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Hi

 

Can someone please provide some advice?

 

We currently have a boiler with a cold water tank in a vented system with no hot water pressure upstairs, and due to the way the existing system was installed, pumps don't help. 

 

As part of house works we're installing under floor heating, and the plumber suggested a mega flow with a system boiler and he has said I don't need to change my boiler, just the cylinder. He will also make the system unvented. 

 

Is it more sensible for a system boiler over a combi? I have been offered a nearly new combi boiler by family for free. My house is a 4 bed with 2 bathrooms (1 is rarely used, which will probably change in the future?

 

Also, would a conventional boiler do the job of a system boiler?

 

Thanks

 

 

 

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If your cold water pressure and flow is OK, the free combi would be tempting and as long as it doesn't cost too much to fit.  You could always change later.  Unvented cylinder and system boiler would be more standard for your house.

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There's two ways to look at this.

 

You take the combi and use as a combi, just make sure its not massively oversized for your needs, over wise it will short cycle on heating duty.

 

The way is make sure it's not undersized and the DHW is then rubbish.

 

But as you are used to using a cylinder and already have space, upgrade to unvented cylinder.

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On 09/04/2022 at 15:00, Taztaz said:

My house is a 4 bed with 2 bathrooms (1 is rarely used, which will probably change in the future?

If you are a growing household then you may eventually struggle with a combi, but there's me ( showers ), the missus ( 1800x800 bath to fill ), and 4 kids ( mix of 2 showers + 1-2 baths ) at ours and I have a combi and am happy with a few cons for the pro of not having to have a cylinder.

 

We're happy to observe a "nowt else running whilst someone's in the shower" policy, and that works just fine for us tbh. I will change that this year, hopefully, by adding a type of thermal store to the cold feed that supplies the combi, eg so it receives a stable input of pre-heated 'cold' water, which will remove my one major niggle with the combi. That is the fluctuation in hot water delivery ( associated with any type of instantaneous water heater, not just a combi ) when the hot or cold does get used elsewhere during a shower. That will be further offset by the installation of a cold mains accumulator ( just a 100L one to cover the <30L of draw-off from the mains from other use eg WC#2 being flushed + hand-wash or the washing machine / dishwasher filling etc ) which will be more than suffice for us then in terms of demand, reliability and stability.

 

If you need a hot return circuit to get instant hot water to outlets reasonably far away from the hot water source, then a cylinder would be preferable as you cannot ( sensibly ) achieve a HRC with a combi.

 

If you have or are having solar PV, then a cylinder is a no-brainer to get as much FOC DHW as you can each summer by feeding excess generation into an immersion. 

 

What boiler do you currently have, that your plumber has said can be converted to 'sealed and pressurised'? Is it a modern, condensing boiler? There is, essentially, no need to convert the boiler, unless you want to get rid of both the F&E and the CWS ( small and large ) plastic storage tanks in the attic(?) You do 'need' to change the cylinder to an UVC to get the DHW coming out at cold mains potential, which would then remove the CWS tank to boot. If you do ever want to convert the attic, now would be a good time to get rid of both tanks.

 

How much area of UFH will you install, and will it need to run independently? Will you still have rads upstairs? Gas boilers do not like to run UFH directly, as when the floors are warmed up, the heat load is then quite small to keep them there, so the boiler can short cycle. Type that into the search bar here for LOTS of discussion on that ;) 

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To expand on what Nick has said.

 

Our system is pretty much as described above, minus the accumulator.  Which I have thought about also.

 

Our combi heating circuit is connected to the heating coil in the thermal store only. The cylinder is heated to 34 degs with a 4 deg hysteresis. Boiler heating temp is set to 35 degs with a max temp of 55 degs.  On our boiler we are able to set the gradient of the heat input, so have set as low as possible.  The boiler starts when the cylinder is at 30 degs, and takes around 20 mins to heat the cylinder to 34 degs, very slowly ramping up the temperature output of the boiler.  Boiler stays in full condensing mode at all times.

 

UFH pipes are connected to thermal store.  UFH manifold temp set at 30 deg.

 

We have an immersion in the cylinder also connected to solar PV and will be adding solar thermal in the summer, to displace some gas usage.

 

DHW goes through the DHW coil at the top of the thermal store prior to going in to the boiler.

IMG_20211027_132126.jpg

Edited by JohnMo
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Thanks for the thorough answers guys. We have a conventional boiler at the moment fed from water tanks, I think it's a oil Worcester boiler under a different name. The plumber didn't say anything about converting it, just said he'd install a unvented cylinder. Our underfloor will be about 30m2, all the rest of the house rads. The UFH room also joins another room with no divide and that has rads. When you say directly, do you mean as in what a combi would be doing? I guess it would run independently.

 

The HRC sounds nice but I don't know if I need it, the main bathroom is probably about 20m away from the cylinder. 

 

I've read up on short cycling and dont see how that would be avoidable without a thermal store. Also seems on the expensive side to run

 

Im definitely not brave enough for 4 kids either. 

 

12 hours ago, JohnMo said:

To expand on what Nick has said.

 

Our system is pretty much as described above, minus the accumulator.  Which I have thought about also.

 

Our combi heating circuit is connected to the heating coil in the thermal store only. The cylinder is heated to 34 degs with a 4 deg hysteresis. Boiler heating temp is set to 35 degs with a max temp of 55 degs.  On our boiler we are able to set the gradient of the heat input, so have set as low as possible.  The boiler starts when the cylinder is at 30 degs, and takes around 20 mins to heat the cylinder to 34 degs, very slowly ramping up the temperature output of the boiler.  Boiler stays in full condensing mode at all times.

 

UFH pipes are connected to thermal store.  UFH manifold temp set at 30 deg.

 

We have an immersion in the cylinder also connected to solar PV and will be adding solar thermal in the summer, to displace some gas usage.

 

DHW goes through the DHW coil at the top of the thermal store prior to going in to the boiler.

IMG_20211027_132126.jpg

I'm a real novice, do you mean you have a thermal store and a cylinder, or are they the same? What's the white box in your picture? I have space for a cylinder (or similar) and the boiler in my outside store) cupboard. 

 

thanks again

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The green/blue cylinder is actually a thermal store, which I referred to as a cylinder.  Sorry this may have confused.

 

The white box on the left is the combi boiler.  The one on the right is part of the ventilation.

 

 

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