# Condensing Boilers - optimum return temp

Flow 0 deg return 0 deg, extremely efficient, no steam no alum in the atmosphere blah blah blah

Its generally recommended that the boiler output on a system boiler is at least 10*C above the target stored water temperature which is recommended to be 55-60 rather than the 60-65 often quoted here.

While in leafy Bristol area there will be many system boilers, in smokey London they are a rarity as combi boilers take up so little space.

A big advantage of a condensing combi is that the CH temperature can be set quite low without affecting the DHW delivery.

Many of the latest condensing boilers can operate with a temp diff of greater than 20*C which can give better economy with good heating performance.

Tony

Agile said:
Its generally recommended that the boiler output on a system boiler is at least 10*C above the target stored water temperature which is recommended to be 55-60 rather than the 60-65 often quoted here.

Many of the latest condensing boilers can operate with a temp diff of greater than 20*C which can give better economy with good heating performance.

Tony

Tony
Thanks for your response. I will look at reducing the Megaflow HW temp in view of your comment.

Not sure if my boiler - Vaillant ECOmax VU 226 E - is designed to run at a greater than 20* difference but but it is rarely in that region the return often being 30* less than the flow.

111

How do you decide how much to oversize the rads to get a reasonable balance between effy and cost Do you just estimate 10% or can it be easily worked out.

Heat Emission from Radiators has two on-line calculators.

The first calculates how the output of a radiator changes under different conditions of flow, return and room temperatures.

The second calculates the return temperature if a radiator is used to provide more or less than the quoted output.

One interesting thing, which many do not realize, is that it is not essential that radiators have to be balanced so they all have the same temperature drop. That "rule" assumes that the radiators are exactly the correct size. The drop can be adjusted to provide the desired output - within limits. A higher return (smaller drop) will give a greater output and vice versa. The drop at the boiler still needs to be within mfr spec.

Have fun

I'm dim, but what is an average return temp?

The return (and flow) temperatures fluctuate with time; it's probably the average return temperature whilst the heating is on.

A condensing boiler serving radiators is most efficient when used with weather compensation. The radiators are sized for design conditions (-5ish degC outside); they are oversized for any external air temperature above this, i.e., most of the heating season. The flow (& return) temperature can be reduced most of the time, while still achieving the indoor design temperatures.

Domestic hot water should be stored at 60 degC to prevent the growth of legionella. The legal requirement (HSE L8 document) is that the water temperature should be 50 degC within 1 minute at any hot water outlet.

Most people use the water at about 40 degC for washing and they mix it in the mixer tap or in a basin. The requirement to fit a TMV only arises if the users are vulnerable (very young, very old or disabled). Some people cannot feel that they are being scalded or can feel they are being scalded but are unable to move out of harm's way. In this case, TMVs would be fitted at the point of use, NOT at the outlet from the cylinder.

Most employers fit "very hot water" signs adjacent to unregulated hot water taps.

This post is nearly 3 years old, why bring it up again

I know this is a very old post, but still relevant today.
I have tried very hard to get a 20 degree differential across the flow and return on the boiler. I've set the flow temp at the boiler at 55. When it gets warmer in the house I reduce this to 50 sometimes even 40 depending on the weather outside. I know I should get a weather compensator. I'm looking into that.
I'm using a Anton Sprint which gives me a accurate reading of the temps.
I've started by creating a 11-12 degree on all the rads, but that failed.
So I connected 1 clamp under the boiler on the flow and another on the return.
After trying very hard to adjust some LS valves I managed to get a 10 degree difference at the boiler.
Any idea what I'm doing wrong here.
I have 9 radiators all with TRVs expect the bathroom hallway where the room stat is.
I have a Vaillant Eco Tec Plus 831 previous model.

Not sure why you are posting here, better in the cc for gas reg people.

Obviously you have too much flow but you should know that. But your over rated boiler is not going to be happy with that and the internal bypass is probably going to operate and reduce the delta T even further.

Tony

Hi Tony

If I am understand this correctly I should be reducing the KW heat load to reduce the flow? am I right? If so should I be working out the BTU's of the heating system then set the KW according to that?
I would be gratful if you can guide me in the right direction as I would like to set my home heating system correctly and future customers.

to optimise boiler efficiency a good control system seperates a call for heat for hot water (normally around 80c) from heating (variable according to demand).

Vaillants VR 65 does just this with a modulating room controllers..

if we add up all things we don't bother to chase for efficiency..we end up with an inefficient system with a boiler that could be more efficient...

you have to start somewhere and optimising boiler effiency by working round as low as flow temperature as possible with out compromising comfort is a good starting point....

I have a Eco Tec 831 Combi boiler, so the VR65 does not apply to me.
Regards to the flow temperature, I set it between 40-55 maximum according to the temperature outside. I have the maximum setting for heating reduced to 60 from the diagnostic codes. But I can not get the 20 degree difference on the flow and return. I would like to know how I can achieve this.

You can't ... 20c is the maximum most heat exchangers will tolerate, but less is perfectly acceptable...that's why the way to enhance boiler efficiency is to lower the flow temperature, which brings down the return...

Vaillant's own modulating controls float the flow and return temps according to demand.

Vaillant's own modulating controls float the flow and return temps according to demand.

How are you suggesting they do that?

If a certain flow temp is needed then to provide the required temp in the house the only way the return temp could be changed is by modulating the pump speed.

Whilst a few Vaillant and Kestons do have limited step pump speed modulation, as far as I know none have completely variable speed control.

Tony

because they are modulating controllers...the programmer looks at the actual temperature and tells the boiler what temperature the radiators need to be to sustain the room at the programmed room temperature.

If the deviation is big i.e. the room is 18 and the target temperature 20c the rads will be as hot as is programmed in the diagnostic codes of the boiler allow.

if the deviation is small say room 19.5 and the set room temperature 20, the flow will be much lower

on my boiler even in the this cold weather the flow temp hovers around 54c and the return at 48, and the room at the programmed 21c...

what happens is with the cold weather it fires more often and for longer, at these temperatures rather than at higher ones..all to maintain a constant room temperature

in milder weather the flow temp is about 48c and the return 42c...

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