the rate of improvement around 54 degrees.

There is interplay between the amount of water remaining in the flue gas, and its temperature. Less water may give less scope for visible plume, but lower exit temperatures means any water present may form a visible plume closer to the flue where it will be more concentrated and visible. So a notable difference in the amount of visible plume may not indicate much actual change in efficiency. In much the same way a cool damp day will also have an effect on the amount of visible plume, but not indicate less efficiency in the boiler).

You have a few layers of requirement:

Flow need to be hot enough to heat the house to the desired temp Return wants to be cool enough to boost heat recovery

However, if you engineer that temperature too low, you may force the boiler to start cycling to hit that target flow temperature. That actually lowers efficiency rather than increases it, and also causes more wear and tear on the boiler.

The ideal solution is weather compensation and well designed control systems to fine tune the operation in real time.

Not quite as simple as this. Many modern boilers may still only have a

Try this link

Conventional systems were typically balanced for an 11 degree drop across the rads. Condensing system usually want a wider spread like ~20 degrees.

For a good proportion of the year, more than likely... in the milder bits of the year, mine often runs flow temperatures under 50.

Split temperature operation sorts that, but not all boilers support it, and not all installers seem aware of it. (probably why they like installing combis!)

Yup, mine takes it to 70 one day a week.

All this talk of balancing for a specific return temperature is fine for a system without TRVs but I can't see (but would appreciate being enlightened) how it can be feasible in a modern system with individual control of room temperatures. Mechanical TRVs are an example, but a more extreme example is the Honeywell Evohome system which fits a motorized valve and temperature sensor on every rad. The sensors report back to a controller which sends commands to the valves, based on the programmed schedule for that room (without checking I think the granularity is 10 minutes and 0.5 degreesC) and sends commands to the zone valves when there is a call for heat. I have about 20 areas running on Evohome, each with their own schedule, several of which are set to a low temp unless we have visitors. How can it be possible to "balance" a system with a load that varies so widely?

My Vaillant was set to 60C flow during the frosts; it's now on 55C and will be 50C later in the week. I control the temperatures in various rooms with wheelhead valves and the valve where the 'stat is situated. Due to misty weather it's difficult to use the plume so I just feel the temperature at the flue - if it's coolish then condensing is OK>

Do you actually have 20 zones or just 20 radiators controlled by "smart" TRVs? A programmable TRV with a schedule is fundamentally no different to a manual TRV as far as the boiler is concerned.

The system is first designed without reference to TRVs because worst case is with them all fully open. Balancing traditionally means that you adjust lock shield valves so that all radiators reach the same temperature with the TVRs open.

These days you don't necessarily have to wait for the heating to come on and then run around like a blue arsed fly checking the temperature of each one. The exercise can be performed on paper by those designing the system properly. You can get Auto Balancing TVRs

Modern CH pumps can operate on proportional pressure control and change flow when TVRs or zones shut down.

Weather compensation input to the boiler's control can adjust the output of the boiler for different outside temperatures.

Yes, but surely that's just to size the boiler and pipework ...? Balancing traditionally means that

Interesting. I hadn't hear of them but found this video:

Yes, mine does that but how does that help with balance?

Unfortunately my W-B boiler can't be fitted with wx compensation - something I overlooked when spec'ing.

I've just realised that I mentioned "balance" but what I should really have queried is how it's possible to design for a certain dT at the boiler with a Smart system.

Last year I had a new Baxi 830 installed. Was your older model similar to this one?