Boiler undersized question?

[vessy_73]

Hi,

Recently moved into a 70's house (detached 4 bed 1700 sqft) and now winter is here the central heating system is struggling, it's fuelled by a Baxi Solo 2 60pf boiler supplying heating and vented HW using S plan plumbing.

I've pretty much come to the conclusion that the boiler is undersized, the system feeds 12 rads, these together place a 59000 BTU load on the boiler, the system takes over an hour to heat the flow output to 60 deg, and eventually gets to 68 deg but no higher (boiler set to max which is about 75 deg), subsequently the house takes a long time to heat, once the system is up to 68 deg flow temp (about 1 hr 40) the house temperature rises at a decent rate, just takes an age to get to 68 deg!

I have fully balanced the system, getting 10-11 deg drop across each rad, the boiler return temp is around 11-12 deg below flow. The lounge rads are furthest away and lockshields are fully open, pump is on speed 2. With 68 deg flow temp I have 57 deg return at the boiler.

TRV's are fitted to the upstairs rads, once the house is above 20 deg (after 2-3 hours) the upstairs rads start to shut down, then the system is able to reach 75 deg and the boiler shuts off and cycles every 3-4 mins supplying the down rads until the room stat temp is reached.

My confusion and question is if I replace the boiler for a larger BTU output this will surely increase the flow temp from the boiler? taking it away from the 11-12 differential, if I increase flow through the system to get the differential back down then the rads will be lower than 10-11 deg drop, struggling to understand what to do!

I really want the rads to get to temperature quicker, so the house warms quicker.

Any advice welcomed. Thanks

 

[mcmoby69]

Is the boiler cycling often? Is the pump running fast enough? Are there any cold spots on the rads? Are the convectors clear? What s the insulation like?

Get a couple of local guys in to advise. Put your postcode in here, might be a good guy close to you.

 

[vessy_73]

Boiler only starts to cycle after about 2 1/2 hours (once trv's upstairs close), once the flow gets to 75 deg it shuts down, then comes on again around 69 deg, prob after 3-4 mins, fires for 3-4 mins until temp again reaches 75. When heating HW only the system gets to 75 deg after about 5 mins and probably cycles every 3-4 mins to maintain 75 deg flow temp.

I've had two people round, one thought it needed a flush and the other wanted to fit a new boiler, i'm pretty good at understanding how something should work, once I have the rads at 68 deg flow the house warms at about 1 deg in 20 mins, ive just never known a system to take an hour to reach 60 flow temp, and then nearly 2 hours to reach 68!

rads are warm all over, everything clean and good insulation, house loses about 1/2 deg per hour with current weather conditions (0 deg).

should a boiler cycle or should it remain constantly on when supplying CH? I thought cycling to maintain system temp was normal?

I've tried pump on 3, this slightly lowers the temp differential across all rads and boiler, so imo not required, pump on 1 gives a 14 deg drop across rads and boiler, oh new pump fitted 3 months ago

 

[DP]

Switch off the hot water zone while heating is on. See if heat up is quicker now. If it is ( most likely will be), then stagger the ch and HW run times

A service would bea good idea to I.e. A proper service not a just an operational check

Boiler changers and power flushers are a waste of time- they would still insist regardless of what the issue was. Have proven them wrong enough times for it to get boring

Boiler generates heat (obviously), pump moves it. Boiler thermostat monitors boiler emperature. If boiler flow temperature exceeds boiler thermostat set point, burner shuts down until flow temperature below set point.

Your boiler goes flat out for said one hour and still flow temperature has not reached set point.

Might have a solution for you. Do the check with HW zone off

 

[Servotech]

It seems to be the boiler that's at fault... it needs a good check over. You could take the thermostat Phial out of the boiler and boil the system for a short time.

 

[muggles]

A 60PF is capable of producing 60,000 BTU, although it may have been adjusted downwards. Don't let the powerflush guy back in your house, he's an idiot. If it needed powerflushing, you'd be experiencing frequent cycling. You've said you have no cycling at all for the first hour. I'd suggest that it's most likely a boiler issue and that the gas valve isn't opening properly, so you're not getting the heat output the boiler is rated to.

 

[45yearsagasman]

Has the boiler been gas rated?

 

[jeff the gasman]

You need to gas rate your boiler. All will become clear.

 

[vessy_73]

Morning all,

Thanks for the replies, DP I have done the staggering as you said, if both HW and CH are demanding the effect is even worse, all the figures I have given are with CH only.

I too did think about getting the gas checked, if the pressure is low and it is increased then I'm back to my original question of keeping flow and return at the boiler to 11 deg difference, surely this will become a larger split as the boiler is providing more heat to the flow?

What drives me mad is when I give in and call someone round they just feed you BS!

 

[muggles]

You can re-balance the system once your heat input is correct

 

[vessy_73]

You can re-balance the system once your heat input is correct

OK Thanks, So I can get my head round this, if I presently have an 11 deg drop across the rads then the flow through them is correct for the size, if the boiler output is increased (either by gas rate or new boiler) then what do I re-balance? where does the extra flow go that is needed to bring the boiler differential back down to design drop?

 

[vessy_73]

A 60PF is capable of producing 60,000 BTU, although it may have been adjusted downwards. Don't let the powerflush guy back in your house, he's an idiot. If it needed powerflushing, you'd be experiencing frequent cycling. You've said you have no cycling at all for the first hour. I'd suggest that it's most likely a boiler issue and that the gas valve isn't opening properly, so you're not getting the heat output the boiler is rated to.

This does make sense as my current load is 59000, this is without HW factored in, if the boiler is maxed out already then my only option is a bigger boiler, I'm thinking 80000 to give me enough power to heat rads and water together and a little headroom if any rads are added or upsized in the future, or should I go bigger?

 

[muggles]

A 60PF is capable of producing 60,000 BTU, although it may have been adjusted downwards. Don't let the powerflush guy back in your house, he's an idiot. If it needed powerflushing, you'd be experiencing frequent cycling. You've said you have no cycling at all for the first hour. I'd suggest that it's most likely a boiler issue and that the gas valve isn't opening properly, so you're not getting the heat output the boiler is rated to.

This does make sense as my current load is 59000, this is without HW factored in, if the boiler is maxed out already then my only option is a bigger boiler, I'm thinking 80000 to give me enough power to heat rads and water together and a little headroom if any rads are added or upsized in the future, or should I go bigger?

I'm afraid you've missed my point entirely. Your boiler is capable of 60,000BTU, you only need 59,000BTU, ergo your boiler is already correctly sized for your system, especially given that one would normally size a boiler to have a smaller output than the total of all the radiators, on the basis that it's generally unlikely that every single radiator is going to be needing the max. possible heat load at the same time. You could try using a Whole House boiler sizing calculator to see what your house technically needs...

I still maintain that you have a problem with your current boiler, and that it's not putting out the 60k BTU it's supposed to. Time to get an experienced boiler engineer in to have a look at it.

 

[muggles]

You can re-balance the system once your heat input is correct

OK Thanks, So I can get my head round this, if I presently have an 11 deg drop across the rads then the flow through them is correct for the size, if the boiler output is increased (either by gas rate or new boiler) then what do I re-balance? where does the extra flow go that is needed to bring the boiler differential back down to design drop?

Balancing is partly fine art, partly experimentation, so you'd need to sort the boiler first then play with the system. You'd most likely need to change your pump speed and lockshield settings, but let's worry about getting the main problem sorted before you get too concerned about whether the system is properly balanced or not

 

[vessy_73]

A 60PF is capable of producing 60,000 BTU, although it may have been adjusted downwards. Don't let the powerflush guy back in your house, he's an idiot. If it needed powerflushing, you'd be experiencing frequent cycling. You've said you have no cycling at all for the first hour. I'd suggest that it's most likely a boiler issue and that the gas valve isn't opening properly, so you're not getting the heat output the boiler is rated to.

This does make sense as my current load is 59000, this is without HW factored in, if the boiler is maxed out already then my only option is a bigger boiler, I'm thinking 80000 to give me enough power to heat rads and water together and a little headroom if any rads are added or upsized in the future, or should I go bigger?

I'm afraid you've missed my point entirely. Your boiler is capable of 60,000BTU, you only need 59,000BTU, ergo your boiler is already correctly sized for your system, especially given that one would normally size a boiler to have a smaller output than the total of all the radiators, on the basis that it's generally unlikely that every single radiator is going to be needing the max. possible heat load at the same time. You could try using a Whole House boiler sizing calculator to see what your house technically needs...

I still maintain that you have a problem with your current boiler, and that it's not putting out the 60k BTU it's supposed to. Time to get an experienced boiler engineer in to have a look at it.

Thanks for clearing that up, I didn't realise that it was normal to slightly undersized a boiler, I read somewhere on here that you usually add 9000 btu for water demand, that's what has lead me to believe it was undersized. It would be nice to have HW and CH running together, 3 boys and us use a fair amount of hot water daily!

However my main goal is still to get the system/radiators up to a heat producing temp in less than an hour!

I will get the gas checked and report back