Combi or unvented - based on my property

[aimno12]

Hello

Have read numerous posts on this subject (especially the famed unvented and accumulator theory/proven system argument ones), and it all still leaves me guessing on my property

Have old system, 50% efficient most likely, time to upgrade. Current system is 20yr old boiler, vented, hot tank and CWT, gravity fed. Shower head is 1 - 1.5 from bottom of CWT, poor poor shower.
Measured flow rate at garden tap of 13L per minute on 3 seperate morning, weekend and weekdays when people would likely be using their wash facilities. (garden tap tees off after the 15mm pipe into the property from stopcock)

Measured flow rate in kitchen of 13.6L per minute on 3 seperate evenings, weekend/weekdays peak periods again.
Waiting on a pressure testing kit to turn up in post, no idea what I have standing/dynamic just yet but I'm keen for that parcel to drop on my doormat.

Received a number of quotes now, 3 to be exact. 1 is to keep my hot tank and CWT, upgrading the boiler to an efficient one and making the system fully pumped. This will do nothing for my shower flow/pressure.
Can get a pump but hot tank is in cupboard of a bedroom so will end up waking someone up too many times for me to consider it worth my sanity dealing with the outcome.

Another quote was for combi, upgrade the mains pipe from stopcock into the rest of my system from 15mm to 22mm.
Pros are lose CWT, get mains pressure, don't need noisy pump
Cons are flow rate through combi I read to be maxed around 14L per min, loss of pressures if other outlets used during shower, possible the boiler might get ragged quicker through general use, if mains flow/pressure is poor then I can't get pump to improve it and will have a weak shower (though better than my 0.1 to 0.2 bar I have now)

Final quoter said to go for unvented cylinder system with decent boiler, again upgrade pipework of 15mm to 22mm.
Pros are lose CWT, lose hot tank from bedroom as cylinder will be located elsewhere in property, no noisy pump, mains pressure, flow rate of whatever the mains is, backup immersion heater.
Cons likely to be cost more than other system, if mains flow/pressure isn't good then can't pump to improve, probably some others I can't pull off the top of my head right now.

House details - 1 bathroom with bath (infrequent use), current gravity fed mixer shower over bath (frequent use), a toilet. Kitchen has a sink and washing machine. There's a cloakroom toilet with sink downstairs rarely used.
3 story house. 3-4 bedrooms. 2 occupants, hope to be a few more in future.

Question is which system do the expert plumbers recommend, I'm asking as I've had 3 expert plumbers come round and all quote 3 seperate systems and pushing them. I can imagine each one feels better installing that system.
Waiting on their prices still, this is massively annoying. However I'd like unbiased opinions so when their prices drop into my inbox I'll be more confident in weeding out my choice.

I know I do not have pressure readings as yet, I read min 1.5 bar and 20L per min for unvented but also have read of people with less than this and report their unvented is rocking their world. Can someone advise for example if I get 1 Bar and say 13.5L then I should go combi and get on with my life, but if I get 1.5 Bar and the 22mm will possibly increase the flow to 20L per min I should chill a few more months and save up for the unvented. Then get on with my life.
Don't want system with CWT and shower pump for sure.

thanks to anyone who had the patience to read the lot of this and carries on to advise on my property

 

[Dan Robinson]

Current system is 20yr old boiler, unvented, hot tank and CWT, gravity fed

You sure?

 

[aimno12]

Current system is 20yr old boiler, unvented, hot tank and CWT, gravity fed

You sure?

Ninja'd

 

[Dan Robinson]

Depending on the budget, I would be looking at a decent combi.

Any "higher performing" options would depend heavily on the logistics of upgrading the water main.

Look at Intergas HRE, Atag A325EC or Vaillant 824 (o 831) Plus. Our favourite 3.

 

[aimno12]

Depending on the budget, I would be looking at a decent combi.

Any "higher performing" options would depend heavily on the logistics of upgrading the water main.

Look at Intergas HRE, Atag A325EC or Vaillant 824 (o 831) Plus. Our favourite 3.

Ok thanks, I'll read up on those tomorrow whilst "working from home"

The combi quoter, I asked him about unvented and he said normally for a house with 2+ bathrooms and combi should do fine in my place so I'll look out for the boiler type when his quote drops. I'll have read up on those you recommend by then.

thanks Dan R

 

[Dan Robinson]

The main thing with Combi's is that you understand their restrictions.

The space savings and unlimited good showers are major benefits for some. the lack of a back is an issue for others.

13l.min is pathetic by the standards in our area, but the cost of upgrading the supply might be a deciding factor.

Personally I would be happy with the Atag and a possible future upgrade of the supply, budget depending; but without seeing the property it is hard to give a fair appraisal - I am inly going by instinct from your post.

 

[aimno12]

Personally I would be happy with the Atag and a possible future upgrade of the supply, budget depending; but without seeing the property it is hard to give a fair appraisal - I am inly going by instinct from your post.

I appreciate your opinion thankyou. I think a combi might do well enough for me, I don't need a shower that knocks me off my feet just one that doesn't mean I have to be hugging a wall to get any water on me.
The Atag combi specs show 15L per min flow, probably be fine for me, I just now decided to measure the flow on my gravity shower and it came in at an amazing 4.4L per minute so in my mind a combi Atag could triple that flow.

Yes, upping the mains pipework is something the future can deal with.

 

[Dan Robinson]

Remember the combi flow/temperature specs are based on a temperature rise. If the flow is slightly lower then the temperature of the water will be higher.

The Atag EC has an energy catcher that makes its hot water performance amazing.

We fitted one in Hitchin recently, and the flue gases were cool to the touch (try holding your hand over your current boiler's flue terminal).

 

[OwainDIYer]

A 20 year old cylinder may not be compliant with Part L and so may have to be replaced along with the boiler.

If the additional occupants are going to be small children they are often bathed until they're old enough to shower.

Just another option to throw into the mix - a combi boiler to provide a mains pressure shower, with a HWC plumbed as an additional radiator circuit to provide stored hot water (whether vented or unvented). My preference would be vented, which can be pumped if necessary.

This also gives you a store of hot water you can use if there is a loss of mains water, gas, electricity, or boiler failure, and you can easily use an electric immersion heater as a backup in case of boiler failure.

Check the temperature rise for the given flow lpm on a combi boiler. You might get 15 lpm during the summer, but in winter when the mains cold is a lot colder, to get it hot enough, you may get a lot less than that.

 

[aimno12]

A 20 year old cylinder may not be compliant with Part L and so may have to be replaced along with the boiler.

Good thoughts, cheers. The HCW the quoters reported to be recent and within regs, good insulation etc. I've tested heating it up, turn off boiler, a day later the water was hot so I think it's good.
Could this cylinder be used as the hot water storage option you mentioned?
Uncertain if it classes as unvented or vented, it lives within a vented system right now.

I think on the temp rise you mention it would be a case of running the shower some minutes before you get in if multiple people needing showers back to back, that right? If so then it costs a little bit more money on the meters, but if you don't make it a habit no problems

 

[flameport]

I think on the temp rise you mention it would be a case of running the shower some minutes before you get in if multiple people needing showers back to back, that right? If so then it costs a little bit more money on the meters, but if you don't make it a habit no problems

No, that is not what it means.

The incoming water in the winter will be cooler, so this means either your shower will be cooler, or the flow rate will have to be significantly reduced to get a decent temperature.

Running the shower before using it will do nothing other than waste water and gas - the temperature of the shower is not affected by how long it has been running.

 

[BigSnoopy01]

one thing you may want to consider is flow restrictors at your fixtures. Would help even out flow rates around the house.

also if you have a combi, make the cold to the boiler the first tee after your mains stop tap, will help keep the hot flowing when somebody opens a cold tap.

 

[aimno12]

The incoming water in the winter will be cooler, so this means either your shower will be cooler, or the flow rate will have to be significantly reduced to get a decent temperature.

Thanks there for the explanation

one thing you may want to consider is flow restrictors at your fixtures. Would help even out flow rates around the house.

also if you have a combi, make the cold to the boiler the first tee after your mains stop tap, will help keep the hot flowing when somebody opens a cold tap.

Appreciate the suggestions, if I go combi route I'll run these by the installer

 

[CBF]

sorry to disagree with some, but @ 13 ltrs/min i wouldn't consider a combi or unvented

 

[aimno12]

sorry to disagree with some, but @ 13 ltrs/min i wouldn't consider a combi or unvented

Thanks CBF, can you give some more explanation as to why if you have time please

I've set aside a few hours later today to read through Dan R's combi recommendations, specifically Atag, as well as the others recommendations on temp rise and flow restrictors.

Quotes came back in and the combi is about £600 cheaper than the unvented option.
If the flowrate was more then I'd see that as something to save a bit more money for and get unvented, as heard is cheaper on gas bill but might also add value to house if I wanted to sell at later date.
But right now the combi seems better option than uv-cylinder and upping the pipes to 22mm from the mains. Upgrading the mains pipe in street to house not an option as of now.
Combi will use more gas meaning higher bills but TRVs should help control that.

Tested the mains pressure also, standing was 3.5bar ground floor and little over 3Bar 1st floor, so guessing 2nd floor would be about 2.5bar as have no fitting up there I can attach to. Dynamic I sat the gauge on 1st floor then ran some taps on the cold feed, also emptied some of CWT so it would refill, during this time reduced pressure from 3bar to 2bar.

Also on combi I could keep existing HWC and use it as a hot store right?