Rate of increase in boiler flow temperature

[bw65587]

Hi,

I have a Glow-worm Ultracom 38hxi open vent boiler that is set to 82C.

It was downrated to 16kW by BG as it had been massively oversized by a Glow-worm recommended installer that didn't know what they were doing!

Anyway, BG had to replace the Grundfos pump on Saturday as there was little/no circulation and the boiler was cutting out.

However, when the heating came on at 05:30 this morning, it was only at a flow rate of 60C by 06:15.

Question is, how quickly should a boiler reach its set temp (82C in my case)? It seemed to be only creeping up very slowly. Is this because the circulation has improved???

Any advice/thoughts/opinions would be most welcome!

 

[Agile]

You have not described your system. Heat loss of house as calculated, number of rads and heat output of rads.

In a typical three bed semi the boiler flow will reach the set temperature in 10-20 minutes. The house temperature should be within a degree or two within 30-50 minutes.

Most three bed semis need about 10-12 kW.

It seems that BG have not correctly identified or set your system.

Tony

 

[bw65587]

Hi Tony,

I have the calculations at home so would have to provide them later but it's a 4-bedroom detached property with 11 rads.

Every rad has a Westherm TRV except the towel rail in bathroom and hall rad where the stat is.

Appreciate the assistance.

Cheers,
Neil

 

[Agile]

In that case then 16 kW is likely to be about right if reasonable well insulated.

I really do not understand how installers can work when they are so inept at what they are meant to be advising on!

It does sound as if there may be a flow problem!

Measuring the flow and return temperatures at the boiler might give an indication of the problem.

Tony

 

[bw65587]

Thanks Tony.

Yesterday, I reduced the boiler down to 75C and the return temp was 55C. I put it back up to 82C as that's what BG left it at. I think it was set to 80C when they arrived. Not sure what the return temp is at 82C though.

Originally, the GW-recommended installer hadn't even fitted the pump etc in a close-coupled arrangement and it was pumping over!

Thanks,
Neil

 

[bw65587]

I've calculated that I have 14,376 watts of existing radiators.

I also have a 48 x 18 DHW cylinder (162 litre) that I would need to add on. Is that 2kW to add on???

Now, an online whole house calculator comes out with 12kW.

So, do I rate the boiler for 12kW or rate it for 14,376 + whatever I need to add on for the DHW cylinder? Say, 17kW?

Many thanks,
Neil

 

[Agile]

The whole house seems very good in practice.

If you have correctly calculated the rad output at 75/55 then I would use 14 kW plus 2 kW for the cylinder. That should ensure a quick warm up and a very efficient boiler operation ( less the rubbish oversizing of the boiler ! )

Its clearly got a problem. Set the flow to 75 C as that will be most efficient.

Could you watch the boiler for 10-15 minutes while its warming up in the morning and see if the boiler is cycling?

Next, and the probably fault, can you time the hot water to start 30 min before the heating? The aim is to have the cylinder satisfied before the heating starts.

Is the cylinder stat correctly set to about 55-60 C ?

What size of cylinder do you have and how many occupants?

Tony

 

[bw65587]

Hi Tony,

Boiler rated to 16kW and set to 75 C. DHW is set to approx. 55 C. The cylinder is 162 litre 48" x 18" and says on it that heat loss would be 2.74 KW per KWH/24h whatever that might mean! We are a family of four.

Advanced it on at 11:02 this morning and this is what it did:

• 29 C starting temperature at boiler at 11:02.
  Immediately boiler flow temp rose to 52 C before dropping down to 33 C.
  After 10 mins at 11:12, flow temp 40 C.
  At 11:15, flow temp suddenly rose quickly to 53 C.
  Between 11:15 & 11:45, flow temp rose to 75 C at about 1 C per minute.
  At 12:02, flow was 75 C and return was 57 C.

So, it took 43 mins to reach 75 C from a starting temp of 29 C and everything appeared well, until...

Rightly or wrongly, I wanted to see what would happen once the cylinder stat setting was reached so I turned it down slowly until it clicked at around 48 C. At that point the boiler went off despite the CH stat being at 25 C and the house was at 22 C according to a thermometer next to the stat. Maybe that's what it should do, I don't know?

Anyway, since then, the boiler didn't seem to be able to stay on when there was no demand from the cylinder despite the CH stat being set to 25 C. Raced up to 75 C+ before switching off.

When I increased the cylinder stat back up just now to 55 C, the boiler seems happy again, until the cylinder stat is reached I suspect!

What do you think it could be?

Cheers,
Neil

 

[Agile]

Next, and the probably fault, can you time the hot water to start 30 min before the heating? The aim is to have the cylinder satisfied before the heating starts.

Tony

You apparently did not do that. The objective was to ensure the cylinder was satisfied before the heating was required and then the whole boiler output could go to the heating.

But you have identified that there is another fault, no call for CH to the boiler !

That would usually be a faulty CH motor valve head or its not correctly wired.

Tony

 

[bw65587]

Hi Tony,

I hang my head in shame! You are quite correct. I didn't do that bit. I thought I'd report back to you as a two stage process. That's my excuse and I'm sticking to it.

Anyway, I will alter the timed settings as advised.

However, I am beginning to suspect there is something wrong somewhere with the set up of the boiler and wiring etc. Originally, there was an options board fitted and we had a Climapro RF but we had to get rid of it as it kept losing touch with the boiler which kept coming on at silly o'clock in the morning!

I have just called GW to see what could be up after the uninstallation of the Climapro RF. They weren't much help but tried! They confirmed the boiler should be at it's set temperature within 20 mins though as you said.

I think I'm going to plump for a wired Climapro unit as I have the options board and see if that sorts it out.

Know anyone who wants a cheap Climapro RF and receiver???

Cheers,
Neil

 

[Agile]

Dont think too much of those who answer makers help lines.

Whilst they can look up data and know all the common faults their boilers get they are not normally gas registered and will not have the same skills that the experts who post here who are working enginers fixing real boilers.

Tony

 

[ianniann]

They confirmed the boiler should be at it's set temperature within 20 mins

That's a little optimistic with your boiler downrated to 16kW, especially with your moderately high set flow temperature. I would suggest it could easily take an hour to reach 82C, maybe 50C-60C in 20 minutes. You can easily calculate a theoretical time to reach that temperature if you know how much water is in your heating system. Your boiler at 16kW will raise nearly 70l of water by 50C in an hour, assuming no losses. Of course there are considerably losses since it is heating your house, so that is an absolute minimum time.

Possibly it will never reach the set temperature, depending on the heat output of your radiators. For example, if your radiators output 16kW at an average temperature of 50C (say input 55C and output 45C) then your boiler will only ever be able to heat up to that temperature. The output of the radiators will increase as the temperature increases, so often the boiler would reach a particular temperature and then stay there. 16kW is a not unreasonable output for a set of radiators at 80C. If TRVs start to kick in then the total output of the heating system will decrease and the boiler will be able to increase the water temperature.

 

[D_Hailsham]

You can easily calculate a theoretical time to reach that temperature if you know how much water is in your heating system. Your boiler at 16kW will raise nearly 70l of water by 50C in an hour, assuming no losses.

My physics is a little rusty. How did you work that out? What's the formula?

 

[bw65587]

Next, and the probably fault, can you time the hot water to start 30 min before the heating? The aim is to have the cylinder satisfied before the heating starts.

I altered this evenings timed setting so DHW on from 17:00 to 17:30 followed by CH from 17:30 onwards.

In the half hour, the DHW rose from around 40 C to 48 C, not to the required 55 C. Additionally, the rads warmed up slightly?

Following the DHW period, the boiler did seem fine heating the rads up to the CH stat setting but since then does seem to have fired up and gone off very quickly on a number of occasions?

Neil

 

[ianniann]

My physics is a little rusty. icon_sad.gif How did you work that out? What's the formula?

Ballpark figures, 16000 (W) / 4 (joules per g of water per degree) / 1000 (g/l) * 3600 (secs/hour) / 50 (degrees C)

I get 288l now, which is a lot of water! Did I divide by 4 twice in my head? Oh well, so much for that. Factoring in the continuously changing heat loss to get a better calculation would need calculus and I'm not up for that at this time of night! You could also get a quick figure using the combi water heating rule of thumb, but I couldn't remember it

Still, a 16kW boiler will take best part of an hour to fully heat a decent sized heating system. I know, I've tried it It is warm after 20 minutes, but it certainly isn't 80C!