Central heating pump sizing

[Reddwarf4ever]

Hello, until this February it has not been in condensing mode since new 2009, only just realised how it should operate. Can’t see how the heat exchanger will get blocked ? It’s just a case of efficiency

 

[Reddwarf4ever]

Last week, I had a phone conversation with a Vailant engineer. I asked why my boiler cuts out at a DT of only 3oC between flow and target temperature & not 5oC as I have been told it should be, he replied it varies, depends on how hard the boiler is firing, I replied that doesn’t make sense, the cut out temperature is a specific value or % of range, either way a definite value, he just repeated it again, then said it controls on DT of inlet and outlet temperatures D41-D40, I said that doesn’t make sense either, it can only have one control parameter, which is D5.....he kept on about things not being specific and depends on this or that.......I completely gave up in the end, none the wiser......send one FINAL long email to Vailant re that phone call and asking for clarification......don’t suppose I will get any....

 

[John.g]

Overall, I think you are doing pretty well, you can get the return temp down to ~48C by increasing the flow temp to ~ 75C and reducing the flow rate but then the boiler will exceed the 30c max deltaT on fire up due to the reduced flow, and trip, if you keep reducing the flow temp then the rad outputs will fall below the minimum that you require and below the minimum output of the boiler.
Also, in non condensing mode, 10C difference in flue gas temperature (~ same difference in return temp) will only result in ~ 0.45% difference in boiler efficiency.

 

[John.g]

Just had another look at those numbers and I can see now why you can't get into condensing mode at a demand of 5.2kw.

If all your rads were in service and assuming a rated output of 18kw then with flow/return of 75/58 & 0.75m3/hr flow the actual output will be
15kw. IF the heat demand now fell to 5.2kw and you reduced the flow temp to 45C then you will get a return of 39C and 5.2 kw output.

Now you only need to heat 4 or 5 rooms so these rads which I reckon have a rated output of 7.2 kw are required to output 5.2kw so for the same flow rate of 0.75m3/hr then flow/return of 65/59 will give you the required 5.2kw but the return is way above condensing and there isn't much you can do about that IMO.
If you reduce the flowrate to the minimum required of 0.33m3/hr to maintain a boiler deltaT of 25C for the required 9.6kw boiler output for 1 minute on fire up then to give the required min output of 5/5.2kw requires a flow temp of 68C which results in a return of 54C, still no condensing.
You would have to oversize the rads by a factor of 1.25 (9kw) to even begin condensing at 50c (flow of 66C) and by a factor of 1.8 (13kw) to get meaningful condensing at 42C (flow of 55C)

It seems very apparent that no matter what the combination of flow temp and flow rate is that it is thermodynamically impossible to have a boiler in condensing mode with the rads outputting their rated output and have to be outputting only ~ 56% of their rated output to get a return of ~ 42C and real condensing effect.

 

[Reddwarf4ever]

I agree John, tried all your ideas and no matter what I do, either the boiler continually cuts out due to over firing / over temperature, or the inlet temperatures continues to be around 55-58oC. Think condensing boilers work best when the heating system is designed from new for a condensing boiler.

looking at the condensing efficiency chart, I am at 86-87% with the boiler in Non condensing mode, if I change 2 lounge radiators to increase heat output to 30% for Each, not sure how much difference this would make to the return temperature, even if I could achieve 45oC the efficiency would only be 4-5% higher, so wonder if it justifies the expense......

thanks

 

[John.g]

I don't think it's worth the hassle/expense, I have always been skeptical of the actual condensing benefit achieved.
On oil fired boilers, the flue gas temp on a SE boiler is around 230/250C and 80/100c on a HE so a real gain of ~ 6 to8%, I would like to see typical flue gas temperatures for SE/HE gas boilers, I wouldn't expect a SE gas boiler temperature to be as high as that of a oil fired boiler.

But there are lots of ways of skinning a cat, I ran and maintained relatively large 50MW, 45 bar superheated steam boilers and because of the high water temps involved (259C) regenerative rotary air heaters were used to heat the incoming combustion air with the outgoing flue gas and the boilers had ~ 85% efficiency....your next project.

 

[Reddwarf4ever]

I don't think it's worth the hassle/expense, I have always been skeptical of the actual condensing benefit achieved.
On oil fired boilers, the flue gas temp on a SE boiler is around 230/250C and 80/100c on a HE so a real gain of ~ 6 to8%, I would like to see typical flue gas temperatures for SE/HE gas boilers, I wouldn't expect a SE gas boiler temperature to be as high as that of a oil fired boiler.

But there are lots of ways of skinning a cat, I ran and maintained relatively large 50MW, 45 bar superheated steam boilers and because of the high water temps involved (259C) regenerative rotary air heaters were used to heat the incoming combustion air with the outgoing flue gas and the boilers had ~ 85% efficiency....your next project.

I worked most of my life on refineries, where heat recovery was paramount, especially producing 600# steam from furnaces to drive turbines. I love thinking of ways to be more efffcient, but sometimes the odds are weighed against me. I envisaged a system, where a separate cold water tank ( feeding the DHW cylinder, could be preheated, using an other boiler heat exchanger, just before it vents outside, to extract the remaining latent heat, would only need a very small pump, only running when the boiler is on......the ‘heated’ DHW storage tank would be the size needed for a days use of DHW, if it fell low, it would be made up from the main DHW storage tank, would need to be foam insulated also......just a dream......

the way Energy is going, won’t be long before burning gas is a thing of the past, we will all be on electric heating, so much simpler. There is so much more better ways to use the gas, or at least the oil it was produced from, than just burning it. Burning fossil fuel for energy should be the very last thing we do, when it can’t be re used or recycled anymore IMO

take care

 

[king of pipes]

Products do exist but they are costly. Kop

 

[Reddwarf4ever]

Shaun corbs said “Need to keep it condensing else it will break the boiler in a year or two as the heat exchanger will get blocked as there’s nothing to move the waste”

but dont see why this should be or how

 

[ShaunCorbs]

You will get particles that will build up in the burner chamber that won’t get flushed through with the condense and will build up to a point where it just blocks the hex and could cause it to split