Gas central heating thermostat / timer settings

Scouter

Thanks Malc, interesting reply.

So, if I've understood you correctly, you are saying the most efficient way for me to run the system is to keep the timer and temperature settings as they are, BUT turn down the boiler temperature?

To be honest, I've been wondering about the water temperature. I turned it up last winter when the radiators just didn't seem to be getting as hot as they should. Eventually, the pump failed, so I guess the radiators weren't getting hot enough because the pump wasn't operating at full capacity before giving up the ghost. By this time it was at the tail end of needing the heating as summer (finally!) arrived, so the temperature has remained set to max since then, despite having a new pump.

Anyway, I'll turn that down and see how we get on.

Talking of temperature loss, you've just reminded me: The first October we had the current boiler fitted, we went away for a week at half term. I turned the temperature down quite low (12 or 14c possibly, I can't remember now), on the basis the house was going to be empty, and the heating was unlikely to come on. During that week, the weather changed from being quite mild (like now) to having the first frosts.

If I remember correctly, the average day time temp outside was around 10c when we got back, so the boiler still kept running occasionally to warm the house to the low setting (no saving there) and then took an age to warm it back up to our prescribed settings, to the point where I'm certain it will have wiped out any savings from lowering the temperature while we were away!

Scouter

Cardew: (sorry, you posted while I was typing!)

From memory, and without checking, I'm sure somewhere in the bumph from Worcester Bosch, it suggests the water temperature should be set to maximum, where possible (or words to that effect).

Cardew

Scouter wrote: »

Cardew: (sorry, you posted while I was typing!)

From memory, and without checking, I'm sure somewhere in the bumph from Worcester Bosch, it suggests the water temperature should be set to maximum, where possible (or words to that effect).

Further to my post about boiler water temperature. This Worcester Bosch publication gives advice on the best way to use their boilers - nowhere does it mention turning down the water temperature:

http://www.worcester-bosch.co.uk/homeowner/boilers/boiler-controls/how-boiler-controls-could-help-with-fuel-bills

Similarly on their latest regular boiler there is nothing in the user manual about setting a low water temperature, in spite of a section on economy.

http://www.worcester-bosch.co.uk/homeowner/boilers/gas-boilers/greenstar-40cdi-classic-regular/literature

My understanding was always that a condensing boiler regulated the return temperature of the water by controlling the water flow.

malc_b

Wikipedia or just look at sedbuk data. Condensing (oil) boilers are ~8% (for oil, gas is a bigger difference AFAIR) more efficient. That because they condense but if the return water temperature is too high they don't condense so they become non-condensing boilers and thus lose efficiency.

I guess you could also set the flow rate low to get more than the normal 10C drop i.e. 75C out, 55C return, rather than 65C out, 55C return. 55C return is reckoned to be the cut off point for condensing.

FYI the normal radiator is ~85C, 75C out, average 80C, 60C above a 20C ambient. That is when it is 100% of its spec figure (i.e. a 600W radiator gives out 600W). At 65C, 45C diff, it is ~70%, 40C diff it is 60%.

As you can see there are conflicts here. Unless you have over large radiators you cannot get a condensing boiler to run in condensing mode if the outside temperature is very cold. There is also a conflict with hot water temperature. Legionella multiplies 20-45C and dies 60+C so having a boiler temperature too low risks Legionella.

Putting on my cynic hat I would say that running the boiler at low temperature is not in the manual to avoid the making being sued over Legionella or to save people complaining their house is cold. Lack of efficiency is hardly likely to cause complaints since how would the average person know?

Cardew

It is not in dispute that in condensing mode the efficiency is higher than non-condensing boilers or condensing boilers in non-condensing mode.

However, with respect that wasn't the exam question!

I really am not trying to score a point here, but you made the statement below, and I have seen it quoted before on MSE.

Condensing boilers ... lose significant efficiency
when you run them with high radiator temperatures.

I have seen no technical data to back up that statement, and why wouldn't such an important point be highlighted in all user instructions - especially those concerning efficiency?

Your 'cynical' point about Legionella is not supported by this extract from user instructions:

Turn the central heating temperature control [6] to the
desired level, between approximately 35°C and 90°C.

Surely if your contention about running at low water temperatures is correct, there would be some mention of it in all the brochures; and if the OP is correct, his user manual tells him to run at max water temperatures.

Also I have about 20 radiators(most doubles) in my house. On occasion I might want them all turned on and the water would be dissipating a great amount of heat, on another occasion perhaps just one or two would be on.

That would be quite a normal scenario in many houses, how could you set a water temperature, to return cool enough to enter condensing mode, to cope with varying demands on the heating system?

Scouter

I should just clarify, as I alluded to in post #2, my boiler is actually gas, but I posted in the wrong sub-forum.

malc_b

Cardew it depends on your definition of significant then. Given that the difference between running 24/24 and not is a small percentage then 8% becomes a significant amount. Again I've done the maths for this in another thread.

As for Legionella why don't you google it? Try safe water temperature which gives this from The Chartered Institute of Plumbing & Heating Engineering as 1st hit.

You have a strange way of running your heating system. All my radiators are left on all the time (but they do have TRVs). I let the main thermostat sort out how long it runs for. Turn off radiators just upsets the balance and at the end of the day the insulation in the house shell, not inter room. Turn off radiators and the others just have to heat the rest of the house as well as the room it is in.

But, why should it be easy to achieve best efficiency? When I learnt to drive the car fuel and ignition timing were mechanically controlled, now every car has a computer under the bonnet to get max fuel economy because it is not easy to do. Running a condensing boiler always in condensing mode falls into the same area. It isn't easy and really we probably need a computer and lot more controls.

Ideally you need ~4x the design radiator area (by which I mean the area needed with 80C rad, 0C or -1C outside). That's 2x to allow for intermittent running and thus needing to top the house as well as balancing the load, then another 2x so you can run with 60C radiators. That's a massive overkill except for mid winter so you need to compromise and accept that you don't running in condensing mode in mid winter.

But the OP's question was how best to run an existing gas CH system. The answer to that is to try to keep the return water low to get condensing mode (but higher enough to kill legionella). Running longer is likely to give a saving over turning up the temperature. I would be tempted to install a PID controller like the CM907 which works out when to turn the heating on to hit a design temperature (i.e. you set 20C at 8am and it works out when to start the boiler). It should learn the house with the low water temperature and then adjust if you need to increase the water temperature as the weather gets colder.

But, if you want to make large fuel savings then you need to look at more insulation. A house typically has a very long thermal time constant so there isn't a big heat loss saving to be made changing times. Last thing at night, house is 20C, outside is 0C, heating goes off. How cold is it 8hrs later? I doubt it is even 10C, mine was 15C AFAIR so average night time loss is 17.5C to 0C, not that different to 20C to 0C.

Cardew

Malc,

Can you please read my post #16 again.

I clearly stated:

It is not in dispute that in condensing mode the efficiency is higher than non-condensing boilers or condensing boilers in non-condensing mode.

I am seeking technical data/explanation to support your contention that to achieve that condensing mode, and thus achieve significant efficiency savings, the boiler water temperature must be set low.

This despite of the OP’s statement that his instruction manual states to set water at maximum temp, and the linked brochure details I posted simply states:

Turn the central heating temperature control [6] to the desired level, between approximately 35°C and 90°C.

Despite masses of information on how to achieve efficiency savings, there is no mention of setting water temperature low in any brochure I have read.

I cannot understand why you think I don’t know about Legionnaire. You raised that subject suggesting that was the reason the OP’s brochure stated max water temperature should be used. Yet as Worcester allow water to be set as low as 35c – that doesn’t seem to be the reason.

Also allowing the water to be set to 90C seems to conflict with your contention that low water temperatures are necessary.

Lastly why do I have a strange way of running my heating system? I turn off the radiators in the rooms not in use with a TRV – how else would you turn them off? That includes all bedrooms during the day. The point being that on occasion all my radiators could be on and demanding heat, and on another occasion only one or two radiators. The load on the boiler thus can vary tremendously.

malc_b

Cardew wrote: »

Malc,
I am seeking technical data/explanation to support your contention that to achieve that condensing mode, and thus achieve significant efficiency savings, the boiler water temperature must be set low.

I'm sorry if I did explain myself well. In order for a condensing boiler to condense the return water temperature must be pretty cool. See that wikipedia where it gives a figure of 55C. It the low water temperature that allows the boiler to condense. Any boiler will in fact condense if the water temperature is low enough. However, condensing boilers are stainless so will stand this. Old style steel boiler would rust out. The efficiency gain is just down to condensing, otherwise there is no difference between a condensing and older style boiler.

If you have balanced your radiators as you should then you will have set a figure of ~10C drop for in/out difference. Hence if the return water is 55C it follows that the output water is at 65C. I would agree that there is a period at the start of burn when condensing always happens, when the water is still cool. This happens on condensing AND non condensing boilers. It doesn't last very long (unless the water temperature is set low), which is lucky for old style boiler else they would rust out, wouldn't they.

Cardew wrote: »

Despite masses of information on how to achieve efficiency savings, there is no mention of setting water temperature low in any brochure I have read.

I can't say why they don't but surely the logic is inescapable? We want to run in condensing mode, thus we need a return water temp <=55C. Output water is return +10C therefore output must be set to <=65C for maximum efficiency.

BTW for other readers this assumes a modulating gas boiler, on/off types bring short cycling into play, so I'm ignoring that complication.

Cardew wrote: »

I cannot understand why you think I don’t know about Legionnaire. You raised that subject suggesting that was the reason the OP’s brochure stated max water temperature should be used. Yet as Worcester allow water to be set as low as 35c – that doesn’t seem to be the reason.

Sorry I misread the post as you asking why the manual said run at max (which is what the OP said not you). But I would point out that the Worcester manual is hardly trustworthy if it says it is ok to run HW at 35C.

Cardew wrote: »

Lastly why do I have a strange way of running my heating system? I turn off the radiators in the rooms not in use with a TRV – how else would you turn them off? That includes all bedrooms during the day. The point being that on occasion all my radiators could be on and demanding heat, and on another occasion only one or two radiators. The load on the boiler thus can vary tremendously.

A good point but I thought I cover it. Let's suppose you have a completely open plan house and turn off all but one radiator, what would be the effect? That one radiator would have to heat the entire house so would need to get very hot to give maximum output. Return water would thus be hot and we would not be running in condensing mode. Agreed? So, let's say you now have a normal house with rooms and turn off all but one radiator. The question now is what is the U value between rooms, what is the air flow? The answer is quite a high U and quite a high airflow. Internal doors are sealed like external doors and internal walls are insulated like external walls (neither are floors). So although not as bad as completely open plan neither is your one room the same as a one room house. You are going to heating the rest of the house to some degree with this one radiator, hence it will need to work hard and the return water will be a high temperature.