Fit two combi boilers or go unvented?

[Illuma]

Hi all, I am not a heating engineer but I am an engineer and I have done a fair bit on plumbing but much has changed and I am hoping some useful advice and opinions can be given please.

I am currently in the process of buying a 5/6 bedroom house it was built around 1920 possibly earlier when we get it I will be making sure it is as fully insulated as possible. The property currently has two combi boilers, one to feed upstairs and one to feed downstairs, I believe the previous owner wanted to put a granny flat in and also extend into the loft space and that is all I really know about the system. The previous owner ended up loosing the property to the bank and kindly decided to remove a few essentials one of which was all the radiators (boilers are still there) there is damage to some of the pipework but to what extent I do not know as of yet.

What I would like to do is get some sensible opinions on where to go from here, we have no plans for adding to or extending the property and we are looking to live in it for quite a while, I need the system to cope with two adults and two young children, baths more than showers and it has one main bathroom (big bath), one en-suite (shower), and a downstairs WC / Shower (not bothered about keeping the shower). Is it wise to keep the two combi system (replaced with new condensing units) or install a single unit and possibly go with an unvented cylinder? If the cylinder is a good option is it worth getting one with two coils to allow for the possibility of solar heating? I do know the main water feed to the house had been changed to what was described as "a new large one" in preparation for the previous owners plans to extend so I assume we will have a good pressure / flow rate.

Also I am swaying towards Viessmann or Valliant systems any good / bad things to know about these or any suggestions on other models?
Energy saving is quite high on my list so if any methods of controlling zones other than TRV's (which will be fitted) will also be welcomed.

I am happy to replace any piping if required it is currently in 15mm copper to the rads, is this still a good medium for things like heat loss? or is new plastic / pushfit better?

Thanks in advance

 

[Agile]

You are asking a lot of very basic questions which you should read up on.

Little point in changing boilers if they are copper heat exchangers as the saving in efficiency is small.

Do put well sized rads in though as that will aid efficient condensing boiler operation.

Often if insulation is greatly improved then existing rad sizes using the best high output types will give enough output but the number and location needs to be checked by an expert as some installations were rather lacking.

All these things could be advised on by an expert at your property within an hour!

Tony

 

[BertCoombes]

The cheapest is to keep the two combis. Split the load of the DHW. Have one combi supply DHW to the en-suite shower room and the other the bathroom. Then one does not interfere with the other in pressure/flow. Two showers can be run at once with no problems. You could combine the two DHW outlets using check valves to get both combis supply the bath fast, but you have to know the boilers and what you are doing.

Ensure the cold mains supply to both is separate, not one 15mm pipe snaking around the house and all the cold taps and combis teed off it. Have the cold mains water pipe to each combi run back to the stop tap (two pipes) and replace the stop tap with a full bore tap to keep flow up.

Have one combi heat the upstairs rads and one do the ground floor rads. Have a clock on each so they operate independently. This will save gas as the upstairs can be off while the downstairs is on.

You have the added advantage that you have heat backup if one combi fails. There will be heat and DHW somewhere in the house until the combi is back up.

Plastic or coper pipe is suitable. Ensure most of the DHW pipes are lagged. This saves quite a bit of energy and gives faster DHW at the taps.

 

[Illuma]

Thanks for the info, I think I will keep the two combi system and do as Bert says and combine the two for the bath. I am doing a mini survey on the house in the next couple of days so I will get a good chance to find out what the boilers are and see if there will be any leads to getting the system working again! I am hoping I may find some info on the company that did the install, if not I will figure it out myself.

 

[picasso]

I would go for the unvented cylinder, it sounds like you have the pressure and flow on the mains so no problem there, combis struggle in the winter as the incoming water is cold unless you put in a big kw boiler which means upgrading the gas pipework, as you have said the cylinder can be solar ready, I cant see any benefits of having two combis over an unvented cylinder other than you have heating in one part of the house if one of the combis breaks down,

dont forget you will have an immersion heater as back up so will always have hot water.

 

[BertCoombes]

I would go for the unvented cylinder, it sounds like you have the pressure and flow on the mains so no problem there, combis struggle in the winter as the incoming water is cold unless you put in a big kw boiler which means upgrading the gas pipework, as you have said the cylinder can be solar ready, I cant see any benefits of having two combis over an unvented cylinder other than you have heating in one part of the house if one of the combis breaks down,

dont forget you will have an immersion heater as back up so will always have hot water.

He has two combis so will always have hot water if one combi is down. Spending on an unvented cylinder when already has two combis in place is economic madness. It will give him absolutely nothing extra.

Modern combis do not struggle in winter. One will do one shower fine in winter. He is combining the two outlets to fill the bath quickly. Two combi will fill a bath very fast.

He has automatic CH zoning and can save a hell of a lot in gas bill by having upstairs off.

To put in an unvented cylinder and zone the CH will cost a fortune - for no gain.

 

[BertCoombes]

Thanks for the info, I think I will keep the two combi system and do as Bert says and combine the two for the bath. I am doing a mini survey on the house in the next couple of days so I will get a good chance to find out what the boilers are and see if there will be any leads to getting the system working again! I am hoping I may find some info on the company that did the install, if not I will figure it out myself.

What make and models are the combis? Combine the DHW outlets for the bath only. Use a check valve on each DHW feed just before the tee to the bath tap. Also have a small shock arrestor between the tap and two check valves, if the taps are lever taps. They cost around £10 and the size of tennis ball.

The cheapest way is keeping the two combis and doing it right, as I have suggested, will be cheap to run and deliver the DHW. Running two showers at once using a cylinder means you may need a large expensive cylinder. The combis will never run out of hot water. No waiting or running out of hot water.

 

[Tibbot]

Thanks for the info, I think I will keep the two combi system and do as Bert says and combine the two for the bath.

Oh dear, Dr Drivel has created another set of ID's to post between himself.

Next time why not go by your real name, 'Norman Bates'.

 

[picasso]

FFS drdrivel/watersystems rides again this thread is doomed to sink into the usual dross now that bert is involved.

 

[picasso]

Oh dear, Dr Drivel has created another set of ID's to post between himself.

.

Never thought of that

 

[DeltaT2]

But Dr Drivel aka Big Burner, Water Systems....etc, etc, if you/he has two combis that's two boiler services per year. Now, your normal mantra is fit a 'sludge bucket', not an unvented cylinder - due to the annual service costs etc. What has happened has Gledhill given you your P45, again???..................

 

[ChrisR]

I'd keep the two combis - depending a bit how old they are etc.
If you plan a major refurb any time then look again.
If the "bath" one is 28kW or more I'd keep the shower/bath combis separate or the shower will suffer if someone turns the bath on. Add sharing valves for emergencies.

It can be difficult or mean a major replumb to arrange the CW supplies to them to be optimal, and if you have a high flow shower it's still likely to suffer if someone turns the "wrong" tap on somewhere else.

But as long as it's done right, then any old plumber will be able to maintain it - not the case for some of Bert(etc)'s more esoteric sclutions

 

[Dan Robinson]

Loathe that I am to type this... I agree two combi's would be better in this instance.

I am trying to see it as agreeing with Chris rather the Dr Drivel though.

 

[doitall]

Assuming the gas meter is big enough, and the mains pressure/flow will supply, it does seem rather silly to change it.

Perhaps DrDrivel could connect the two together and have one BigBurner for the WaterSystem.

Taxi.

 

[Illuma]

Hi, sorry for not replying sooner for some reason my update notification is a bit hit and miss! I got into the house today and I know allot more on the system. All the radiators have been taken and it also most of the copper piping has gone even from under the floors, but they were nice enough to screw the floorboards back down! Both boilers are the same and they are Glow worm xtrafast 120 E1 manufacture date 2003, they appear to have survived as I see no signs of damage. The first boiler is in a utility room near the kitchen (approx 5mtrs away from sink, 10mtrs away from shower) the other boiler is in the main bathroom which is located above the kitchen (approx 3mtrs away from bath and 12mtrs from en-suit). Given this extra info what do you think about this type of boiler? I have googled and there is not allot of info on them so I can only assume they are either quite good or really bad and only manufactured for five mins! It looks to me they can modulate from about 12 - 35Kw according to the manual. I have added some photos for reference. View media item 47234 View media item 47235 View media item 47236 View media item 47237