Replacement boiler - TRVs and 55 degrees ?

[thesled]

Hi Folks

I'm looking at getting a replacement combi boiler. Just that, nothing else (apart from possibly a gas pipe re-route). Probably be a Greenstar of some variety.

I'm seeing a lot of stuff implying that new installs must have all TRVs (I have one) and limited to 55deg flow temp. This could be a problem with my 8mm microbore !

I think I've knocked the TRV one on the head as it seems this only applies when radiators are replaced. However still unclear about the 55deg business.

Any clarification gratefully received.

 

[oldbuffer]

1. For a condensing boiler to operate at high efficiency, the RETURN temperature at the boiler should not exceed 55 degrees. At or below 55 the boiler will work in condensing mode. Above 55 it won't.
2. With a condensing boiler, the temperature drop across each radiator should be about 20 degrees, which means the FLOW temperature should not exceed 75 degrees.
3. It is likely that your existing system will run at higher return and flow temperatures. If you drop the temperature for efficiency, you will not get as much heat out of the system as you do currently. However, it is quite possible that you will not be able to detect the difference, as the system may just run for a little longer before the desired room (air) temperature is reached.
4. It is my understanding that if the boiler is replaced, the system controls should be updated, and that this includes fitting TRVs to radiators that don't have them already. Given the relatively cheap improvement in control this gives, you might wish to reconsider not having them fitted.

 

[Johntheo5]

Hi Folks

I'm looking at getting a replacement combi boiler. Just that, nothing else (apart from possibly a gas pipe re-route). Probably be a Greenstar of some variety.

I'm seeing a lot of stuff implying that new installs must have all TRVs (I have one) and limited to 55deg flow temp. This could be a problem with my 8mm microbore !

I think I've knocked the TRV one on the head as it seems this only applies when radiators are replaced. However still unclear about the 55deg business.

Any clarification gratefully received.

I think all new rads outputs must now be based on a 55C Flowtemp basis, rad outputs are based on a "50 deg" (or T50) basis which is the mean (average) rad temperature - 20C (20C is generally taken as the required room temp).
so a rad with flow/return temps of 75/65 would be a T50 rad (75+65)/2 - 20 and will give its 100% rated output, if you ran that same rad at 75C/55C then its output is 87.2%.
Condensing only begins at 50/55C so if you can get the return temp down to say 40C then you will get very meaningful condensing so if you ran the above rad at 75/40C then its output is 68.8% and it would have to be oversized by a factor of, 100/68.8, 1.45, say 1.5 to give it a T50 basis.
Now running a rad at 55/50C will give 57.1% output, 55/45C = 51.5% and 55/40C = 46% output so the rad should be oversized by a factor of 2 at least or a factor of 2.5 based on the T50 basis. I don't know if the manufacturers make people aware of this or not, some I think do give T50 outputs based on a "T30" so the rad running at 55/45C is a T30 rad and should be oversized by a factor of 100/51.5, 1.94, say 2.0, IF the manufacturer doesn't clearly state this, I have seen T30 rated rads I think since ASHPs are now appearing and require fairly low flow temps to achieve good efficiencies, Coefficient of Performance or COP. One might think that a T30 rad should be oversized by a factor of 50/30, 1.67 but that is incorrect as the calculation is (50/30)^1.3, 1.94.

 

[muggles]

The 55ºC flow temperature requirement is only for new systems, although I don't see why you think having 8mm microbore would make a difference. If anything it's probably more suitable. That being said, the lower you can go the more efficient the boiler will be. TRVs are a good idea for improving energy efficiency too.