New Boiler: is this the right size?

I rent a 4 bed semi (9 rads and 1 bathroom, open vent CH but mains pressure water cylinder) with an ageing Baxi Solo WM50/4RS (12Kw) boiler. The boiler has been cycling (on for 2 mins, off for a min) and sometimes intermittently making a clicking sound (sounds like a solenoid). I asked the landlord to get an engineer in whose advice was to replace the boiler with a GlowWorm Flexicom 24HX.

I'm keen on the new boiler (hopefully my heating bills will go down) but am not sure why a 24Kw boiler is required when the Baxi performed well enough at 12Kw. Isn't this going to cause problems as all but one rad is TRV fitted? Also, don't want my landlord, dear old chap, getting ripped off.

Any advice would be gratefully received as I know little-to-nothing about boilers (but understand plumbing well).

Tony

TonyTT said:

ageing Baxi Solo WM50/4RS (12Kw) boiler. ... an engineer in whose advice was to replace the boiler with a GlowWorm Flexicom 24HX.

Click to expand...

A 24kW would be a minimum if you were having a combi, but not with your existing set up. So the engineer either has it lying about in his shed, or he does not know how to size a boiler and is relying on the boiler's modulation feature to knock it down to 12kW (they can go down to 9.3kW). However, as Flexicom HX are available in 12, 15 and 18kW sizes, I wonder what his game is?

If you want to know what size boiler is required use the Boiler Size Calculator

If there is any problem with your existing system, and there may not be any, then it seems that engineer is more interested in doing a bigger job rather a proper repair or assessment of any problem.

You already have an unvented cylinder which is good and the normal solution would be a 12 kW boiler to replace the existing one.

Tony

Is the boiler cycling because of a fault or because it only needs to be on for two thirds of the time to produce enough heat?

Anyways, assuming that 12kW got you through the last winter OK then you don't need anything bigger.

Wow! Thanks for the great responses. Truly appreciated.

Using the Boiler Size Calculator, and allowing 3Kw for the water, I get 14Kw. Last winter the house was perfectly warm but we don't use all the rooms and I time the water to come on at seperate times from the heating.

I'm thinking 15Kw should be fine and I shall suggest a second engineer is called in.

I fitted the TRV's myself 3 years ago. Could that be the cause of the cycling? Still like the idea of a new class A boiler for economy.

Most modern boiler size calculators INCLUDE the allowance for hot water which should be 2 kW.

Tony

Agile said:

Most modern boiler size calculators INCLUDE the allowance for hot water which should be 2 kW.

Tony

Click to expand...

Hi Agile

I used D_Hailsham's link to the calculator. It gave 2Kw default for water, as you say, which I increased to 3Kw as the cylinder is quite modern (Salton Premiere).

You should not do that!

TonyTT said:

I time the water to come on at separate times from the heating.

Click to expand...

In that case you do not need to allow the extra 2kw for the hot water. Though as it is a rented property, it's probably safer to include the extra 2kW as the next tenant may want to have HW at the same time as CH.

I fitted the TRV's myself 3 years ago. Could that be the cause of the cycling?

Click to expand...

You mention the boiler cycling a lot. Does it do it all the time or has this only started to happen now the weather is warmer? If only now it is warmer its cycling because that is the only way the boiler can produce the lower amount of heat required. Unfortunately they are either full on or off. Modern boiler can modulate, which means automatically adjusting the output to meet requirements. Similar to cruise control on a car!

there's nothing wrong with that boiler a good repair engineer shouldn't be able to fix. What was his reasoning for suggesting a replacement?

The cycling happens when either CH or water is on. Not sure when it started. I only noticed it last year and as it doesn't seem to affect performance didn't think much of it until recently. Perhaps I should try putting the heating and water on simultaneously and see if it does it.

mickyg said:

there's nothing wrong with that boiler a good repair engineer shouldn't be able to fix. What was his reasoning for suggesting a replacement?

Click to expand...

His diagnosis went something like "it's knackered, mate" accompanied by some teeth sucking. Too technical for me. But as has been suggested, maybe he has a 24HX laying around he wants to shift.

TonyTT said:

The cycling happens when either CH or water is on. Not sure when it started. I only noticed it last year and as it doesn't seem to affect performance didn't think much of it until recently. Perhaps I should try putting the heating and water on simultaneously and see if it does it.

mickyg said:

there's nothing wrong with that boiler a good repair engineer shouldn't be able to fix. What was his reasoning for suggesting a replacement?

Click to expand...

His diagnosis went something like "it's knackered, mate" accompanied by some teeth sucking. Too technical for me. But as has been suggested, maybe he has a 24HX laying around he wants to shift.

Click to expand...

Touch the 2 pipes coming off the boiler when it's in cycling mode, if they're hot (60-70 deg), then everythings fine. if they're only luke warm then there's a problem. Dont get talked into changing it, great little boiler.

Both pipes hot.

With Water only the cycling starts after about 10 mins, 2mins on/1 off. I just tried with both water and ch on, all TRVs full on, room thermostat up to 30, boiler started cycling after 25 mins and the cycle time is longer, about 4 mins on/1 off.

Pump is set to "2".

bet theres nowt wrong with it! sounds like the boiler is just getting up to temp

With Water only the cycling starts after about 10 mins, 2mins on/1 off. I just tried with both water and ch on, all TRVs full on, room thermostat up to 30, boiler started cycling after 25 mins and the cycle time is longer, about 4 mins on/1 off.

Click to expand...

That is classic cycling caused by the boiler output exceeding the required output. Not a fault. The room stat won't affect this although the TRVs will. The room stat will turn the boiler off completely until the temperature inside drops enough to turn the boiler back on again. The TRVs as they shut down will reduce the demand for heat and the boiler will cycle more frequently.

Just for fun, this can be used to calculate the output level of your radiators. Turn off the water, turn on the heating, TRVs all wide open, and see how often it cycles. Let's say 3:1, means your radiators can only push out 75% of the heat the boiler produces, which would be around 9kW. Very believable. The rooms are presumably quite warm right now, so when they are at 15C on a winter morning they may be able to handle the whole 12kW while the house warms up, but not a whole lot more. You could get them to put out more power by increasing the boiler temperature but this isn't something you want to do with a condensing boiler except as a last resort when the next ice age arrives.

A higher power boiler will exceed the ability of your radiators, so is a bit of a waste. They will heat up more quickly initially, then would cycle more frequently. A modern 24kW boiler would likely modulate down to around 6kW as needed (I'm too lazy to look up the one you mentioned!) and so it might not cycle until TRVs start closing.

It would be nice to knock perhaps 30% off your gas bill with a new boiler, but the cost of a new boiler makes that a rather borderline proposition financially. Of course of someone else is paying for the boiler and you're getting the cheap bills ...