Factors affecting combi boiler kW-rating choice

[mikej2005]

Hi guys.
I'm intending to replace my ageing, inefficient 'Glow-worm 67/75' (67 or 75k BTU/h) conventional boiler and vented cylinder set-up with a modern, condensing combi, like the 'Worcester Greenstar 30CDi' or '37CDi'.

A quick, rough online calculation for the total radiator capacity (15 rads) required to heat my 4-bed detached house to a comfortable level is around 14.7kW (~50k BTU/h).

The 30CDi is rated at between 8-30kW and the 37CDi between 9.8-30kW for central heating output.

On paper, both these boilers should be more than powerful enough to heat the house so my questions are, if I was to choose between these two combis :

1) Each combi is powerful enough to heat all the radiators effectively, so should the model rating decision be based on the DHW flow rate requirements ?

2) For central heating alone, would the 37kW boiler cost more to run than the 30kW model or should the cost be similar, based on the fact the energy requirements would be identical ?

3) If the answer to (1) is 'yes' and (2) is 'similar', should I, in fact, go for the 42kW model, to get the fastest DHW flow rate possible ? (minimum domestic inlet pressure permitting) Would a 42kW model help with the 'turning two hot taps on at once affects the flow' problem associated with combis, or will nothing get you away from that limitation ?

I intend to replace one of the two electric showers in the house with a thermostatic shower, so a better flow rate would be desirable. This en-suite is also at the opposite end of the house, so anything I can do to reduce the delay caused by the long pipe run would be good too, presuming that higher DHW flow rate = water gets hot quicker ?!

Any thoughts or comments would be much appreciated - cheers.

I've got many more things to sort out, but I'll save those for another time - I'll no doubt be scanning the forum for help and advice in the coming weeks !

 

[Agile]

A combi boiler is rated on the DHW output.

A combi boiler is limited by its output or the mains water flow rate whichever is lower!

A 42 kW would be quite good and almost run two taps together! You will need at least 20 li/min mains flow.

You will need a very large gas pipe from meter to boiler.

A more powerful combi will be slightly less efficient for CH because its minimum power will be above the power needed to run your CH and the boiler will cycle on/off .

You need to seriously need to consider the requirements for HW based on the number of PEOPLE !

Keeping electric showers is a good idea if you have a lot of people.

Why dont you take the advice of a competent CORGI engineer? Or do you think that they are all "ignorant plumbers" ?

Tony

 

[mikej2005]

A 42 kW would be quite good and almost run two taps together! You will need at least 20 li/min mains flow.

Thanks - I'll check on that. Boiler specs mention minimum inlet pressures, not flow rate as such (1.4, 1.6 and 1.9bar respectively for those 3 models)

You will need a very large gas pipe from meter to boiler.

A 22mm connection from the meter to the boiler won't be a problem.

You need to seriously need to consider the requirements for HW based on the number of PEOPLE !
Keeping electric showers is a good idea if you have a lot of people.

Yep - that debate has kept us going for a while ! Bath hardly ever used and a cylinder full of hot water is mostly wasted every day. Electric shower will be retained for guests in main bathroom, to avoid problems trying to run two showers off one combi.

Why dont you take the advice of a competent CORGI engineer? Or do you think that they are all "ignorant plumbers" ?

Woah there - no need for that at all. I'm posting here to get as much background information as I can and you've very kindly given me some help with a lot of my questions, for which I'm very grateful. Isn't that what this forum is all about ? There's no need to be so presumptuous.

 

[Agile]

You will need a very large gas pipe from meter to boiler.

A 22mm connection from the meter to the boiler won't be a problem.

A 22 mm gas pipe will run a 42 kW boiler as long as its no more than about 3m from the meter!

A 35 mm gas pipe or larger is "large" !

You need to seriously need to consider the requirements for HW based on the number of PEOPLE !
Keeping electric showers is a good idea if you have a lot of people.

It sounds to me as if all you need is a 28kW or 30 kW combi.

Why dont you take the advice of a competent CORGI engineer? Or do you think that they are all "ignorant plumbers" ?

Woah there - no need for that at all. I'm posting here to get as much background information as I can and you've very kindly given me some help with a lot of my questions, for which I'm very grateful. There's no need to be so presumptuous and so defensive.

I am not being defensive but a competent engineer could much better advise you on site where he can see exactly whats required and he will understand the relationship between dynamic pressure and flow rate. I hope you will be using a CORGI installer so why not appoint him now and take advantage of his experience.

Tony

 

[mikej2005]

A 22 mm gas pipe will run a 42 kW boiler as long as its no more than about 3m from the meter!

Yep - about 3m and one or two elbows will do it. Garage with meter is adjacent.

I am not being defensive but a competent engineer could much better advise you on site where he can see exactly whats required and he will understand the relationship between dynamic pressure and flow rate. I hope you will be using a CORGI installer so why not appoint him now and take advantage of his experience.

Yep - I'll be getting it properly and legitimately installed. I was anticipating, however, that I might have trouble getting a Corgi engineer onboard from day 1 if I'll be doing all the non-gas pipework and radiators myself, as it won't be a huge job for them. Either way, it's definitely helpful to get some advice on here beforehand.

 

[gas4you]

If you fit a 42Kw I would go for 28mm minimum from meter to where it joins the boiler gas tail. Are any other gas appliances coming off this short length of gas pipe, if so their gas rate needs to be taken into consideration.

The 37Kw Vaillants I fit use 4m3 gas /hr so I would expect the 42CDi to use more than this, although I don't know this boiler.

You also need to bear in mind your domestic gas meter will only give 6m3/hr maximum, so watch out if you do have several other gas appliances

Unless you are going to run nothing but baths all the time I would stick to the 37Kw at max. Can you range rate the worcesters as I can my Vaillants for the ch? This would make it slightly more efficient to use.

 

[Agile]

Be aware that every elbow is equivalent to 0.5 m length of pipe!

As Dave says, use a minimum of 28 mm pipe size.

To Dave and others, someone I know upgraded a 24 kW to 32 kW and founf that the incoming gas supply to the house was NOT capable of supplying the 6 m² that the meter is rated for. The pressure started dropping off at little more than 24 kW.

He was placed in a difficult position as the customer expected the boiler to be working properly and the gas supplier wanted about £1000 to upgrade the supply pipe to the meter! I have not yet heard of the final outcome.

Have any installers come across a "shuddering" effect when a premix boiler on a very short pipe is "pulling" ont he meter governer?

Tony

 

[gas4you]

I would have thought that it was NG resonsibilty to correct foc. How could this have been passed and connected originally

I've had 2 or 3 customers now that have had to have new services put in due to failing, or not up to scratch existing services. They have all been done foc. Were they the lucky ones

 

[Agile]

I think they must have been lucky!

I had considerable discussion with people in Transco and they were adament that it would be chargeable!

There is no regulation that says the supply should meet the meter rating!

I also asked how one could find out the supply capability for any given property and was not given a straight answer.

Nor is there any official test we are permitted to do to find out for ourselves! I do know how I could do it though.

Based on my friends experience it would be quite worrying if fitting a much larger boiler. His customer was in Berkshire and I suspect in a country area where long pipe runs were involved.

Tony

 

[gas4you]

But how can Transco knowthat it is not a blockage or fault in their supply up to the meter. Their responsibility only ends at the ECV If the pressure drops so much when in use then it could constitute a danger as is low working pressure at the govenor and should then be called in and addressed as a fault. They have a responsibility to supply a working pressure of at least 19 mbar at all times surely, I mean Tony, surely

 

[Agile]

They have sent people out to it at least three times!

Until I encountered this problem I would have expected the supply to be available up to the meter capability.

I now know otherwise! Dont assume anything!

I went to one near Warwick that would only supply about 14 kW but that house has a long front garden. Transco offered to upgrade the pipe but the owner decided he did not want his garden and driveway disturbed. It was not clear who was going to pay in that case.

Tony

 

[mikej2005]

Thanks for alerting me to the potential gas supply issue, guys.

Sounds like the 30 or 37kW model might be sufficient for our usage, so I'll avoid the 42kW model and the problems it might bring (unless expert, on-site advice advises differently)

There are currenly no other gas appliances off the supply, although I will be adding a gas hob at some stage in the future.

Back OT - does anyone else have experiences of combi ratings based around our usage/situation ?

For info : 4-bed house with 2 adults and 1 toddler, two bathrooms - 1 with an intended thermostatic shower and the other with a bath (hardly used) and electric shower over, downstairs toilet, kitchen and utility room sinks.

 

[simond]

You are asking an impossible question for us to answer. Some people use HW more than others and for longer.

If you buy a flow cup you will be able to simulate exactly how much water will come out of your repective boiler choices. Bear in mind you have to use 42C temp rise not 35 that is measured in the specs.

The 'gas main problem' is not normally a issue. Gas is delivered at a much higher pressure on the supply side of the meter and a 42Kw load is not a big deal. Every battle we have taken on with the gas supplier to date has been resolved with them uprating the supply FOC

 

[gas4you]

If you intend to shower mostly then a 30/31kw would be adequate. if you intend (I know you say you dont) to run a lot of baths then the 37Kw would be better, Either of them will only efficiently supply one hot outlet at a time.

If I visited you to survey and quote my initial recommendation would be to NOT fit a combi on a 2 bathroom house.

 

[mikej2005]

Thanks for the comments - much appreciated.