Can a dual coil cylinder be used with single heat source?

[EliteAV]

This is my first post in this forum, but I have been lurking here for a couple of years as have previously found it to be a mine of useful information.

My current system comprises a Vaillant ecoTec 637 (37kw) boiler with Vaillant eBus controls (VRC 470, VR61 2 zone centre, VR81) and a vented 225L cylinder.

I intend to replace the vented cylinder with an unvented one to allow me to lose the cold water storage tank in the loft as I am planning a loft conversion that will add another 2 bedrooms and 2 ensuites (bringing total for house to 6 bed/4 bath).

I had originally intended to go with a Vaillant Unistor 310L unvented cylinder as I have been very happy with the quality & reliability of my current Vaillant system. However one of the plumbing companies I have asked to quote (who installed my current system a couple of years ago) suggested using a dual coil cylinder and feeding both coils simultaneously from the ecoTec 637.

The cylinder suggested was a Grant DuoWave Indirect 300. This is marketed as a solar cylinder with the (smaller) top coil fed from a primary heat source like a gas boiler and the bottom (larger) coil from a solar system).

The plumbing company advised that the Grant cylinder, because of it's dual coils, has twice the surface area of the single coil in the Unistor 310L which should significantly reduce the cylinder recharge time (& reduce the amount of time that heat is diverted away from the space heating with the system configured in DHW priority mode).

This all seems sensible to me, however, a 2nd company that I asked for a comparative quote advised that this wasnt a configuration supported by the cylinder manufacturer.

This does make me wonder, if feeding a dual coil cylinder from a single heat source could provide the above benefits, why manufacturers dont support this?

I have searched this & other forums and cant find another example of anyone else using a dual coil cylinder in this way, hence the reason why I thought I would throw the question open to the experts in this forum

Thanks a lot for your help.

 

[ALEC1]

I always combine two coils in series...its not an issue at all..

But there are pitfalls... the primary temperature will need to be adjusted in hot water mode as the extra coil area can cause the set temp to be much higjer tan the set point..by lowering yje flow temp hysteris is reduced consuderably..

 

[EliteAV]

I always combine two coils in series...its not an issue at all..

But there are pitfalls... the primary temperature will need to be adjusted in hot water mode as the extra coil area can cause the set temp to be much higher than the set point..by lowering the flow temp, hysteris is reduced considerably..

Alec1, thanks for the quick response, this is reassuring to know. The Vaillant controller (VRC 470) has an NTC sensor (VR10) that fits in a dry pocket in the cylinder so it can accurately monitor the actual vs target temperature of the water in the cylinder so I would hope that this should help with the issue you have flagged. Note: I specified a dry pocket when I ordered my current (non Vaillant) vented copper cylinder but the Grant DuoWave unvented cylinder has 2 pockets as standard (top/bottom of cylinder) so I would use the top pocket for the Vaillant sensor.

Re your 2nd comment (about the installer not having the knowledge), I think you are right!. However I did email Grant for confirmation on this and never received a reply (but then I have previously sent similar emails to Vaillant in the past & never received a reply either; I think manufacturers prefer to deal with trade enquiries which is understandable).

My dilemma is that if I go with the (Grant) dual coil cylinder installed by the 1st installer, it is going to cost me about £1200 more than going with a Unistor single coil cylinder installed by the 2nd installer (total cost will be approx £5K vs £3.8K - the job involves relocating the cylinder from the airing cupboard to the garage and running extra pipework to the loft, hence the prices). I have used both installers before so can vouch for their work but I think the 1st installer has more experience with non standard configs. Ultimately I want a solution that works as efficiently as possible so am leaning towards going with the 1st installer.

 

[EliteAV]

An update:

Grant Technical Services emailed me back to confirm that there would be no problem feeding both coils in their DuoWave Indirect unvented Solar cylinder from the same heat source (i.e. a boiler). I must admit I was pleasantly surprised to get a reply as it wasnt a trade enquiry.

I have therefore decided to install the Grant twin coil cylinder rather than the Vaillant Unistor single coil cylinder (which just as an FYI to anyone who is considering installing a Unistor, Vaillant has recently revised the design of the cylinder & it now includes a dedicated secondary return - there may be other differences between the current & previous models but this is an obvious one. The latest user guide/install manual is on the Vaillant site).

I have also been able to negotiate the price of the job down by about 10% which helps.

 

[Corgigrouch]

Alec, why in series and not in parallel?

 

[ALEC1]

in series lowers the return temperature...and works with the tank stratification...