What temperature is the most efficient to run condensing at?

[Useroftheforce]

I am soon to invest in a Worcester Bosch 15ri Condensing boiler. Since before hand, I need to add four rads to the existing 8kw (total) rads and replace 1 rad; it seems sensible to account for the new condensing boiler in the calculations for these new radiators.

There was no easy way to get excel to import into this posting format (you've escaped!) so in very rough terms to demonstrate the process used (so it can be critisised)

CH loss calculations - in excel.

Room || Loss W hr || RAD Rated Watts || / 0.6 **to allow for lower temps see notes below**

A || 946w || 1699w || 1019w
B || 1049w || 1230w || 1137w
C || 2485w || 4533w || 2719w
etc totals..

losses 10.97kw - so house is cold at the moment due some rads being too small and not enough heat in the right places.
Emitters 18.17 @ 50 differential - which they are rated at when sold.
Actual yield 10.90kw due - 38 degree differential as per notes.

Tank - number X power pr l X difference / three hour reheat.
Litres = 166 X 1.16 watts X 56 / 3 = 3.59kw

Total KW 14.18 "This is the required boiler size, unless I can improve insulation = 15KW"

• 
  House leaks 11 kw per hour and currently the rads input 8.5 kw per hr in the bedrooms, leaving me to add some downstairs to get it right.
  I have sized a new condensing boiler against the loss not the total RAD capacity.
  
  *I have read that a condensing boiler should be run with the return below 50 degrees. But Radiators are designed to produce their rated output at a 50 degree temperature differential.
  *This means normally the rads are at 80 and the room can be at 30 or below and were getting what we should be or more. However with the condensing boiler in super efficient mode our temperature differntial is only
  *70+50/2 - 22 = 38 degrees difference (assuming a room temp of not more than 22 degrees and a 20 degree drop across the rads). So we need to apply a reduction factor as stated by the rad manufacturer of about .6 to *find the rated output.
  *Then compare this new output figure against the loss on each room to see if the RAD can cope or is too small.

1. What is the correct temperature to run the boiler at? - the manufacturer quotes maximum efficiency at below 30 degrees - yikes.
2. Is the principal I have used in sizing the rads ok?
3. Have I arrived at the right boiler size?

 

[oilman]

You have made assumptions in your figures which have given you misleading results.

You say your house requires 10.97kW. This is true for a given temperature difference between outside and inside. It is not true for any other temperature difference.

If you MUST have set temperatures from your heating system everywhere inside the house, then you will pay for the luxury. If you have a boiler and a few rads, then you can keep the chill off. If it gets colder, heat fewer rooms, put more clothes on etc. These things might seem like carp now, but it will not be many years before this thinking will be the norm.

 

[spudkey]

Even with oversized rads return temps of less than 50 with a 70 flow temp will only really be achieved on initial heat up and at outside air temps of -1.
Look at fitting weather compensation control wich will lower flow temp and hence return temp as outside air temps increase

 

[Useroftheforce]

I really appriciate the inputs made from you both - I think everyone on here is so helpful.

Oilman, we be of one mind. The truth is even now I sit here in 3 layers heating 1 room. This is OK for me but not when I am entertaining, the missus is here etc. The house simply will not warm at the moment with the existing setup (it's not a mechnical or sludge fault, the system is in perfect order). I presume my boiler / rad sizes are correct for the design temperature?

spudkey - how does the weather compensator work - is that a Worcester-Bosh thing or an aftermarket thing - clueless ! I presume it provides information to the boiler and some adjustment is made against that data...?

 

[spudkey]

most heating systems are designed for an outside air temp of -1 however for the majority of the year the air temp is well above -1, so heat loss on the house will be less in which case a lower radiator temp can be used to maintain room temps. Weather compensators monitor outside air temp and adjust the boiler flow temp accordingly.
At present i do not think worcester have any weather compensation controls available but manufactures like vaillant & viessman do. IMHO i think these are far superior boilers to worcester bosch and are worth looking at

 

[Gasman28]

20 degree drop across the rads? Normally about 11 degrees.
I'm not an installation engineer, but all the latest bulls**t that I have read recently about weather compensators makes me laugh. Install the system correctly with TRV's etc and correctly placed room stat and of course, correct sized rads. Adjust the new boiler CH output to suit - no probs.

 

[Useroftheforce]

Gasman, as I now understand it, that approach would mean most of the time the condensing boiler was not condensing. I read one has to adjust the system to return a low as possible to the boiler to help with creating the condensing environment within the boiler. This makes a significant difference 9% or so to the kw the boiler produces for the same gas consumed. www.condensingboiler.org has a digestable, version of several other larger works on the subject I have studied.

This is the whole point of my post...! how to get the most out of that new Govt enforced techology were required to use.

Of course your right about the rads trv's etc.

spudkey - just checked out the vaillent ecoTEC plus 415 thanks for the pointer.. I was only considering Worcester-Bosch on the stength of the reliability sticky post in this forum - upon which Vaillent are not in the "top box" ?

 

[oilman]

If you can measure the temperature at each end of the radiator, you could see how much heat they are giving. You may be surprised (or then again you may not).

 

[Useroftheforce]

I balanced the system to give exactly 11 degrees drop so when it was at freezing point outside a few winters ago. But this is against a fanfare with an 80 flowstat as normal.

 

[Agile]

It is possible to set the flow through each rad to give the ideal 20°C differential. Few installers bother though.

If you do that including the HW cylinder ( few installers even fit an adjustment on it! ) then you will achieve an outstanding efficiency.

Add weather compensation AND use it properly and you will have a system as efficient as possible.

HOWEVER, the difference between condensing or not is just 12%.

You would achieve about 20% saving if you bought two pullovers* at £20 each and wore them and turned the temp down just 2°C.

* One for you and one for the wife. Or double savings if you both wear two pullovers.

Tony

 

[oilman]

You would achieve about 20% saving if you bought two pullovers* at £20 each and wore them and turned the temp down just 2°C.

Matalan sell fleeces at around £4 each. Buy two each, wear both and turn the temp down even further. Damart thermal undies will allow further reductions. Balaclavas and gloves would allow you to have a zero heating bill.

 

[Useroftheforce]

Many thanks - one and all - all your time, suggestions end efforts (yes even the sarcastic ones) are appriciated in this post and many many other good ones I have read on this forum.

 

[Agile]

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.

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

Tony

 

[BigBurner]

most heating systems are designed for an outside air temp of -1 however for the majority of the year the air temp is well above -1, so heat loss on the house will be less in which case a lower radiator temp can be used to maintain room temps. Weather compensators monitor outside air temp and adjust the boiler flow temp accordingly.
At present i do not think worcester have any weather compensation controls available but manufactures like vaillant & viessman do. IMHO i think these are far superior boilers to worcester bosch and are worth looking at

Yes, a good boiler with weather compensation is the key. Broag Avantaplus are well priced, excellent quality, and have excellent control systems - OpenTherm.

Fit a 3-way diverter valve on the system with quick recovery coil in the cylinder giving DHW priority. The Avantaplus control system will run the boiler to maximum temperature to re-heat DHW ASAP. When DHJW satisfied it returns to weather compensation for CH control keeping the return temperature as low as possible.

 

[D_Hailsham]

I need to add four rads to the existing 8kw (total) rads and replace 1 rad; it seems sensible to account for the new condensing boiler in the calculations for these new radiators.
...
losses 10.97kw

Presumably this loss has been calculated on a room by room basis. It might be a good idea to check your calculations against the Recommended Boiler Size calculator at Sedbuk

Tank - number X power pr l X difference / three hour reheat.
Litres = 166 X 1.16 watts X 56 / 3 = 3.59kw

That is assuming you are heating a full tank of cold water up from 4°C to 60°C. Cylinder thermostats usually have a differential of 8°-10°C. Which would only need 640 watts for a 3 hour reheat. The normal allowance for hot water is 2kW, which is included in the Sedbuk calculations.

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

I have sized a new condensing boiler against the loss not the total RAD capacity.

Good

I have read that a condensing boiler should be run with the return below 50 degrees. But Radiators are designed to produce their rated output at a 50 degree temperature differential.

The actual temperature at which the boiler will condense will depend on several factors and is usually between 50°-60°C. Having the return below 50 should guarantee condensing all the time.

70+50/2 - 22 = 38 degrees difference (assuming a room temp of not more than 22 degrees and a 20 degree drop across the rads). So we need to apply a reduction factor as stated by the rad manufacturer of about .6 to *find the rated output.

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

What is the correct temperature to run the boiler at? - the manufacturer quotes maximum efficiency at below 30 degrees

The glib answer is: the lowest that will warm the house up in an acceptable time and keep it warm. That will depend on the boiler and how it controls its modulation. You can only experiment - providing the mfr will allow you to adjust the temperature.

I think the 30°C figures are something to do with the way boiler efficiency ratings (Sedbuk A -F) are calculated as all mfrs seem to use this.