Boiler flow temperature

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We have a Glowworm 38hxi downrated to 21kw as we have radiators + dhw totalling about that. We have the flow temperature of the boiler set to 75.

We had a new radiator installed the other day so the system was drained down for that (I increased rating to 21kw from 19kw due to the new rad being 2152 watts.

However, last night, when checking the rads were balanced, I put the thermostat up to 30 and after 1.5 hours the boiler flow temperature was only showing as 55 and the temp in the house had only risen from 21 to 22. All of the rads warmed up evenly so they do appear balanced but none of them ever really got hot. The return temp at the boiler was about 12/13 less than the flow temp.

Why didn't the flow temp get up to 75?

Many thanks!
 
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We had a new radiator installed the other day
Great stuff but have you asked your installer who made changes to your heating system,they have actually seen your heating system layout. Every heating system is bespoke, your system could be just about working during the current climate and the current setup.

who knows :!:
 
I asked our installer (BG) at the same time I posted here as I've always had great help on here over the years and wanted a second opinion. The BG engineer has recommended I bleed each rad to ensure that there's no air in the system following the drain down and refill. I'll see if that helps.
 
There was hardly any air in the system when it was bled and all of the rads appear to warm up equally. They just don't get as hot as I think they should get despite the boiler being set to 75. The boiler just seems content to flow at 55 after hours of operation. It does occasionally fire up to 75 for some seconds before returning back to 55. BG engineer is coming back at the weekend to check but does anyone have any thoughts on it so I have something to discuss with him? I have added up the rad output for all of our rads and added 2kw for the DHW and it comes to just shy of 21kw which is what the boiler is currently range rated to. Any advice would be appreciated!
 
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However, last night, when checking the rads were balanced, I put the thermostat up to 30 and after 1.5 hours the boiler flow temperature was only showing as 55 and the temp in the house had only risen from 21 to 22.
Was the house very cold when heating started? That might explain the time taken.

But apart from that, if the boiler is outputting 21kW, and the rads are dissipating 21kW with a flow temperature 55° and room temperature about 20°, the system is in equilibrium. Maybe the boiler is outputting less than 21kW, or 21kW is not enough and needs increasing. There's plenty of scope for raising it as it's rated 38kW.
 
Was the house very cold when heating started? That might explain the time taken.

Not particularly. The house doesn't generally lose a lot of heat.

But apart from that, if the boiler is outputting 21kW, and the rads are dissipating 21kW with a flow temperature 55° and room temperature about 20°, the system is in equilibrium. Maybe the boiler is outputting less than 21kW, or 21kW is not enough and needs increasing. There's plenty of scope for raising it as it's rated 38kW.

I did try experimenting with increasing the boiler output to 24kw and then 30kw but it became excessively noisy and seemed to me as if it was overrated on those settings from its behaviour.

Thanks for your comments.
 
I did try experimenting with increasing the boiler output to 24kw and then 30kw but it became excessively noisy and seemed to me as if it was overrated on those settings from its behaviour.

Thanks for your comments.
Is it possible to increase the pump speed?
 
Is it possible to increase the pump speed?

I've looked and it's set to position 3. Is it possible that the pump just isn't big enough? We've built an extension on the house so now have 4 more radiators than we did a year ago. There are a total of 14 radiators and the length of pipe run in the house (large 4 bed detached) is now a lot more than it was. The pump is a Grundfos (blue in colour with British Gas logo on). It says it's a type Multihead G.
 
I've looked and it's set to position 3. Is it possible that the pump just isn't big enough? We've built an extension on the house so now have 4 more radiators than we did a year ago. There are a total of 14 radiators and the length of pipe run in the house (large 4 bed detached) is now a lot more than it was. The pump is a Grundfos (blue in colour with British Gas logo on). It says it's a type Multihead G.
Maybe higher output would be OK (no noise) if the pump flow was higher. I changed my pump early this year as it was very noisy, and when I took the old one apart I was surprised how much crud was inside and behind the impeller (that wasn't the cause of the noise, a stainless steel ring had fallen out in the casing). I cleaned it up and keep it as a spare. Old pump, UPS15/50 needed speed 3, new one UPS2 is OK at speed 1. It might be worth seeing if yours is clagged up. Easy enough, only 4 Allen head M5 bolts, no need to dismantle pipework.
 
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Maybe higher output would be OK (no noise) if the pump flow was higher. I changed my pump early this year as it was very noisy, and when I took the old one apart I was surprised how much crud was inside and behind the impeller (that wasn't the cause of the noise, a stainless steel ring had fallen out in the casing). I cleaned it up and keep it as a spare. Old pump, UPS15/50 needed speed 3, new one UPS2 is OK at speed 1. It might be worth seeing if yours is clagged up. Easy enough, only 4 Allen head M5 bolts, no need to dismantle pipework.

Thanks! I'll let you know!
 
21 kw is pretty much at the top end of a 15/50. I would try a 6 mt head pump.

BG changed the pump on Monday to a Grundfos Alpha 2 15/60 with Auto Adapt but they said that there does seem to be a lot of crud in the system as the magna clean was pretty dirty again within the few hours they were here after cleaning it on their arrival. They've suggested a powerflush next year when we replace the last 3 old steel rads in the house.

What is surprising to me is that I've still had to lower the boiler output to 18kw as it still doesn't like the boiler being set to 21kw even though I've done all the calcs again and again and come up with 21kw as the total wattage of all of our rads including 2kw for DHW. If I set the boiler to 21kw it does exactly as I described in my first post, never wishing to go above a flow temp of 55. With it set to 18kw, it rises slowly to the set flow temp of 75. Does anyone understand why that may be? Is that what you experts would expect?

Many thanks!
 
With it set to 18kw, it rises slowly to the set flow temp of 75. Does anyone understand why that may be? Is that what you experts would expect?
I wouldn't expect, it seems counter-intuitive, but I'm a DIY amateur, not an "expert". We'll maybe see what others think.

Out of curiosity, did you keep the old pump? I would have, for a spare, as it was presumably OK since the new pump hasn't improved things. Was the old one clagged up internally?
 
Out of curiosity, did you keep the old pump? I would have, for a spare, as it was presumably OK since the new pump hasn't improved things. Was the old one clagged up internally?

BG took it away with them. I didn't think to ask how it looked. I'm going to experiment with different boiler outputs just to see how it affects things. The boiler is 10 years old next year. I may just replace it. I have a love hate relationship with it. My children say I know the boiler better than I do them! Ha-ha! At least it never answers me back!

Thanks for your comment.
 
BG took it away with them. I didn't think to ask how it looked. I'm going to experiment with different boiler outputs just to see how it affects things. The boiler is 10 years old next year. I may just replace it. I have a love hate relationship with it. My children say I know the boiler better than I do them! Ha-ha! At least it never answers me back!

Thanks for your comment.
OK thanks
 

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