Use of Combi but with HWC for bathroom.

[gordyc]

We have a large 4 bed, 1 bath, 1 showerroom bungalow. Ideal Mexico rs65, yes that old!, has run faithfully since 1987 when the house was built. It provides more than adequate heating and hot water and most of the year the boiler thermostat sits at 1. Only over the very coldest spells does it get turned up beyond 2. It's Scotland so we are entitled to a bit of additional gas consumption!! This boiler is rated at 18kW. I'm considering changing to a combi as the hot water run of 10m from the cylinder to the kitchen and utility means it takes a while for taps to run hot. Using a combi to provide DHW to these taps reduces the run to about 3m max and provides a flow rate that should be greater than we have put up with. I would plan to simply keep the HWC connected in the system as it is but remove the pipe runs along to the kitchen/utility. I would lose the 3 port valve currently used and run with a 2 port to control the HWC heating feed off the CH with a cylinder stat in circuit. Combi ratings all seem to be much greater than the Mexico is. I get they need to also heat water instantaneously.
Questions:
1. Do I really need anything with a rating greater than 25kW? Showers/bath would stay on the HWC circuit.
2. Am I missing something key about the more modern combi design that mean it has a higher rating but is somehow smaller and more efficient?
3. Which makes deliver the best trade off between cost and reliability? Ideal have gone through a long period of poor press in general, as have many others, otherwise I would consider a same make replacement. I have seen much better reports recently about ideal but also, viessman and intergas. The latter do not seem well supported up here though.
Sorry about the length of this story but I'm trying to give what I hope is relevant background, knowing that it's a bit like asking how long the piece of string is!!
Gordon

 

[picasso]

Hi Gordon yes its feasible to have a combi and hot water cylinder but you would have to keep the 3 port valve or install 2 x 2 port valves to control the heating and hot water separately , not sure I would fancy sealing 35 year old pipework though, the intergas can run open vented so worth a look for your job, a 24 kw combi would be more than adequate going with the information you have given.

 

[ianmcd]

The Intergas are good boilers as are a few others, you are correct about modern Ideals though , they have a problem with their heat exchangers, you dont say where in Scotland you are, I am in the central belt and there is an Intergas spares stockist in Livingston and Glasgow that have most of what you need off the shelf, all the big Merchants would be able to get spares next day if you needed them, there are probably more but I use these two, biggest consideration for you is the heat exchanger of the boiler you choose as after 35 years the water quality is likely to be poor and will need a proper flush out when new boiler is fitted, low water content heat exchangers do not like dirty water, no matter what new boiler you go for you wont get 35 years out of it, something else you may want to consider is secondary return on your HW circuit with a bronze pump and keep your old boiler, that will cure the time lag for HW draw off

 

[gordyc]

Thanks for the replies guys. I'm in North Cumbernauld.
Picasso: I was planning to use a 2 port valve to act like a TRV. The HWC is currently connected like this with the 3port valve on the Flow to split the DHW/CH circuits. The cylinder return is just Teed in to the CH return beside it. It's worked fine all these years. The other way I have seen is TRV style valve which has a remote sensor. This basically means the cylinder Flow connection can be taken off the CH Flow adjacent to the cylinder and it then acts just like a radiator in a cupboard.
The combi CH would get connected up directly to the CH Flow and Return connections.
I wondered about the boiler rating in case the smaller HE in the combi just couldn't cope with the capacity of a larger system like mine.
The pipework is virtually all plastic (Bartol at the time if you remember them!). I'm currently working my way through replacing it all with the up to date stuff, Barrier pipe and demoutable fittings. I have had zero problems with the plastic in 30 odd years and my daughters system is a combi with plastic pipework and it's been going about 10 years with no issues. Of course, I may just have been lucky!!

Ianmcd: I have read a lot about Ideal HE issues. They seem to have changed again and sell at least two different ranges with different HE materials.. They at least have the balls to warrant their products for a decent time, up to 10 years if I'm not mistaken. I'm under no delusion that anything I replace the Mexico with is not going to give the same reliable lifetime that it has delivered. I would love to keep it going but it is probably on borrowed time now and I can't help but feel my luck is going to run out sooner rather then later. I was thinking about buying a new GCV for it as these are still available and probably the only thing other than the HE that might pack in. TCs are easily sourced and not much else!
I also like the more efficient boilers around nowadays, even if the majority of the increase is down to not running a pilot light 24/7!
I have drained and flushed out the system a number of times over the years, most recently just last year when I was replacing some of the rads due to sludging up. I intend to do another major clean in the next few weeks, once the worst of the winter is behind us.
I'm not following your suggestion about the secondary return on the HW. Can you explain a bit further please?

 

[ianmcd]

I'm not following your suggestion about the secondary return on the HW. Can you explain a bit further please?

yes more than happy to, if you think about large hotels , and the like you wouldnt want to wait for ever for your HW to come out of the tap, so what you do is make a circuit where a pump moves the HW from your cylinder around the loop , and the draw off to any tap, shower etc , is only a short teed off stub , so when you open the tap you are only drawing cold water from the very short stub , before you get the HW, we have fitted loads of types, for a commercial property you would want the pump timed or constant depending on demand, on domestic we sometimes fit PIR sensors in bathrooms or halls etc that the pump only runs when someone is likely to want instant HW, so many go on about energy efficient boilers but using the energy you have , and controlling it properly is the most efficient way

 

[gordyc]

Ianmcd, Ahh! Yes, I understand the technique now.
Other than intergas, do you have suggestions. I guess i was close to the mark when i voiced my concern about the HE size in combi boilers. I agree that it's more important to have a system which runs efficiently than an efficient boiler, although the latter can only help! Are you happy to give my system a look over and advise on a way forward?
When I put the system in originally, I chose the most practical options at the time. Newer boilers and better controls are now around but it's a potential minefield and things are not as simple as they were back in the 80's!!!

 

[ianmcd]

Ianmcd, Ahh! Yes, I understand the technique now.
Other than intergas, do you have suggestions. I guess i was close to the mark when i voiced my concern about the HE size in combi boilers. I agree that it's more important to have a system which runs efficiently than an efficient boiler, although the latter can only help! Are you happy to give my system a look over and advise on a way forward?
When I put the system in originally, I chose the most practical options at the time. Newer boilers and better controls are now around but it's a potential minefield and things are not as simple as they were back in the 80's!!!

where in Scotland are you ? if nearby yes happy to give you some advice

 

[gordyc]

Hi Ian, I'm in North side of Cumbernauld, Balloch area. I'll need to check how to message you privately rather than on this forum.

 

[CBW]

Not sure if you figured out the private message, if you click on your inbox and select start conversation,it should be straightforward from there.

 

[gordyc]

Thanks Novice. I would have struggled to find that!