Vaillant Ecotec Plus 637 or Worcester Bosch 8000 35kW

[windsortg]

If you've been successfully heating your home with a boiler capped at 24kW there's absolutely no way you need anything bigger than that. I don't know what software BG use to calculate heat loss but it sounds like it's not very accurate. Did you try doing your own using Heat Engineer?

no, I didn't try it on my own but I did get another heating engineer who came over last week to carry out a heat loss calculation from which I should get the results any time soon.

From the two heat loss calculations that have been carried out, British Gas only took measurements internally of the house, whilst the other heating engineer only took measurements on the outside of the house. I guess there are many ways to skin a cat!

The other thing is, I have lowered the power output of the boiler to 24kW to eliminate the S.53 message I was getting on the boiler. What I do not know is whether 24kW is the right output as I do not have anything else to compare it with. I could possibly find a 30kW output providing a better heating experience.

 

[windsortg]

The biggest problem with whichever boiler is installed will be running the UFH on its own as its only a 2 loop system but ironically, the easiest to see its heat requirement, all thats needed is the flowrates through the loop flow tubes and the manifold dTs.

thanks @Johntheo5 - TBH, I am no really sure how to go about measuring the flowrates through the tubes and the manifold dTs . I do not have any design details of the UFH set up.

I did call the Vaillant technical team and they mentioned that the reason my boiler is cycling is because the flow rate through the boiler & system is not enough when only the UFH is operating. They said for a 24kW boiler, the min flow rate needs to be 17.2L/min, and for a 30kW boiler, the flow rate needs to be minimum 21.5L/min. I have no idea to know what the flow rate is for my UFH. Their recommendation is therefore to install a Low Loss Header along with 1-2 additional pumps.

Oh, and also since this week, I have found that my existing boiler is 'kettling' so whatever happens, I will have to change my boiler soon.

 

[Johntheo5]

Just put on the UFH with the rads, let the system heat up for say 15 minutes or so.
UFH: there are two flow meters on the top (flow) manifold which read 0 to 5 (L/min), to the left of these, under the red handled isol valve is a temperature gauge, on the bottom (return) manifold under the blue handled isol valve is another temperature gauge. When the system has been on for 15/20 minutes read off the two flows and the two temperature readings and post back.

 

[windsortg]

thanks @Johntheo5. I have attached a photo (taken last October) of my UFH system. As requested, I will take a reading of the flow meters along with the two temperature readings once the system (both, central heating and UFH are both on) is on for 15-20 minutes.

The blending valve temperature is currently set between 45 and 50.

I know a couple of heating engineers have tinkered with the flow meters when observing the UFH running on its own for a couple of minutes.

 

[windsortg]

@Johntheo5 - I took some readings of both, the boiler and UFH. The readings are shown below (I also attached a file showing these readings incase the image below did not come through).

I took the readings across 5 minute intervals. What I found is that when I had both, CH and UFH turned on, the boiler was happily firing (without any sound of kettling which there was earlier in the week). The flow meters on the UFH was showing 2.5 (range 0-5). When I turned off the UFH, the flow meters on the UFH went to 5 and these flow meters stayed on 5 when I turned on the UFH and turned off the CH.

After about 10 minutes of the UFH only being on, I could see the boiler short cyling (every 2 mins) and the boiler flow temperature would rocket from 45 to 72 degrees. I hope this helps but happy to do some further tests to get to the root cause of the problem.

 

[windsortg]

@Johntheo5 - one thing I forgot to mention, I did share the above readings & observations to another person and their response was:
Simple - fix a 2 port buffer of around 25 to 50 L capacity (closer to 50L is better), across the flow and return loop to the boiler or across the flow and return from the UFH, where ever is easiest to install. No need for low loss headers or 4 port buffers and additional pump (s).

Parts list
- Two Tee pieces,
- A buffer or volumiser https://www.theplumbshop.co.uk/products/expansion-vessels-intermediate-buffer, these are cheap enough, you need to insulate yourself.
- Two isolation valves

Is this a possible solution to stop boiler from short cycling when only the UFH is on?

 

[Johntheo5]

Some strange readings there, if you take test 6 (T+15), CH+UFH ON, it shows blr flow/returns of 54C/36C. and UFH flow/returns of 48C/27C with combined flows of 5LPM, this gives a UFH output of 7.3kw but the temp gauge above the pump reads 58C which should be the same as the manifold flow temperature (48C).
Test 11, (T+40) UFH only on shows blr flow/returns of 71C/36C. and UFH flow/returns of 49C/34 with combined flows of 10LPM, giving a UFH output of 10.5kw, however the boiler is cycling, but even if the UFH requirement is only 5kw then the cycling shouldn't be excessive.

Very strange as to why the UFH flow increases to 10LPM and even more strange that with the CH on and the UFH off that the flow was still 10LPM.
When the CH is on but UFH off is the manifold pump off and is there a UFH zone valve somewhere that switches this pump on/off?.

What was the boiler flow target (setpoint) temperature for the above tests?.

Don't know how a buffer works just connected between the flow and return.

 

[windsortg]

@Johntheo5 - I decided to run a fresh test today and the readings are shown below. The boiler temp is set at 65 degrees. When the UFH is off, the flow meters are 0 so I must have taken a wrong reading yesterday. Also, on T+40, the flow temp on UFH was either 40 or 50. I wrote down 50 at the time but I think that was wrong and it should have possibly been 40.

Are the readings still strange? No kettling sound from the boiler when I was running these test readings.

I have also attached photos of the main pump and UFH pump when both, the CH and UFH are on

 

[windsortg]

@Johntheo5 - just to run some further tests, I reduced the speed of the main pump to speed (1) from speed (3) (photo attached) and took the following readings with just the UFH on.

 

[muggles]

The other thing is, I have lowered the power output of the boiler to 24kW to eliminate the S.53 message I was getting on the boiler. What I do not know is whether 24kW is the right output as I do not have anything else to compare it with. I could possibly find a 30kW output providing a better heating experience.

Which is why you need a room by room heat loss survey to find out. Not sure what you mean by "a better heating experience" though. A more efficient and cheaper to run heating experience is what most people want