What temperature is the most efficient to run condensing at?

[Agile]

[

currently the rads input 8.5 kw per hr in the bedrooms

Presumably this is the rated output (not input) at 75°/65°/20°, according to mfr specs. The output will be less if you run at a wider differential

Something would be very unusual if the output did not equal the input!

In a practical situation the best reasonably achievable efficiency is obtained if the system is designed to run with 70/50 °C at the rads. Of course that is the maximum temp which would give adequate heating when its -1 ° outside. For the rest of the time the flow would be reduced ( ideally under the influence of weather compensation ).

In reality its irrelevant if the boiler is not condensing just on those very few days during the year when its below zero outside.

I made a mistake when advising a friend on his rad sizes because I did not expect the 1920s house had a cavity wall. Not only that, but the cavity had been insulated! So the rads were about 25% oversized. His wife loves it because its so warm!

Tony

 

[D_Hailsham]

[

currently the rads input 8.5 kw per hr in the bedrooms

Presumably this is the rated output (not input) at 75°/65°/20°, according to mfr specs. The output will be less if you run at a wider differential

Something would be very unusual if the output did not equal the input!

A 100% efficient radiator!

I suppose I should have pointed out that the output is measured in watts not watts per hour?

 

[Agile]

Its not correct to say 5.7 kW per hour.

But on the other hand its not actually wrong either if the rads being heated for a whole hour or day for that matter!

I think the OP is trying to be a little over technical!

But at least its refreshing to see that he does not want to oversize his boiler which is the most common cause of reduced efficiency.

Yesterday I was driving in a 1.4 litre Peugeot beside a 4.2 (?) Lambourgini at 40 mph. After a bit he accellerated up to about 80 just to pass a couple of cars on the inside! I still thought I had a more efficient car which cost under £500 and did over 40 mpg compared with his £50k car doing perhaps 16 mpg!

Tony

 

[simond]

If you want to lower your energy usage;

Fit weather comp (that rules out the entire Worcester range)

Consider only heating the parts of the house you are using at that time to sendentary temperatures.

The latter will save you the most money.

This can be achieved by Honeywell rondoStat timed TRVs, Pipe zoning with motorised valves and prog room stats, Honeywell CM Zone, or Honeywell Hometronic. In order of cost

Because environmentally friendly doesn't come cheap.

If your house is reasonably well insulated look at the Mitsubishi Ecodan system, we like these. Tests in UK properties have shown a significant reduction of energy cost compared to gas, but you will need some big radiators.

 

[BigBurner]

If you want to lower your energy usage;

Fit weather comp (that rules out the entire Worcester range)

Consider only heating the parts of the house you are using at that time to sendentary temperatures.

The latter will save you the most money.

This can be achieved by Honeywell rondoStat timed TRVs, Pipe zoning with motorised valves and prog room stats, Honeywell CM Zone, or Honeywell Hometronic. In order of cost

Because environmentally friendly doesn't come cheap.

If your house is reasonably well insulated look at the Mitsubishi Ecodan system, we like these. Tests in UK properties have shown a significant reduction of energy cost compared to gas, but you will need some big radiators.

Ecodan is an "air" sourced heat pump using a Gledhill unvented cylinder You need "very" larges radiators or UFH, or both, and in mid winter expensive to run resistance heating will be needed to get high enough DHW temperatures.

All this is not cheap. Spending the same money on a decent weather compensated boiler using two heating zones (or two combis doing the same) and heavy insulation and air-tightness would give cheaper bills and superior comfort conditions.

 

[Useroftheforce]

I finished applying more insulation to the areas of the house that were a little lacking. Hate that job, so dusty and dirty.

Bigburner – thanks so much for that I am seriously considering the Avantaplus now.

D_Hailsham – thank you so much for your excellent response – (not that others weren’t also really helpful too.)

I have checked out the house on sebduk – and I was about 1kw over in total. Trouble is, the house is not symmetrical and has a small extension, but hey ho were in the right ball park. I am fortunate that the existing Rads are oversized by enough to make this entire suggestion a workable proposition.

If the rad mfr is quoting for a 80°C flow temperature, they are not following British and European Standards which specify 75°C flow, 65°C return and 20°C room temperature. If 70°/50°/22°C is required the output will be reduced to 68% of the nominal output. You will therefore have to oversize your radiators by 48% [1/0.68] to give the required output.

You can do this with your new rads, but what about the existing ones? Are they already so oversized that you can retain them? 8kW will reduce to 5.44kW

I see that I confused myself, now that I recheck my calculations. Total existing Rad output = 12.5kw (75/65/20). From this it’s easy to see there is more heat being generated than lost. However all the existing Rads are at LEAST 50% oversized and incorrectly distributed, hence the potential exists and the reality exists for some areas being freezing. (This is also causing the boiler to short cycle, with roasting rads simply unable to meet the demand). Again, AFAIK the system is in A1 condition this is simply a sizing loss equation. The system at 75/65/20 will produce 18kw when I have completed this exercise, against the stated 10.97kw loss. But by your calcs 18X.68=12.24kw, but my calcs say 11.3kw, about right. I used the stelrad heatline output figures and table against which to perform my calculations. These state at a mean temperature difference of 38 degrees (70/50/20) the factor to be applied is .748 or 75 percent which is not 68 percent – where am I going wrong?

Everyone – I meant OUTPUT of the existing rads is 8kw with the standard 75/65/20 – not input.

Simond
I do have a CM zone system in the house with the existing rig. It’s exceptionally accurate at temperature control – much more so than the (other rads in the house) rads which are fitted with traditional TRV’d (honewell heads) most of which are off all the time. The rondostats seem to be unavailable in the UK – I was going to invest in a further CMzone controller to create a third zone and run three TRV’s off that for the lounge area (the problem area with a loss of 12,000btu with one 7500 btu rad in it at the moment, and lots of draughts) – but I might get a few rondostats where does one purchase these in the UK? They appear to be Germany only, god bless those techy Germans.

 

[oilman]

Damart thermal undies will allow further reductions. Balaclavas and gloves would allow you to have a zero heating bill.

I associate Damart with old people and would never wear any of their products.

I hope you don't hold such irrational prejudices when dealing with heating installations, or customers for that matter.

When I get saga holiday mail shots I return them saying I would not want to go on holiday with fat old women!

Tony

I would think this might be caught by age discrimination legislation.

It does confirm irrational prejudice though. Saga deal with over fifties. There are several television presenters in this age group who are not fat, and even putting looks aside, are otherwise attractive. One whose name I would have mentioned, sadly died in September. Several years ago even at 50, she was attractive to men half her age.

 

[WDIK]

if you want these levels of predictable temperature standards, then look towards scandenavia in that the temperature inversions they experience are of the extreme
walls 2' thick with 18" of insulation. triple glazing.
point being get the containment right and you could enjoy the heat from the lightbulbs.


as for your fancy calcs well done, hope they make you feel all warm inside.

 

[WDIK]

I associate Damart with old people and would never wear any of their products.

when did this happen Tony?

 

[Agile]

Everyone – I meant OUTPUT of the existing rads is 8kw with the standard 75/65/20 – not input.

With all the thinking you have been doing, I am surprised that you have not realised that radiators are always 100% efficient and the OUTPUT will ALWAYS be equal to the INPUT !

Even Dr Drivel would ( probably ) know that!

Tony

 

[WDIK]

Tony rethink that answer, 100%?

that would mean it radiated 100% of its total heat input,

fourtunately for the rest of the system they dont,

radiators are not rated as being efficient, they emmit what they are designed by size to emmit there is no loss due the radiator, its the flow temp drop across the rad that dictates how much heat they can emmit subject to there design criterior and how close to ther design output is a matter of correct balancng,

 

[BigBurner]

Tony rethink that answer, 100%?

Think? There lays the problem.

 

[Agile]

Yes, radiators emit 100% of the energy supplied to them.

The input always equals the output! Conversely the output always equals the input.

There is nowhere else for the power to go to or come from.

Tony

 

[kevplumb]

ARGI REGISTERED ENGINEER

means what ?

oh yeah ACOP REG

ACS REG

or apart from that / / the space is for the answers BTW

 

[D_Hailsham]

But by your calcs 18X.68=12.24kw, but my calcs say 11.3kw, about right. I used the stelrad heatline output figures and table against which to perform my calculations. These state at a mean temperature difference of 38 degrees (70/50/20) the factor to be applied is .748 or 75 percent which is not 68 percent – where am I going wrong?

The table of factors given in the Stelrad catalogue assumes that the only thing being changed is the room temperature, i.e the flow and return stay at 75°C and 65°C but the room temperature rises to 32°C. You are changing all three temperatures so the table is not suitable.

If you want to see the maths and play around with an online calculator go to Heat Emission from Radiators. The only problem with the online calculator is that is used 80 Flow, 60 Return and 20 Room as the comparison point, If the BS temps of 75, 65 and 20 are input to the calculator you will find that a 1kw rad will produce 1014 watts. This needs to be taken into account. So if we put in your temps of 70, 50 and 22 the calculator gives 685 watts for a 1000 watt rad. Dividing this by 1.014 gives 0.6755. I rounded this up to 0.68.