Another flow and return temperature thread.

[Mottie]

I have a Vaillant EcoTEC plus 418 heat only boiler. 10 rads, I have the max output set at 15Kw and the flow temp at 62°. My 3 speed circulator pump is set at 1 - the slowest speed. After watching a YouTube video the other day showing how to check flow and return temperatures, I checked mine. I opened all rads up fully and run the system until the reads were too hot to touch and then checked the flow and return temperatures on the boiler using the boiler screen display. Flow was 61° and the returned was 45°. I thought I read somewhere that the return needs to be 20° less than the flow to condense? Is that right and if so, should I turn the max output down lower?

 

[Johntheo5]

Not really, its horses for courses, You will be condensing a bit with a return temp of 45C, if you increased the flowtemp to 63C and reduced the flowrate to give a dT of 20C, the return will be 43C which will give a little more condensing, the rad output in both cases will be 58.9% of its max T50 rating but if you required the full 100% rad output with a dT of 20C then you would end up with flow/return temps of 80C/60C, no condensing at a dT of 20C so generally speaking, its best to oversize rads by a factor of 2 if installing new rads, in the first instance above where you have flow/return/dT/output of 63C/43C/20C/58.9% then by increasing the rad by a factor of 2.0, you will get 44.5/34.5C/10C/29.45% so almost full condensing (even though the rad is only emitting 1/2 of the original output, you have doubled the heating surface so same required output).
the 100% rad at 80C/60C/20C will emit 50% at 54.4C/44.4C/10C so still give excellent condensing, again you have doubled the heating surface so the same required output. So even with a dT of just 10C you are getting excellent condensing with those 34.5C & 44.4C return temps. those two numbers would be returns of 29.5C & 39.4C at dTs of 20C so obviously a further increase in condensing effect.

 

[Mottie]

My head has just exploded but thanks.

 

[Johntheo5]

IMO, some people get carried away with all this jazz even though I like doing the numbers, I'd say quite alot of people are quite happy with a return temp of 50C even though no condensing, if the return temperature for whatever was say 60C then the loss of efficiency is only about 0.5%, about a fiver a year, however, you will save a extra say 4% in the mid condensing range,

 

[Harry Bloomfield]

I have a Vaillant EcoTEC plus 418 heat only boiler. 10 rads, I have the max output set at 15Kw and the flow temp at 62°.

Same boiler here, but 9 rads, but with the fancy, clever Vaillant control system. We arrived back home yesterday, after a week away, and with no call for any heating by the house, for that entire week, it felt a little chilly, despite the system being set to resume service, the day before. So I nudged it up temporarily to 20C, rads became hot, and the place became comfortable, then it went back to it's default hall temperature of 18C day temperature.

Today, the boiler has hardly needed to fire, the house is warm, the radiators are barely lukewarm, living room is at a sweltering 22.4C.

 

[Notch7]

I’ve been amazed by how low flow temps down as low as 30 deg actually warms the house so we’ll, our roads are only ever slightly warm and the house feels cozy with a temperature of around 21-22 degs (too warm for me, but if it’s much lower my ears start burning from all the moaning)

 

[Madrab]

with a return temp of 50C even though no condensing

John, a little confused by this comment??

Dew point of the flue gases in the HEX of a condensing boiler is around ~53deg C, so if the return into the HEX and therefore the bottom of the HEX is at that point or lower then condensing occurs, any lower and the condensing increases, maybe I am picking you up wrong when you say no condensing @ 50deg?

 

[Johntheo5]

Thats right but the gases will be a fair bit higher than the returning water temperature, you can see from the enclosed that based on a return water temperature of 50C that condensing is just about beginning and full condensing doesn't occur until ~ a return temp of ~ 20/25C?.
I,m surprised that someone doesn't actually measure the condensate into a pot when the boiler is firing continuously at a steady output and return temp.

Why do I have to zip all my files/screenshots before posting??.

 

[Madrab]

Of course and I'm not suggesting that @ 50deg that the boiler would be condensing @ 100%. Though the suggestion was that there would be no condensing @ 50deg and that's what I was wondering about.

Really depends on the temp on the face of the HEX I would suggest, rather than the actual return temp - given these HEX's are highly efficient as far as conducting heat is concerned. If the return temp is 50deg as it enters the HEX then it's entirely possible the HEX itself would be @ 50deg at that point and therefore any gases touching the HEX would condense.

Most condensing boilers I visit condense to one degree or another given the fact that I see it condensing and there is condensate flowing down though the traps and into the outlet, again though if the return was lower then of course they would condense more

re the .zip file, I believe the forum only supports certain file types, I think it's do with anti virus and the like.

 

[Johntheo5]

The above trend shows a boiler efficiency of ~ 85.5% at the dew point of 53Cish and a boiler efficieny of 87%ish at 50C, a difference of 1.5% or a efficiency gain/saving of 1.72% so definitely some gain. I will try and put a few numbers on this re the actual condensate volume.
The big gain with a HE vs SE boiler is the sensible heat gain in the lower flue gas temperatures, I have a 19 year old SE 20kw Firebird Oiul fired boiler and its flue gas temperature has been 230C since day 1, the HE boilers should have no problem in getting down to 75C or so ~ a efficiency gain of ~ 7%, at least, without even trying.

 

[Johntheo5]

water vapour gives up 0.61kwh in condensing 1L of vapour, or give up 1.0kwh in condensing 1.639L (fully condensing), the "wet gas" loss of natural gas is 10% so each 1.0kwh of boiler input will condense 0.1639LPH, a 24kw boiler with a input of 26kw should condense ~ 4.26LPH, fully condensing.
Vaillant give a figure of 2.2LPH with flow/return temps of 50C/30C, or ~ 52% of my calculated fully condensing rate.

It would be interesting if someone would measure the condensate into a bucket when runnng at say a steady 10kw and whatever return flow temp the boiler is running at.

 

[Madrab]

Yes - I certainly don't think it is an exact science though as there are sooooo many variables at work with so many different systems. I've had new boiler fitted where the condensate hadn't been run but the client needed heat so the condensate would run into a standard bucket (shhh .... don't tell anyone) and the clients have had to empty that bucket in the evening as they were over 3/4 full.

What I do tend to see too is that the boiler condenses a lot more while it's getting up to temp due to the much lower return temps that it does once it is fully up to temp for obvious reasons