Conflicting Advice, help please

[Julian Such]

Hi,

Homeowner without a clue. I am replacing my Valliant Turbomax 28KW system boiler and have had three quotes. Two plumbers have said a 30kw condensing system boiler should be fine (Bosch, Valliant or Viessmann).

However the last plumber has said that I need to be at least 40kw or more as I have an 18kw coil in my HW cylinder and if I don't get this I will get a drop in CH temp and 'cycling' and a loss in efficiency etc. I started to get a little confused at this point!

I live in a 4 bed house with 14 rads and have a tado smart control. The HW is only on for say an hour a day.

So, the question is did the first two plumbers miss this or is the third a genius or over speccing my requirements???

Cheers

 

[dancinplumba]

i would go with third plumber

 

[dancinplumba]

Don't go WooShitter bosch or any other boiler that has ali to system water heat exchanger

 

[ShaunCorbs]

In short the last one is allowing for your hot water to be on aswell as your heating

But I would be surprised if your rads add upto 20kw

Just put the hot water an hour earlier or go with the bigger boiler but heatloss calc the house (how much heat to heat your house)

 

[Chalked]

An oversized condensing boiler will cycle. And thus use more gas.
Has anyone actually measured the Heatloss for the house?

 

[Chuck]

If you are happy with the performance of the current boiler there is no reason to up-rate the new one by 40%. Renewing the boiler is not going to change the energy requirements of the house. The HW tank will only draw 18kW for a relatively short period, probably less than 30 minutes, while the tank is heating from cold.

 

[king of pipes]

I would spec something in between 30 - 40 kw , I have no loyalty to a brand they all have their good and bad points and costings, you need to find a engineer you feel comfortable with can deliver the job you expect and offer you long term back up service . Kop

 

[Rob Foster]

I have a 5 bedroom house over 3 floors, 3 bathrooms an 2 showers, in past 4 kids inc. daughter, wife etc. 30 kw is fine even on coldest night. I recorded minus 14 a few years ago and we were fine except outsidevtap ! The big test for a system is a cold Xmas day when everyone gets up early esp little ones and wander around in pyjamas fir hours. 30 kw has done us fine. Do as KOP says get a good local respected installer and follow his advice I like ATAG boilers but do not have one yet as moving to new one which will have an ATAG Rob Foster aka centralheatking

 

[Welder]

As already suggested, an oversized boiler will both cost you more to install and, ultimately, to run/service.

It is frankly vital the installer you choose does a proper heat calculation and shares its results with you so you understand that they understand!

In terms of water heating, my own boiler is set to heat our water when we are not using it to heat. Until today, another story, I used Tado too.

As already stated, all boilers have their issues. The most important thing for your new system is how its prepped and what additional kit is specified to accompany the boiler to deliver longevity (ergo reliability) such as deaeration, filtration and as importantly appropriate expansion vessel sizing as your system, AFAIK, will need an additional vessel to just that in the boiler.

Some people who install are installers. Some are engineers. Choose an engineer. Ignore price (you know what I mean) as that will see you suffer long term.

HTH

 

[Rob Foster]

As already suggested, an oversized boiler will both cost you more to install and, ultimately, to run/service.

It is frankly vital the installer you choose does a proper heat calculation and shares its results with you so you understand that they understand!

In terms of water heating, my own boiler is set to heat our water when we are not using it to heat. Until today, another story, I used Tado too.

As already stated, all boilers have their issues. The most important thing for your new system is how its prepped and what additional kit is specified to accompany the boiler to deliver longevity (ergo reliability) such as deaeration, filtration and as importantly appropriate expansion vessel sizing as your system, AFAIK, will need an additional vessel to just that in the boiler.

Some people who install are installers. Some are engineers. Choose an engineer. Ignore price (you know what I mean) as that will see you suffer long term.

HTH

One might say installers do regular stuff like many times same system in 100’s of semis,
Engineers will design fit and maintain your bespoke system
York’s Dave talks total sense
Chking

 

[Welder]

One might say installers do regular stuff like many times same system in 100’s of semis,
Engineers will design fit and maintain your bespoke system
York’s Dave talks total sense
Chking

Thank you.
Cheque is in the post Rob

 

[fezster]

However the last plumber has said that I need to be at least 40kw or more as I have an 18kw coil in my HW cylinder and if I don't get this I will get a drop in CH temp and 'cycling' and a loss in efficiency etc.

Only for as long as it takes the cylinder to heat up. I assume it's an unvented cylinder with a coil that size, which usually have a re-heat time of around 30 mins. Most systems are setup as HW priority so that the cylinder gets all (or most) of the heat from the boiler to heat it up as quickly as possible and minimise the time the CH is not being heated. And given unvented cylinders are insulated very well and lose little heat, most people will have it heated up before the CH comes on.

So plumber #3 is advocating a much larger boiler for the odd occasion where you might have HW and CH on together, and even then for just 30 mins. Yet he isn't considering that for 99% of the time, when the HW will not be on, the boiler is cycling and losing efficiency.

As said by others above, size the boiler according to the heat loss of the house.

 

[Rob Foster]

Only for as long as it takes the cylinder to heat up. I assume it's an unvented cylinder with a coil that size, which usually have a re-heat time of around 30 mins. Most systems are setup as HW priority so that the cylinder gets all (or most) of the heat from the boiler to heat it up as quickly as possible and minimise the time the CH is not being heated. And given unvented cylinders are insulated very well and lose little heat, most people will have it heated up before the CH comes on.

So plumber #3 is advocating a much larger boiler for the odd occasion where you might have HW and CH on together, and even then for just 30 mins. Yet he isn't considering that for 99% of the time, when the HW will not be on, the boiler is cycling and losing efficiency.

As said by others above, size the boiler according to the heat loss of the house.

I think you know the answer now. Unless your house is an open sided bbq area 30 kw is plenty . Keep,us informed what you eventually do please Rob Foster aka centralheatking

 

[Julian Such]

Thanks everyone feeling a lot more confident about the descision.

 

[fixitflav]

If you are happy with the performance of the current boiler there is no reason to up-rate the new one by 40%. Renewing the boiler is not going to change the energy requirements of the house.

Exactly. If boiler is increased to 40kW the next thing he'll have to worry about is whether the gas pipe is big enough. Sounds like somebody trying to feather his nest.

 

[Rob Foster]

Thanks everyone feeling a lot more confident about the descision.

Now just go for it ,all decisions must be made on the 80,20 principle. 80% info 20% enthusiasm Rob Foster aka centralheatking