Combi v Conventional Boiler?

alps5000

Has anyone any thoughts or experience on replacing a conventional boiler with a combi, particularly with reference to the radiator system, which would have functioned at atmospheric pressure with a conventional boiler, but will now have to bear 1.2 -1.6 bar depending on model?

Snow_Dog

We replaced a 10 year old (ish) Potterton Myson conventional boiler with a Vokera Linea Combi boiler in our old place about 8 years ago. The property was three bed, 1 bath.

The reasons for having a combi was, leaking cold tank needed replacement, very little hot water pressure so would have needed shower pump if replacing with std boiler, diverter valve and pump were also on their way out.

The Vokera despite a couple of teething troubles over the 8 years was fine though running a completely full bath probably took a bit longer (didnt bother us much as we shower mostly.

Savings were measured at approx 30% of gas usage over the period of 1 year measured, so fairly good savings and so I reckon the boiler probably paid for itself over about 4 to 6 years.

There were no problems with pressurising the rad system that was already there. The pressure is only generally 1 bar with a bit of fluctuation (higher when hot).

Problems with combis:-
1. Not so good un a multiple bathroom house, anything over 1 bath, 1 en-suite certainly wouldn't consider it.
2. sometimes under sized for the application, BTU O/P needs to be higher than conventional boilers.

If you do go for Combi there is an italian make whose name evades me at the moment but they operate in an unusual way compared to most combis (they dont have a diverter and small DHW tank, they heat the DHW directly through the heat-exchanger and these boilers are very prone to scaling up).

Hope that helps and answers your q's.

Steve_PS

As a Corgi CH engineer I would recommend you go down the condensing non combi boiler vented system route. Presurised systems operate at 1-1.5 bar cold, so with thermal expansion your heating system can easliy have to cope with 28+ psi. Extras required with a pressurised system include expansion tank and safety valve. Combi boilers have many extra working parts i.e. diverter valve, plate heat exchanger. You CANNOT beat a full tank of hot water.

harryhound

I too will be forced to make a decision on boiler replacement soon, my old sheet steel job is 30 years and is overdue to fail.

Back in the 1960's, I worked in the Trianco service department. We could achieve 80% efficiency burning gravity fed anthracite (at 15 GBP per ton) even then. Trouble was it required the customers to brush their flue ways weekly and caused external chimneys to fall over as the condensate attacked the mortar.
Customer's could not be bothered with the work, especially when we suggested they kept the thing alight in the summer for hot water. They also thought a stainless steel chimney liner a shocking waste of money. So we redesigned the product down to 75% efficiency and it probably still has a 25 year life.

Judging by this link the production may have moved from cannon ball range of Hampton Court Palace to Sheffield (or China ?) but its the same product I helped to prototype.

http://www.trianco.co.uk/trg.cfm

Then along came natural gas and everyone in UK has been living in a fools paradise squandering the stuff ever since.

During the 40 years of self sufficient squander, I was responsible for two boilers. The family home's 80K Btu Potterton and a late 1980's Volkera combi in a buy to let student "3 up 2 down plus rear kitchen & bathroom extension", Victorian terrace.

The Potterton was 2 boxes of cast iron with knobbly bits on the inside and two burners, similar to those down the sides of a kitchen oven. It ran from the mid 70's until April 2003 during which time it was NEVER SERVICED. I would squint at the flame from time to time, it was a nice blue colour. Eventually I discovered that the drain c o c k (sorry net nanny) on the front had been weeping onto the burners and rust had rotted a hole in them. (Some local installer gave me a load of guff about modern regulations and manometer readings on the chimney and an estimate of 2.5K, so I thought I'd double check with British Gas - best part of 4K for a combi and a load of ugly pipes to get to an outside wall. I managed to find a "little man" who rebuilt the rusty pipes leading to the burners for 400 GBP. That saw my mum out and the house was sold in 2005).

The 10 year old Volkera gave half a dozen more reliable years of service.
Getting the annual certificate was like obtaining an MOT on a car of similar age: a pain in the backside and I think I may have been a victim of the swap the pump scam. The tenants could not get the hang of keeping the pressure out of the red zone, so I had to remember to check on every visit !
The house was sold, complete with a newspaper cutting extolling the reliability of the Volkera boiler make a couple of years or so ago.
(I don't know how a combi works, if someone can post a link to (say) the Wikipidia idiots guide to understanding a combi, I would be most greatful).

So what have I learned from a life's experience:

A. Ideally you should be making a 30 year investment BUT will these fancy control systems and micro heat exchangers last that long ? (Cynics say condensing boilers will burn out their choked heat exchangers within 10 years and if they don't you won't be able to find the spare parts and the engineers who know how to fix them (ie like suggesting you want someone to fix your windows '95 PC). They will have an annual test of emmissions like cars ?

B. Wages and overheads will cost a fortune out of your taxed income, so the annual carbon inspection will be something to save up for?

C. Fuel will be taxed according to its carbon footprint ?

D. As from June every property changing hands will be given a 1 - 10 energy rating - can we expect a tax based on this rating and a square merters per occupant measure ?

E. One thing is obvious: stuff your home with as much insulation as possible, it makes much more sense to spend an extra 400 on better insulation than 1000 on a more fancy boiler. [I renovated my own place 35 years ago and put 150mm (6") in the loft of a bungalow and everyone thought I was off my rocker]. I'd recommend an upgade to 300mm. "Which?" seems to like this stuff in its recent report, its paper mache:

http://www.longandsomerville.co.uk/custom/shop/product?productID=0322792

F. A reliable long life boiler will be worth it weight in saved maintenace costs.

G. I don't think they will be able to think of a way to tax a wood burner in the living room, could be useful when the fancy gas boiler breaks down on Xmas eve (I'm not joking, there is a whole estate near me where the centrally provided heating went off at lunch time on Xmas eve and the tenants phoned "a well known multinational company" to be told to ring back in the new year.)
You could also cut out the middle man and annoy your neighbours by burning your own rubbish, avoiding the land fill tax and pay as you go local authority bin collection.

H. I am not yet sure about the value of green measures and I don't have a roof slope pointing south, but I have replace most of the light bulbs with curly wurly low energy ones.

I. Whatever you do, flush the system until it sparkles. I'm not sure about the value of additives. In the 60's we would chuck in some Duckhams soluable oil to mke it run quieter.

If my research turns up anything useful, I'll come back back and post the results here.

Cheers

Harry.

PS
The only real solution is to build a new zero carbon home with a U value of 0.1 W/m2K (watts per sq meter per degree in English) in the roof walls and floor, but we don't all want to live in a southfacing greenhouse built into a mound of earth !

dc

Well said

Ken68

Good one Harry.."flush it till it sparkles".

TOBRUK

alps5000 wrote: »

Has anyone any thoughts or experience on replacing a conventional boiler with a combi, particularly with reference to the radiator system, which would have functioned at atmospheric pressure with a conventional boiler, but will now have to bear 1.2 -1.6 bar depending on model?

Firstly I have to say that I do not have the knowledge of people here about boilers and central heating etc so all I can contribute here is my experience with a conventional and combi boiler. Two years ago my conventional boiler packed in and this was replaced by a Vaillant turboMAX combi. One thing I must point out is that I did not have a choice of the sort of boiler I was having as it came under EAGA group - I was having a grant.

I have been paying £20 a month gas and last month my statement shows that I am over £100 in credit!

The tank in the attic was taken away as was the tank in the airing cupboard. The water pressure is fine and the heating is much better, there is no turning on the hot water anymore which is great - it is instant. The shower I have is not a power shower but the pressure is just as good as it was before. I have been extremely happy with it, hope this helps.