Installer insists on Combi with unvented cylinder

[ant6009]

Hi all, first post here and I hope someone can advise me.

My installer insists on fitting a combi boiler with an unvented HW cylinder instead of a system boiler. Hi argument is that they are the same but the combi boiler is cheaper.

Looking at Worcester 4000, the combi 30 and system 24 are roughly the same price, so I am not convinced.

I am puzzled and I hope someone can help. Is there any scenario where the combi, even though I will never use the instant hot water function, is a better choice in my case?

For context: I am replacing an oil, heat-only boiler and vented HW cylinder with a gas, unvented system. The boiler and the cylinder location is changing.
we have 2 bathrooms, 17 radiators, and solar pv (hence I want a HW cylinder to heat water with solar pv excess), 4 bed 2 reception with and large extension with office and workshop).

 

[bernardgreen]

Find another installer. You are the customer so you decide the type of boiler you have in your property,

Hi argument is that they are the same but the combi boiler is cheaper.

The combi boiler he wants to install might be cheaper but the plumbing work required to convert the system to work with a combi will not be cheap.

 

[oldbutnotdead]

First thing, before you commit to unvented cylinder has anyone checked your cold water dynamic pressure and flow rates- anything less than 20 litres per minute at 2 bar and you won't really feel the benefit of the unvented. (Same argument goes for the combi but the workable volume/pressure ranges are lower).
Second one- if you are in a large house and the boiler is near the kitchen you might find that the cost of a quick combi burn to fill the sink might be balanced by not wasting a load of (cheaper) stored hot pulling through a long run.
Likewise, you might think about valve gear and pipework so you can switch one of the bathrooms between combi and cylinder- come winter when pv is thin on the ground again you can choose.
The combi is probably cheaper because more of them are made than system/heat only types. Generally there's long warranties so the added complexity of the combi isn't a huge worry.
One trap to be wary of (check the boiler specs)- very likely that a combi won't have any facility for hot water priority or differnt running temperature for hot water demand (ideally you want the boiler return temp quite low for max efficiency but you also want to heat your cylinder to 65 odd for legionella prevention as well as reducing the 'run out of hot water' instances.

 

[flameport]

Hi argument is that they are the same but the combi boiler is cheaper.

More like he already has a combi boiler which someone else didn't want, and now wants to offload it.

There are a few circumstances where a combi and cylinder could make sense, but 'cheaper' isn't a valid reason at all.

Did they do a heat loss calculation for the building before specifying the boiler?

 

[ant6009]

Thank you all

@flameport: there was no heat loss calculation conducted that I know of unless he did it off the plans and the walkaround we had with the building contractor. I will ask.

@oldbutbotdead: having different running temperatures was something that, after thinking about it, didn't add up in my head. So thank you for raising this, plus the legionella factor. I will read the literature on the boilers concerned.

@bernardgreen: Good point.

I was wondering, but haven't been able to confirm: is it possible that this has to do with the G3 ticket? would installing a combi be a way to get around it? I don't think anyone would stoop that low, and as far as I understand G3 has more to do with the unvented cylinder. Just trying to rationalise why he is so insistent.

 

[oldbutnotdead]

The G3 is for the unvented,nowt to do with the combi.
If he's being really pushy then its good odds that @flameport is correct- he has been stuck with one and wants rid. Or he needs another combi install to make fitter of the year bonus?
Unless you're married to the bloke (ie used him before and he's been excellent or friends have used him & he's been excellent ) mebbe start casting your net a bit wider....

 

[oilhead]

For the size of the property and assuming the hot water outlets are spread wide, then it may be sensible to go with your installer. controlling the heating function to supply both heating and hot water is no different to controlling them with a system boiler, but as already said above, depending on the location of the boiler and cylinder, you may well save a significant amount of 'lost' hot water on long legs and the fitting of an expensive DHW circulation pump.

 

[Madrab]

Unless the plan is to connect the combi's HW outlet up to say a close by sink etc then there's absolutely no reason to have a boiler where half of the parts inside will be redundant and are just possible failure points that won't ever be needed.

 

[OvertonPlumber]

A system boiler is just a combi boiler with the plate and divertor blanked off.
We used to fit combis on swimming pool setups with the hot water capped off. As yes they are cheaper to buy.

 

[JohnD]

He has to install one more combi before the end of the month to win a week's holiday in Maui.

 

[fixitflav]

Hi all, first post here and I hope someone can advise me.

My installer insists on fitting a combi boiler with an unvented HW cylinder instead of a system boiler. Hi argument is that they are the same but the combi boiler is cheaper.

Looking at Worcester 4000, the combi 30 and system 24 are roughly the same price, so I am not convinced.

I am puzzled and I hope someone can help. Is there any scenario where the combi, even though I will never use the instant hot water function, is a better choice in my case?

For context: I am replacing an oil, heat-only boiler and vented HW cylinder with a gas, unvented system. The boiler and the cylinder location is changing.
we have 2 bathrooms, 17 radiators, and solar pv (hence I want a HW cylinder to heat water with solar pv excess), 4 bed 2 reception with and large extension with office and workshop).

Why not just replace the boiler with another heat-only, and keep open vented primary circuit? Starting from scratch, open vented vs pressurised could be 50:50, depending on house layout, but unlikely to be worth converting from existing open vented. Having boiler, pump etc separate is an advantage in diagnosis and repair in the event of a problem.

You can still have a mains-pressure HW cylinder (if mains flow is adequate). The boiler (primary) circuit and HW (secondary) circuits can be open vented, or pressurised, in any combination.

 

[Eccles]

There are some real thickies out there I'm afraid who don't always know what they should. I had a so called expert insist on installing a vented HW tank in my loft a few years ago against my better judgement so I gave up and let him try. After a few days the tank arrived and he proceeded to try and get it up through the loft hatch. Yes you guessed it it was too tall to make it without cutting a rafter which I was not going to agree to. Eventually I went off and bought my own bespoke tank to fit in the airing cupboard.

 

[Agile]

If I fit a combi and cylinder then I like to supply the kitchen hot water from the combi.

 

[gas112]

A system boiler is just a combi boiler with the plate and divertor blanked off.
We used to fit combis on swimming pool setups with the hot water capped off. As yes they are cheaper to buy.

As per overton says a combi can be several hundred cheaper than a system boiler the guy is giving you good financial advice