KW rating of boiler

[fezster]

Hopefully a simple question for you experts, but Im struggling understanding this in my head.

What does the KW rating of a boiler actually mean in terms of what is going on inside of the boiler. To put context to this question, this is what Im struggling to understand -

1) If the flow temperature (as shown on the display - eg. d40 on a vaillant) is the temperature of the water going out of the boiler, then how does a 12KW boiler outputting water at 70 degrees differ from a 37KW boiler outputting water at 70 degrees ?

2) Obviously, the more radiators there are, the more powerful boiler you need. But if the temperature of the water going out from the boiler is the same on both, and the volume of water flowing is determined by the power of the pump, how does more KW equate to more heat ?

I know there is an answer to this (probably a simple one), but Im struggling to get it using the examples above.

Thanks!

 

[JBPHS]

Simple, higher heat inputs allow the set point temperature to be reached more quickly (depending on sufficient flow rates, etc) and for combis (for example) provides a higher flow rate of hot water.

Look at it as:
2 Cars with similar characteristics, one has 70bhp and the other has 170bhp. Which car can reach 70mph more quickly?

James.

 

[ianmcd]

A 12 KW boiler would be a system boiler where it heats radiators with a HW tank , it doesnt take a lot of energy to heat the primary water in these systems, a 37KW boiler would in most domestic dwellings be a combi boiler where the DHW is heated instantly, in these circumstances you need a lot of energy to heat the cold water to hot water instantaneously, but the energy going to the heating circuit will be set at what is required if installed correctly, that is the basics but if you tell us more of why you want to know we could advise you more for your individuall circumstances

 

[fezster]

Thanks for the explanations.

I have a very long saga, discussed previously here:

//www.diynot.com/forums/plumbing/vaillant-438-with-3-zones-frequent-s53.366928/page-8


Specifically, my issue recently has been that my radiators were taking a while to heat up if both upstairs and downstairs zones were calling for heat. I had range rated my boiler to 18KW (long story, but to stop S53 when only my HW zone was calling for heat). So it makes sense that it takes a while for them to heat up, as the boiler is range rated too low. But using the analogy above, am I right in saying that eventually, the radiators would get just as hot with 18KW, as they would with 37KW ?

Secondly, I was finding that my downstairs radiators were only getting hot at the top, cold at the bottom. They are brand new rads, so it's not sludge (lets rule that out for a minute). It could be inadequate flow, and this has been discussed in the previous thread extensively. But the only thing I could try without spending any money was to increase the KW output of the boiler. I found that turning this up not only allowed the rads to heat up quicker, but also more evenly distributed the heat on the rad. The top is still much hotter, but the bottom of the rads are getting hot too now (previously luke warm at best). Would this make sense, and if so, what is the reason for it ?

Btw - all rads are very large. On average, more than a metre in width, and 600mm in height, double panel, single convectors.

 

[JBPHS]

I do not have time to read the full 15 pages. What have you done or had done to your system, I.e. How is it currently running? Please list everything. I as some others in the previous thread appear to show, have experience in getting systems like yours running with ease (without changing the boiler if it's economical and safe to keep it). I usually guarantee that if my work does not improve the system, then I don't charge for it, that's how confident I am!

 

[ALEC1]

the easiest and quickest way to sort this is put one of the reworked PCBs in...where are you in London?

 

[D_Hailsham]

1) If the flow temperature (as shown on the display - eg. d40 on a vaillant) is the temperature of the water going out of the boiler, then how does a 12KW boiler outputting water at 70 degrees differ from a 37KW boiler outputting water at 70 degrees?

The flow rate from a 37kW boiler will be three times as fast as from a 12kW boiler, provided the temperature difference across the boiler is the same

2) Obviously, the more radiators there are, the more powerful boiler you need. But if the temperature of the water going out from the boiler is the same on both, and the volume of water flowing is determined by the power of the pump, how does more KW equate to more heat ?

But the pump has to be sized to give the correct flow rate, which is determined by the temperature difference between flow and return.

Older boilers were designed for a 10/11C difference; newer condensing boilers use a 15/20 difference. This is so the return temp will be below 55C, the temperature at which condensing starts.

If you have a 20kW boiler working at a 10C difference the flow rate is:

20/(4.18x10) = 0.48 litres/sec.

If the temp difference is 20C then the flow rate is half that, i.e. 0.24 litres/sec.

The boiler size is determined by the output of the rads at the required differential.

 

[fezster]

The system is working "ok", but I know not optimally. The problems are / were:

1) that it takes a long time to heat up. This became more noticeable as the outside temperature dropped.

2) if downstairs and upstairs zones are both open, the upstairs only gets warm. When downstairs zone closes, upstairs gets hot.

3) The upstairs zone is ~ 18KW (rad output), downstairs ~ 26KW, and HW zone is a megaflow indirect 250L (26KW coil I believe). I was finding that the HW zone alone would cause the boiler to cycle before the cylinder satisfied it's stat.

4) The downstairs radiators would only get hot at the top, and lukewarm in the middle, almost cold on the bottom. All are new rads as of a year ago, and all are fairly large.

To begin with, I was getting S53 with the boiler not range rated. (Boiler is a Vaillant 438). To resolve this, I range rated boiler to 18KW. This was to remove the S53 and to resolve (3) above, so the cylinder heated up without the boiler cycling.

Another problem I had was a faulty / stuck ABV. This was continually allowing water to flow. I have since changed this, but I must admit, even now, with only the HW zone, I can feel the bypass pipe getting hot. I dont know if this is just due to convection in the pipes, or if the ABV works gradually, allowing small amounts of water to pass. The ABV is a honeywell, and the setting is now 5.

As of a couple of days ago, I've range rated the boiler to 26KW. I think this has helped in solving (1) and (4). I'm not sure about (2), have yet to test.

(3) has been an interesting one. Initially, when I went to test the HW alone (this is when the tank was probably already almost up to temp), I found the boiler started to cycle almost instantly. The return temp would rise very quickly, and then the boiler would turn off. This was not happening in the first 50 seconds - I.e. the boiler should have had sufficient time to modulate down, but because the return temp was rising quite quickly, it was eventually turning off and cycling. The HW stat did not reach temp for at least 1 to 1.5 hours. I tried adjusting the ABV a little higher, as I suspected water may be passing through it, and causing return temp to rise too quickly. I dared not go past setting 6 in fear of breaking it, so eventually just put it back to its original setting. I tried adjusting the single radiator on the HW zone, increasing flow to it and reducing flow to it. Basically, nothing worked. So I left it.

Then the following day, I decided to monitor the boiler as the HW came on, probably from a much lower temp, as we'd used plenty of hot water. To my surprise, the boiler behaved much better. Initially for the first 20 or so minutes, it increased flow temp slowly, and return temps also crept up slowly, with a differential of about 12 degrees. As the boiler reached max temp (70 degrees), it went very, very quiet. At first I thought it had gone into S53, something Ive seen a lot of, but in fact, it was still running normally, it had just modulated down so much, it was barely making any noise. The return temp was 65 degrees at this stage. The boiler continued firing at 70 for about 10 minutes, and the cylinder stat switched it off - a total of 30 mins, which is what the Haetra Sadia literature says the recovery time should be !

So the long and short of it is that I think the system is working pretty well now, but I need to monitor for a week or so. I think the boiler behaves well in certain conditions, but can sometimes get itself into a state where it results in cycling or S53. Perhaps the new PCB would help, but Ive thus far been unable to get one. I've asked Vaillant about it, but they've not been very helpful. I live in SE London / Kent by the way.

With regards to (2) above, I have a secondary pump on my downstairs circuit, as the last rad was not heating up. I have a 26-55 main pump, then a 15-60 second pump on downstairs. I feel the entire system should probably have a 25-80. Probably a grundfos alpha smart pump, because it could adjust the pump speed accordingly if one zone or all three zones are open. They cost over £400 though, so Im reluctant to invest in one unless Im sure it will help.

Sorry for the long post! Wanted to try and be thorough.

 

[JBPHS]

Right ok.

I'm sorry to say that with all due respect to you, you do not show that you understand how an Automatic Bypass Valve works or most importantly how it should be set up. That's the least of your problems, but enough of the visual equivalent of an ear-bashing.

A low loss header was mentioned in the previous thread. Has this been done? By the way it's not just a case of fitting a big pipe! You need to seriously consider employing someone who understands how to resolve your problem before you waste anymore money on pumps or worse a failed heat exchanger.

I've seen (and rectified) this situation on many occasions. With thorough understanding and as vital, experience, of the matter, it can be resolved. It is possible (as impossible as it sounds) to use a boiler that provides half the heat to what the property requires.

This is very easy to solve, a quote that my high school chemistry teacher used to say "It's like a page 3 model; seen one, seen them all." Anyway I digress, I'll provide information on how to change tap washers, etc. but for jobs like these, what I will tell you is that all of your problems (mentioned above) can be corrected, but you don't get a magician revealing his trick before he performs it, if you know what I'm saying?

 

[Steelmasons]

You were advised to install the circulators in series fester to which you have not done , a 25/80 will solve your issues but you also failed to acknowledge.

 

[fezster]

you do not show that you understand how an Automatic Bypass Valve works or most importantly how it should be set up.

I've read many a thread on how it should be setup. The "problem" is that even on it's highest setting, with only the HW zone open, the bypass seems to be passing some water (although not hot to the slightest touch, which would mean it's fully open). I'm no expert, but the fundamental principles are straightforward enough.

You were advised to install the circulators in series fester to which you have not done , a 25/80 will solve your issues but you also failed to acknowledge.

The downstairs pump is in series to the main pump. Unfortunately, I do not have the skill to change things around myself, and I have a reluctance to get somebody in again without some level of confidence that I will ultimately attain my goal.

A low loss header was mentioned in the previous thread. Has this been done?

No, as above really. To be honest, at this stage, with the system behaving much better, Im not sure it needs any fundamental modifications. I'm pretty certain that some of the problems I described in the previous thread must have been down to an airlock, as the boiler was behaving extremely erratically, with return temperatures barely moving a degree even after tens of minutes. That seems no longer to be the case.

 

[fezster]

The flow rate from a 37kW boiler will be three times as fast as from a 12kW boiler, provided the temperature difference across the boiler is the same

Thanks. Just to expand on this a little - the flow rate of my system is determined by my pump, and therefore is fixed. (Please correct if wrong). Therefore, with the flow rate fixed, a 37KW boiler will heat the total volume of water far quicker (three times as quick even ?) than a 12KW boiler.

I want to understand this point, because in terms of CH only, my system was pretty much working identically at 18KW during summer, to 26KW. Is the answer because the increase in temperature is less, and therefore the difference in time to heat up the water was roughly the same. Now that the outside temperature is much lower, and the increase in temperature required much larger, it is more obvious that at 18KW it'll take far longer than at 26KW ?

 

[fezster]

what I will tell you is that all of your problems (mentioned above) can be corrected, but you don't get a magician revealing his trick before he performs it, if you know what I'm saying?

I did notice you thanked picasso for his post about the low loss header

I have actually spent far too much money on incompetent heating engineers (with all due respect to them, as Im sure they knew what they did know, very well). This size / type of system didnt seem like something they'd come across, at least not often. Drop me a PM and we'll have a chat to see what my options are. Thanks.

 

[Steelmasons]

With all due respect JBPHS there is nothing magical to perform with regards to fezsters heating system.

 

[JBPHS]

With all due respect JBPHS there is nothing magical to perform with regards to fezsters heating system.

To me it isn't magical, it's all in a days work! It's a poor do that this has been going on for so long and no-one has solved the issues.