Condensing Boiler Setup/efficiency

nxdmsandkaskdjaqd

By way of background I have a new condensating Valiant ecoTEC plus 28kw Open Vent boiler feeding a wet 17 radiator system with thermostatic valves fitted (qty 16). With this new system I looking to maximise the return on the investment and have some questions relating to this new technology (for me).

The installer advised that the differential temperature at the boiler flow and return should be 20 degrees centigrade, is that correct? If so, that means that the radiators need to also have a differential of 20 degrees C. But there are many articles on the internet say that the differential on the radiators should be 12 degrees C. What is right?

I found this reference on the internet. For maximum efficiency and for the boiler to condense the temperature of the water that returns to the boiler, the return temperature must be 55 degrees or below. Is this correct?

Being retired, the heating is on for 16 hours a day, with this is mind, would it be more efficient to run the system on the radiator fitted thermostatic valves rather than be controlled via a hall mounted wall thermostat?

Thank you for your feedback.

Cardew

To get the boiler to run in condensing mode, the return water temperature should be below 55C. The difficulty is how to achieve that aim!!

I have been going on about this on MSE for a long while and have phoned manufacturers with no success. I posted this yesterday in another thread.

Good question - I have raised this issue several times and I don't believe there is a satisfactory 'one size fits all' answer.

http://en.wikipedia.org/wiki/Condensing_boiler


Obviously the principle is to have the boiler water temperature as low as possible commensurate with providing adequate heating in the property.

However the heating load on the boiler can vary tremendously; from, say,(1) just a heated towel rail - with all other radiators switched off by their TRV, (2)to every radiator in the house 'demanding' heat(i.e. TRV opened)


Even though a boiler can modulate down, a typical minimum output is 8kW which is far too high for case (1) and the water will return to the boiler at almost the same temperature as it left!


I have also found it impossible to quantify the difference between a boiler in condensing mode and the same boiler in non-condensing mode. However there seems to be a consensus of opinion that the 90% efficiency figures claimed for modern boilers are simply not achievable outside of a laboratory; rather like claims that a V8 engined car will achieve 60mpg at 30mph!

In your case a 20C or 12C differential would not be achieved with a very light heating load.

nxdmsandkaskdjaqd

Cardew wrote: »

In your case a 20C or 12C differential would not be achieved with a very light heating load.

Cardew. Thank you for your reply. Now that I know that the 55C information is correct, I can set-up the system accordingly.

Re the quote above, could you please expand on this, are you saying that I would not achieve a 20C or 12C differential across the radiators and boiler with the boiler running on minimum heat output (5kw according to the specs)?

Andy_WSM

nxdmsandkaskdjaqd wrote: »

Re the quote above, could you please expand on this, are you saying that I would not achieve a 20C or 12C differential across the radiators and boiler with the boiler running on minimum heat output (5kw according to the specs)?

The difficulty you have is keeping the boiler in condensing mode and maintaining those temperature differentials.

So you set it up today, a chilly day in Winter, but not cold. Then the weather turns a little milder - what happens to your set up then? Then we return back to the -7C nights of late and suddenly you find your house is cold through insufficent flow rates and radiator temperatures - yet, the boiler is condensing lovely.

You can only set up a happy compromise on the system - that's just the way they are - as Cardew states above, obtaining maximum efficiency outside of a laboratory is nigh on impossible.

Toyota says my car will do 72 MPG. I achieved that once, on a calm Spring morning, with no other traffic on the road, no red traffic lights and no wind. Under "normal" conditions it's just not possible.

Cardew

nxdmsandkaskdjaqd wrote: »

Cardew. Thank you for your reply. Now that I know that the 55C information is correct, I can set-up the system accordingly.

Re the quote above, could you please expand on this, are you saying that I would not achieve a 20C or 12C differential across the radiators and boiler with the boiler running on minimum heat output (5kw according to the specs)?

As stated above, I am not sure you can set the system up 'correctly' it will always be a compromise..


With a heavy heating load i.e. most radiators on -, you need(initially) the water temperature set fairly high to warm up the house quickly and you will have no difficulty in achieving the differential on the return to boiler.


Other times the heating load will be minimal and there will not be the differential.


What I have not been able to determine is the difference in efficiency between condensing and non-condensing mode for an average boiler; albeit all the literature I have read states that a condensing boiler, even in non-condensing mode, is more efficient than older non condensing boilers.

nxdmsandkaskdjaqd

So today I have spent the last several hours setting up the system. I now have a 11 to 13 degree C differential across all the radiators (a bit more work to do in balancing the pump and some more small radiator adjustments).

For the last few hours the boiler has been running on a minimum gas input of about 5KW and keeping the whole house at about 19.8C.

Across the boiler, in heating mode, I have a similar differential and a return flow of 53 degrees C (sweet). Hot water is a bit more tricky and I have had to reduce the set hot water temperature to achieve a return of less than 55 degrees C.

That leaves me with a final question, is it more efficient to run the system on the Wall Thermostat or on the Radiator Thermostatic Valves?

oldskoo1

Providing all rads heat up and you even heat distribution and you are happy with the time it takes to heat up in the morning from cold I think you have done well to achieve that return temp.

Can the pump go any slower? And make sure it heats all rads if it can.

Keep it on the wall stat and just set the rads to roughly 1 unused, 2-3 on small rads or ones in places like a downstairs toilet. 3 in bedrooms and 4 in living rooms. Tweak as you wish.

Congratulations that 2 hours probably saved you a further 5% on your bills maybe more if it was set up badly before

nxdmsandkaskdjaqd

Cardew wrote: »

I have been going on about this on MSE for a long while and have phoned manufacturers with no success.

Cardew. Have you been able to obtain any Efficiency v Temperature data from Vaillant? My initial enquiry to them resulted in them saying No data was available.

Robert

Cardew

nxdmsandkaskdjaqd wrote: »

Cardew wrote: »

I have been going on about this on MSE for a long while and have phoned manufacturers with no success.

Cardew. Have you been able to obtain any Efficiency v Temperature data from Vaillant? My initial enquiry to them resulted in them saying No data was available.

Robert

I failed to get any information from any source, and to be fair even if manufacturers had carried out an extensive trial, the results would only apply to that boiler, with that particular set of circumstances - building, heating load, percentage of gas used for DHW etc etc.


I have a non-condensing boiler which I run at maximum temperature(82C) for CH as I want the house to warm up as quickly as possible when it comes on.


When the inevitable time comes and a replacement is needed. I would like to know the approx 'penalty' in loss of efficiency for running the boiler at various water temperatures.


A relative with a condensing boiler has his CH water temperature far too low IMO and it takes ages for his house to get warm - particularly in the morning.


Also as said earlier, what is the effect of a very small heating load, or heavy heating load, on boiler efficiency?


When manufacturers claim their 90+% efficiency figures for their boilers, what test parameters are used? e.g. full output, modulated output, water temperature, heating load etc. we know that those efficiencies do not apply to DHW


All I want is some ballpark figures.

matelodave

Here you go = http://www.sesg.strath.ac.uk/Downloads/Report_BRE_E302_040707.pdf and this one - it's American but gives the general info https://michigan.gov/documents/I_Lansing_HVAC_94547_7.pdf

Pincher

You can't "set it up" because conditions are changing all the time. The idea is to have a computer in the boiler that turns the flame up and down (aka modulating) according to


1. The temperature of the house
2. The temperature you want the house to be


and preferably,


3. The external temperature, as measured by an outdoor sensor not in direct sunlight (false reading).


Item number three allows you to activate the weather compensation feature.


If you were able to bolt it all down, so the boiler only outputs at 55 degrees, and it all works fine for 10 degrees outside the house, during the day, what will happen when

A. Night falls, it drops to 5 degrees, so heat loss increases.


B. It starts snowing, it goes sub-zero outside, and the heat output is really not enough.


In fact, it doesn't even work for the static condition, because the TRVs start closing, so the water is not used by all the radiators, and return warmer than what you think you set it up for.


If you don't keep the central heating on 24/7, then you want the computer to go full blast when catching up, and then simmer to maintain steady state.


All you can do is to make sure you get a boiler that is intelligent enough to do the best it can, given the conditions presented to it. An outdoor sensor also helps.