Advice please! Combi v 'thermal store'

Uzima

I'm on the horns of a dilemma. We're upgrading an old pumped CH gravity fed HW system, but what to? My wife wants a combi so she can grab more bathroom space by taking out the cylinder. Her friends have combis. I'm deeply suspicious because I have read so much about unreliability on this site - we have only needed two boilers in 30 years. And would the hot water be enough?

We have an 8 year old non-condensing boiler which works fine, serving a largish 3/4 bed semi with a bathroom, a shower-room and a cloak room - plus rads. Shower in shower-room and shower over bath are currently electric. It's time to make changes because we HAVE to move tanks out of the attic.

1. Combi. My wife has found three or four small combis which could directly replace our wall-hung boiler (just 320mm depth in cupboard). One is an Alpha with 37 or 38Kw output. Is this likely to last ten years? How much will we pay to service it (current boiler has had one service in 8 years)? Should we expect leaks in the CH pipe-work hidden under the floors? We barely use hot water all summer except the showers - but would a combi support any young family that moved into the house after us?

2. Thermal store. I read that we could use an unpressurised thermal store/cylinder, like the Gledhill Torrent, to replace our cylinder. This is unvented but only needs a small header tank in the airing cupboard. Our boiler would support pumping the primary circuit. The mains water is heated simply by passing through this hot store in a second finned coil. Sounds like magic, with mains pressure at the taps (and ?one of the showers!) My wife doesn't think I know what I'm talking about. (She has a lot of experience that she can quote!) So please tell me if she's right, and I'll make sure she doesn't see the thread! Pressure has been checked in past, and was close to 3 bar. I just measured the flow downstairs at over 25 litres/minute.

Any advice on which way to jump?

Premier_2

A thermal store heating system such as that produced by Gledlhill does indeed supply mains presure hot water. A Gledhill Boilermate has the F&E already in position on top of the hotwater tank.

The other advantage of the thermal store system is that the boiler itself is usually much smaller. e.g. a 3 bed detached house may only require a 35,000 - 40,000 btu boiler compared to the 125,000 - 130,000 btu combi that your wife has suggested. This is because the thermal store removes the requirement of a sudden large load e.g. to heat up radiators or heat water from cold; it only needs to be large enough to actually meet the heat lost from the radiators or hot water, not big enough to heat it up quickly in the first place.

The disadvantage with a thermal store system is that the hot water must always be maintained in the tank. If you don't use much hot water (and obviously no central heating) in summer, then the cost of keeping a tankful of water hot all day may be expensive compared to just heating what you need when you need it. The other thing to remember is that the constant hot tank of water will give off some heat no matter how well insulated it is, which may make the house uncomfortably warm in the summer.

Uzima

Thanks Premier. I'm encouraged. I guess the key thing is how much heat we lose from the store cylinder, which is an unknown for me. I guess there are figures for this, with a modern well-insulated cyclinder like the Gledhill. Our current cylinder only has a useless jacket wrapped around it, so the bathroom does get warm in winter!

We don't use the gas boiler at all during the summer. We only use the dishwasher if we have guests, and I hand-wash the rest at cold mains temperature - we don't cook fatty food so it's not a problem. We just use the electric showers to wash. But a 'normal' family coming into the house would behave differently!

I'm inferring from what you say that maybe we should run the central heating from the store water rather than as a separate pumped circuit from the boiler primary.

I'd have no problem with a combi if I thought it would be reliable, and wouldn't cost a fortune in regular maintenance.

espresso

Uzima wrote: »

I'd have no problem with a combi if I thought it would be reliable, and wouldn't cost a fortune in regular maintenance.

I think that you really need to compare the combi maintenance cost with the cost of using electricity to heat the water! :eek:

Uzima

Hi Espresso, good point! - the electricity for the showers. At least I can work out what that is costing. Still have no idea what the real cost of a combi is, since it depends on annual maintenance and how often it needs to be replaced. Even with a combi I'd keep one shower electric in case the boiler hung up for any reason.

Premier_2

Uzima wrote: »

...I'm inferring from what you say that maybe we should run the central heating from the store water rather than as a separate pumped circuit from the boiler primary. ...

Yes.

I think the Gledhill Torrent is not designed for this, but the Gledhill BoilerMate is. It will dump the hot water from the tank direct into the radiators providing instant heat (well usually within about a minute or two) unlike a separate circuit that'll probably take at least 10 minutes or more to get warm.

As you have electrically powered showers, you could just turn the entire system off in the summer, but that would mean no hot water available from any taps. But that sounds like what you have at present unless you use an immersion heater in the summer.

Uzima

Combined electricity cost of showers for two of us is about £60 pa at current tariff - now my son isn't home to take 30 minute showers. Not expensive, but then these aren't luxurious showers!

We do need to run one non-electric shower, and I take the point that it is likely to be wasteful to have a thermal store kept warm through the summer just so people can shower. I'll try to check what the heat loss is from a well-insulated cylinder........

If a combi, for point of argument, costs twice as much in maintenance/early replacement as a basic condensing boiler, I guess that might add up to £200 a year. On this site people have even reported going fully electric, figuring that the extra cost of electricity over gas is compensated by the lower installation and maintenance costs.

Cardew

Uzima wrote: »

Combined electricity cost of showers for two of us is about £60 pa at current tariff - now my son isn't home to take 30 minute showers. Not expensive, but then these aren't luxurious showers!

We do need to run one non-electric shower, and I take the point that it is likely to be wasteful to have a thermal store kept warm through the summer just so people can shower. I'll try to check what the heat loss is from a well-insulated cylinder........

If a combi, for point of argument, costs twice as much in maintenance/early replacement as a basic condensing boiler, I guess that might add up to £200 a year. On this site people have even reported going fully electric, figuring that the extra cost of electricity over gas is compensated by the lower installation and maintenance costs.

I believe the accepted figure for heat loss from a 'conventional' 150 litre tank(with the now mandatory 30mm foam insulation) at around 60C is in the order of 3kWh in 24 hours. Obviously in practice you would rarely keep the tank at 60C for 24 hours.

I am not a fan of combi boilers, mainly because of the poor flow of hot water for taps/baths in winter - but this doesn't seem to be a problem for your requirements.

There certainly is an argument for all electricity in smaller properties.

No maintenance required, rock bottom installation charges and no gas boiler with built-in(so it seems)obsolescence where a boiler needs replacing every decade.

Incidentally is it your wish/intent(wife permitting) to retain your 8 year old non-condensing boiler to use with the thermal store? It is not my understanding that basic(non-combi) condensing boilers are any more reliable than combis.

Lastly, why are you ugrading?

Uzima

Thanks Premier. I'll check on other storage cylinders apart from the Torrent. I can see the advantage of pumping the CH water from the tank. And if the cylinder is indirect, it should return reasonably cool water from the bottom to any condensing boiler we might fit in the future. Gledhill's Boilermate diagrams show the CH being pumped from the boiler primary. Their Systemate cylinder does the full works, HW and CH, but seems to be discontinued - Gledhill Water went bust in October. I have found a Range 'Flowmax' which does the same, and a lot of info on a website belonging to DPS, who also sell 'heat banks'. I guess there must be others.

I'll price up the options. With the thermal store you still need to think about a control system - wiring centre, programmer etc. I suppose that with a combi that all comes incorporated, and you just wire in a room stat.... )I almost wrote cylinder stat, but of course you wouldn't need one!)

Uzima

Thanks Premier. I'll check on other storage cylinders apart from the Torrent. I can see the advantage of pumping the CH water from the tank. And if the cylinder is indirect, it should return reasonably cool water from the bottom to any condensing boiler we might fit in the future. Gledhill's Boilermate diagrams show the CH being pumped from the boiler primary. Their Systemate cylinder does the full works, HW and CH, but seems to be discontinued - Gledhill Water went bust in October. I have found a Range 'Flowmax' which does the same, and a lot of info on a website belonging to DPS, who also sell 'heat banks'. I guess there must be others.

I'll price up the options. With the thermal store you still need to think about a control system - wiring centre, programmer etc. I suppose that with a combi that all comes incorporated, and you just wire in a room stat.... )I almost wrote cylinder stat, but of course you wouldn't need one!)

Uzima

We are too spoilt for choice....
Thanks Cardew. So if a cylinder was kept hot all year, the heat loss would cost less than £100 a year - in cash at least. I want to be environmentally sensitive, but then I also need to enter quickly-obselescent boilers on the balance sheet.

The reason to upgrade is that other changes have meant we are losing easy access to the loft. The header tank is not a problem, but the cistern would be too heavy to perch in the airing cupboard and we would lose a metre of the little pressure we have at the upstairs taps. The primary circuit is unpumped, there's no stat on the cylinder which is poorly insulated, heat leaks from HW into the radiators, and we would like one non-electric shower. The CU is at maximum load, and one of the shower cables is buried under rockwool in the loft. So altogether........

What also nags at me is our growing dependence on specialists. Combis (or other sealed systems) contribute to this. I'd prefer to keep the old boiler - not because I can service it, but because it doesn't often need a service and has never threatened to break down. There are throwaway comments on this forum that even people working in the industry prefer not to have combis even though they presumably cam maintain them cheaply.

We are undemanding in our use of hot water, but we have an eye on the sellability of the house. (I would hate to get an environmental rating on the current system!) The bottom line for us might be that a combi would not have the flow to meet the needs of a busy family of five with kids to bath and shower. It's hard for me to judge this, because I have never used a combi.