New combi question

[badbob]

We're getting the boiler replaced with a combi, he's had a look around and picked out a Worchester boiler. Are they any good? The house has 9 radiators, bath, gas fed shower (thermostatic) 2 sinks, 2 outdoor taps and a toilet. What size would you recommend? Also he said it'll be on the first floor not the ground floor, is this ok and in fact better due to water no longer being pumped up?

 

[tegsy1]

Your plumber should know the size you require,He can't just look around and decide what boiler he thinks should be installed,Radiator sizes need to be calculated using a mears heat loss calculator,this will give you the correct btus required and also the correct boiler size for your property,Worcester boilers are fine but it depends on choice really and if it is suitable for the purpose required.The central heating is pumped no matter where you site it.

 

[newbie1]

We're getting the boiler replaced with a combi, he's had a look around and picked out a Worchester boiler. Are they any good? The house has 9 radiators, bath, gas fed shower (thermostatic) 2 sinks, 2 outdoor taps and a toilet. What size would you recommend? Also he said it'll be on the first floor not the ground floor, is this ok and in fact better due to water no longer being pumped up?

firstly all the cold outlets/taps and toilets etc will become mains fed so toilets may need new internals as the water pressure can overpower them as prev gravity fed
gas fed shower?????
it doesnt matter where you mount the boiler ground/1st floor whatever as the feed again is off the mains
the heating is pumped on a combi and also not on a combi its only the location of the pump combi its inside on regular system its external
need to measure all rooms to get total btu and then convert to kw for boiler size but as you have 9 rads i would say u have a big house so for ball park 30kw boiler is prob ok but the calculation would take a lot longer
the choice of boiler will be very important as is your incoming water pressure
has he tested your water pressure to see if combi is suitable otherwise you could end up with poor hot water pressure
lastly and most importantly is he a gas safe engineer or someone who is going to do the donkey work and then get a mate to sign it off

 

[badbob]

"Gas fed shower" - trying to explain not electrical system, but from the hot water system.

No he did not measure water pressure, or go into rooms except look at gas meter, old boilder and water tank. Company is Gas Line Staffordshire

 

[newbie1]

well i would suggest getting someone who cares a bit more if the system they are going to charge you for is suitable
if the shower has a sep pump this will probable not be suitable with a combi as the pump can pump the water faster than the boiler can heat it

 

[jpm]

Worcester fine,Vaillant fine.I personally don't spec,or quote for anything else.Gas shower?Bin the pump if you have one fit a mira theromstatic mixer and you're good to go.Worcesters you have bottom middle and top.Juniors si's and cdi's take your pick.30 si greenstar is my most common fir but if you need loads of DHW go for a BIG cdi

 

[badbob]

I assume go into details on the second visit? This visit was just a quick look on house layout between meter, boiler and tank. After all a standard combi fitting and thermastat valves should be set price, and things like tweaking pressure and getting right exact boiler are finer details?

Shower is the old style mixer from bath tap, I'll be getting a thermastatic shower.

I'll put those questions to him

 

[Mike Jackson]

You can't tweak the pressure. You've either got sufficient pressure and flow for a combi or you haven't. Part of the initial call should be to test the flow rate to ensure that it is high enough. As a rough guide I always work on needing twice the specified hot water flow rate i.e. with a boiler needing 9 l/min I like to see 18 l/min at the tap. They will work with lower flow rates but not normally to the customers satisfaction.

The size of a combi boiler is dependent on water flow rates required rather than heating output. Most will have an output to heating of 24Kw and an output to water of between 24Kw and 42Kw.

A 24 kw boiler will give 9 l/min at 35C rise, a 42 Kw boiler will give 15 l/min at 35C rise. For a couple with no kids who only shower 9 l/min will suffice. Once you start talking families or baths you need to think about the larger models. You can get storage combis as well which will give higher flow rates for around ten minutes and then drop off to a lower rate.

 

[Bernie2]

A calculation of DHW needs and room heat loss as well as a few other things, are required before you can work out the size of combi required.

Although being fair, combi's now days mostly seem to come in about two or three sizes and its just a matter of picking one that suites the number of rooms you want to heat and a flow rate to suite the number of draw off points in your house.

They are not like the old boilers whose output was virtually constant, a lot seem to have good modulating ability and so have quite an output range.

So you can kinda get near enough on the sizes required by looking at the top and bottom outputs of the boiler and providing you come under and over these, you should be okay. It doesn't mean that you don't have to work out what size boiler you need, its just that perhaps you don't have to be so accurate as perhaps you once did.

The old way using the Mears wheel calculator for heat loss is okay, but there is a free program called "Stars" from Stelrad one of the rad makers that is quite good.

Just put "Stelrad "Stars" program" in your browser and have a look. Its about 33Mb, I've used it on and off for years. I find though, it can tend to oversize the rads a bit, but its better to oversize than undersize.

I must admit my old Mears wheel is still in the draw, you never know do you?

But saying all that, if the installation conditions are not met for a combi i.e. water pressure okay, then its probably better to look at something else.

 

[badbob]

You can't tweak the pressure

Incoming or outgoing? I thought the combi has a pump on hot & cold water outlets, so will provide stable pressure? We're getting new toilets so should have the newer internal parts.

I don't know the incoming water main pressure, as it seems to vary. Would a pre-boiler pump be required?

As for radiators they're rarely on full blast, or all on at the same time. My brothers got a Wochester, spotted a dial on the right side "PSI" His house is roughly the same as ours, so if we get similar model should be fine? Although his area does have higher water pressure.

 

[newbie1]

the pump is for the central heating/the hot water is goverened by the incoming mains pressure and if you have low incoming pressure then i wouldnt suggest a combi