In the end if you drop it too low then in very cold weather the house won't get warm enough, I would go ahead and keep reducing it.

Drop it right down to 35C and work your way up again as winter sets in. You should be running it as low as possible in order to improve efficiency and varying the flow temperature throughout the year in accordance with the weather. The benefits of 50/30 over 80/60 are marked. 35/30 doesn't have much over 50/30 though.

Decent controls - weather compensation - do this automatically in response to external temperature but are alien to most gas safe registered plumbers who masquerade as engineers capable of designing heating control systems. The boiler almost certainly has the connections for them/opentherm/0-10V control.

Intermittently running at a higher temperature to "boost" the heat after a week away is fine. There is a tradeoff between lower flow temperature to improve efficiency vs the expense of increased losses due to increased time at temperature due to the increased warmup time. I wouldn't worry about it too much.

You should also be adjusting the start time on the time clock in order to ensure that the house is up to temperature when required but not a moment earlier. Again decent controls do this "optimum start" time automatically but are alien to most plumbers. (you set the time that you want it 'at temperature' rather than the time you think that you need the heating to come on at in order to get up to temperature)

It's warmup time that will get you eventually, rather than the ability to maintain temperature. You'll notice more favourable temperature gradients in rooms with radiators when the flow temperature is lower: the hot air doesn't shoot quite as high up towards the ceiling quite as fast as it does at 70C!

Who specified and commissioned the current system? Were they given explicit instructions to minimise warmup time without the slightest regard for energy efficiency? Or simply a failure to give a stuff?

The tale is not complicated: 82C is the default temperature and the previous people liked to run the place hot, though I think they had not made too much effort.

I have whacked it down to 40C and will take it from there.

Thanks

Ferdinand

Most ch systems are over-sized and not economically optimum. There are two fundamental reasons for this. Firstly, they are designed to meet extremely cold weather, yet for the bulk of the heating season the demand is a tiny fraction of that. Secondly, cold customers are quick to complain to installers if the ch system is inadequate, but never complain if it is over-sized. So the installer has an inherent incentive to over-size.

The end result is rads that are too big and, worse still, boilers which are far too large. Keep in mind that boilers like most thermodynamic devices (inc all engines) are designed to run most efficiently at a particular parameters.

In general it is more economic to have a system which will meet heating requirements in all but coldest weather, and, when that occurs, do some peak lopping with either an extra jumper or some high cost electric heater, or an open fire.

One other factor: you need to consider response time. That's a bit like acceleration performance in a car. Over-size engine gives huge acceleration but poor mpg overall. A complicated aspect.

Is it worth timing the system from cold to hot, would that give you an idea of how long the system takes to respond and lets you know how much energy is in the system that may be going to waste.

Easier to time the house then set a weather compensation slope/curve.

(follow the commissioning instructions provided with all decent boilers/controls - not difficult unless you have the patience, pride in workmanship, and reading skills of a mainstream gas safe installer)

Attached: readout that you get with relatively dumb controls (£99.99 Owl Intuition) that only do on/off. (the boiler is a state-of-the-ark noncondensing Baxi that only does on/off control and isn't economic to replace) The slope is easy to see though, and tweaking the flow temperature manually every quarter gets most of the benefit at minimal cost.

40/33 will have the boiler condensing very nicely indeed. :-)

Outdoor temperature sensor wouldn't work well in a porch; more insulated than one might think and too sensitive to solar gain. North wall is good.

Sounds like cold feet will limit the minimal flow temperature. Given that it was "fine" at 70C throughout winter, try your 40 at 20C and 70 at -5C outdoor temperatures - a fairly standard looking curve. Viessmann's charts are pretty:

http://www.willsoden.com/6pp.pdf

Evidence says that 1.7 is fine. If you can drop to 1.2 ish (55C flow temps even when cold out) then you'll get that boiler condensing very nicely almost all of the time.

Pump load: the heating will run for longer at the lower temperature, increasing the electricity consumption of the pump. Is it internal or external to the boiler? A-rated? (saves £25/year, allegedly)

Pah. The bunny boiler appears to have reset itself again to a higher temperature.

Outdoor temperature sensor wouldn't work well in a porch; more insulated than one might think and too sensitive to solar gain. North wall is good.

As ever, it is complicated :-o. What I called a "porch" is actually a 10ft by 3ft brick corridor with a dg door at one end facing North. It does not get very warm.

If the point of a well-insulated house is to decouple the inside from the outside thermally, how much benefit is obtained from an External Sensor?

Ferdinand

Borpin - the return temperature limiter is designed to heat the top of the store to a high temperature quickly, rather than heating the entire store to a lower temperature which isn't as useful for DHW. The boiler will supply heat more efficiently at a lower return temperature.

A few boilers run at 75% for a set time before modulating down. (and throw a right hissy fit if they're in a pushing through a bypass against closed TRVs and can't hit 15 minute runtime before hitting the water temperature limit stat) As far as I'm aware this is a strategy to, err, "optimise their performance" for emissions testing purposes. Definitely not gaming the test cycle. (the 15 minute runtime at 75% output matches the NOx emissions test IIRC) The function can be disabled except with some Vaillant models IIRC. Fitting radiators and pump that are appropriately (>boiler output) sized solves the problem.

The more insulated the house the flatter the weather compensation curve Ferdinand and the less useful weather compensation becomes. At near passive house levels you'll be sat at minimum most of the time. It's most valuable in a "typical" shitty semi with undersized radiators that needs the full flow temperature in the depths of winter just to maintain temperature.

Here's an article from the USA on boiler efficiency:

http://m.hpac.com/heating/small_boiler_efficiency

It's got a nice graph in it, showing efficiency versus return water temp at different power levels.
It shows that best efficiency is achieved at low return temp, with lowest power level.

Odd in a way, that most gas boilers don't make it easy to set this- I set a mid flow temp on ours, and a low flow on the pump, so the return temp is low. Bit of a faff though - if I tweak the boiler flow temp down too low, the boiler power required is less than 25%, and it modulates.... best efficiency is found just above the point it modulates I think.
It would be easier to set the boiler flow temp low, pump fast - but doing this I get tepid DHW, and all rads are same temp( we have an old school 1 pipe system, so low flow makes downstairs rads hotter than upstairs, just as we want).

Posted By: RobLHere's an article from the USA on boiler efficiency:

http://m.hpac.com/heating/small_boiler_efficiency" rel="nofollow" >http://m.hpac.com/heating/small_boiler_efficiency

It's got a nice graph in it, showing efficiency versus return water temp at different power levels.
It shows that best efficiency is achieved at low return temp, with lowest power level.

I'm struggling the units in that article, I'm afraid (with an engineering degree :-).

"small condensing boilers, both floor-standing and wall-hung models with inputs of up to 500,000 Btuh"

What's a Btuh in terms of input to a boiler? Or is there a missing slash? It's like a Kilowatt (Hour-squared).

According to me that has the same Dimensions as a Joule-Second, and I'm struggling with the concept.

Equally, further down:

"In this example, there is a 400,000-Btu heating load and two boilers, each with 200,000 Btu, to serve the load. One way to control these boilers is to simply stage them"

What is the meaning of a heating load in BTUs (or Joules or Kwh)? Since when has a "load" been measured in terms of total energy, ignoring time during which it is being delivered?

Or am I just being obtuse?

Ferdinand

Posted By: ferdinand2000What's a Btuh in terms of input to a boiler? Or is there a missing slash? … What is the meaning of a heating load in BTUs (or Joules or Kwh)

Without reading the article my guess would be that both of those are meant to be BTU/h. Americans are very sloppy about that. Most irritating, even more so than British electricians saying mm when they mean mm².

1 BTU ~= 1055 J (depending on which definition you use).
1 BTU/h = 1055/3600 = 0.29 W.
1 W = 3600/1055 = 3.41 BTU/h

"It is not a return temp limiter (implies Max temp) it is a diverter (in that it sends the water round the loop until it reaches the set temp of the diverter that then closes) and yes I *fully* understand that (I designed that part in not my "expert") and is not the question I asked.

I asked what is the most efficient flow/return temp differential/combination for a condensing boiler. Seems no one actually knows!"

A: Flow and return temperatures as low as possible. The boiler will be more efficient without your diverter/minimum return temperature valve, but will be creating heat that's of little use to you.


"If the 75% is done to optimise the NOx emission test, it implies it is running cleanest at that setting. However, is that the most efficient as well?"

A: lowest (continuous) output usually; 30% of nameplate. Sometimes excess air and fan power/pump power start to dominate and you're better off nearer 50%. It won't be 75 or 100%.


"BTW the valve has made a marked difference to the time the boiler runs in a day (reduced it). In addition, elsewhere we discussed mixing/blending valves for TS. I had accidental proof that what I have works. After some work, the motorised valve was left unplugged and I only noticed it when I realised the boiler was short cycling!"

In what scenario was the boiler short cycling? Diverter stuck to "full divert" and water going around and around the loop?


Ferdinand - I second what JonG suspects. Definitely kill the "keep hot" setting if it's been turned on as that'll be chewing gas unnecessarily.

@RobL - Nice graph but a query on terminology; is 'return temperature' to or from boiler?

Yep. Seen these http://www.sheepwalkelectronics.co.uk/