Vaillant unistor reheat time

Hi all,

Have been searching past topics and have not been able to find answers to some questions which I'm currently trying to resolve. Hoping some of the members here can help.

I am coming to the end of a sizable extension which will result in a 4/5 bed semi with 2 full bathrooms, utility, and d/stairs wc. The house is now very well insulated (incl loft with 100mm+ celotex between rafters) : edited for clarity

As part of the overall project the heating system is now to be upgraded to meet the increased current and future demands. Currently 2 adults and 1 soon to be 2 youngsters. We have regular visits from the outlaws and other rellies so often 4 adults plus the kids.

I am currently trying to decide between 2 possible heating solutions.

Vaillant 937 + vrc430

Vaillant 6xx system boiler, unistor 210 and vrc430+vr65.

The 937 would probably fit our needs but has certain compromises re lack of back-up and compromised flow rates where more than one outlet is used simultaneously. Big advantage is endless supply of dhw with no re-heat times (save for 2 mins for store to recharge)

The 6xx system would also appear to meet our needs (and would be my preferred choice due to ability to meet dhw demands at several outlets at once) but I am unsure how the cylinder will perform when a large water draw off is required.

Can anyone shed any light on the re-heat times of the unistor in a real world setting?

Boiler will likely be the 624 (RGI will size). New 25mm mdpe from meter giving 30l/min @ 3.5 - 4 bar. Gas meter within 1 metre of proposed site of new boiler. Gas range cooker in kitchen.

My concern is that with 2 or 3 showers being taken (2x thermostatic mixers) successively that the tank will drain of dhw and the showers run cold. I know that the 210 ltrs of hot in the tank will be mixed with a percentage of cold which will 'stretch' that volume further but for eg 3x 10min showers @ guesstimate 10ltrs/min hot (plus x ltrs cold mixed) would seem, on paper, to be enough to drain the tank unless the re-heat is super efficient. Vaillant quote 22 mins recovery time. I know the cylinder will start to re-heat once the dhw draw off starts but will it be quick enough to prevent the tank 'running cold'

Apologies for the long post, have tried to give a clear picture of potential use patterns etc. I would be very grateful for any comments.

regards

jeff

Sorry, forgot to mention. 6xx system boiler and the unistor 210 would be sited next to each other to minimise heat losses.
regards

jeff

10 mins is a long time in the shower, using 13 litres a minute. Few 4 person families would get through 390 litres in 30 minutes.

You could fit a uniSTOR 260 or larger. You could fit an ACV tank in tank which has a very large heat input, thus very fast recovery.

You can connect VR65 to other cylinders if you use their dry pocket.... not suggested by Vaillant, but we do it where the requirements dictate. Have put standard open vent cylinders with the VR65 and Vaillant solution.

Should just add that 30 litres per min is unusually good, have you measured it?

It seems you want a one-box solution - usually the best way to go.

The Glow Worm Ultrapower is decent enough.
http://www.glow-worm.co.uk/products/ultrapower_sxi.asp

The W-B Highflow (floor mounted) is good too.

The Ethos 54kW combi is the highest flowrate of any instant combi (around 23-24 litre/min) and is better than the Vaillant 937. The 937 is massive and is only an 837 with a 15 litre store behind.

Three showers simultaneously? You have to get dried after, so a number of minutes for recovery in the Ultrapower. Taking three shower after each other would not empty the store completely as it is always recovering as hot water being drawn off.

Backup in case on downtime? Well a number of options there:

1. The house is large, so use Two Combis (this is highly cost effective for what it offers):

1. Two combi's will provide good showers for as long as you want - all day in there with two showers going, if you are that way inclined.
2. The two combis release valuable space taken up by cylinders and gives a natural simple zoned heating system saving money on fuel bills.
3. No contest, the two combis win hands down, on all points to gain a zoned heating system - one combi on a stat/programmer does upstairs and the other on a stat/programmer does downstairs. Totally independent supplying two floors.
4. Two combis guarantees good showers and a bath (combine the outlets for the bath only using two non-return vales and small shock arrestor, and liberating space:

Two combis win on:

• Low capital cost of installation. Combis are cheap. Two combis gives more and cheaper than a system boiler and unvented cylinder.
• Cheap running costs, upstairs heating can be off most of the day.
• No waiting for showers
• Showers never run out of hot water
• Simple zoned heating system with independent time zones.
• Different rad temperatures on different floors.
• Different room temperature stat settings on two floors
• Less controls to go wrong, with accompanying down time and expense. (no zone valves, complex wiring and the likes)
• Always heat and hot water in the house somewhere. Two combis means built-in redundancy (backup).
• Super quick warm up. Two combis heating a house from cold does it in super quick time.
• Releases valuable house space by eliminating a cylinder and tank.
• One combi can supply the kitchen and the other the utility room - always hot water in the food area.
• Fast bath fills as they only combine DHW outputs to fill baths. Two cheap 10 litres/min combis will deliver 20 litres/min.
• Less wear as the usage is spread between two combis.
• Quick installation.
• Simple installation.
• Simple electrics. A mains point to both and one from each to a stat/programmer on each floor. The stat can be a wireless model for even simpler installation.
• Using integral weather compensation in the combi means just two outside weather sensors side by side on the north face of the house, giving maximum condensing economy.
• The two combis can be in separate locations or both together, or both out of the way in the loft.

Only disadvantage is slightly more expensive on regular servicing, which is overwhelmingly offset by the above points. In fact the same servicing costs as a system boiler and unvented cylinder, as an unvented cylinder needs an annual service.

I specified a few installations not long ago using two combis. The Pikies were stealing expensive cylinders. No cylinders and combis on jigs with plastic pipes and removed until final commissioning. They work a treat and customers are delighted with them, especially when two people are using two showers - one shower does not interfere with the other and they can stay in them for ever and no waiting for recovery of stored water.

Look at Broag. Two Avantaplus combis would do, http://www.avantaplus.co.uk which are quality, well priced, 5 year guarantee and with a much superior integrated OpenTherm control system with integral weather compensation. I would seriously consider this route with your requirements and size of house. A 28kW combi is: £645.08 inc. VAT. X 2 is just under £1300 with a few quid extra for the outside temperature sensors.
http://www.tradingdepot.co.uk/DEF/c.../Central Heating Boilers/Broag Remeha Boilers

2. In-line electric instant water heater:

If it is DHW backup you want try a in-line backup water heater for combis. Look at this thread:
//www.diynot.com/forums/viewtopic.php?t=152381

Simple, small, easy and will give backup for a shower and a tap or two, until boiler back up.

Hi Simon,

Thanks for the reply. I have read a little about the ACV but I don't pretend to know much about it as an alternative and how well it compares on a cost/performance basis.

Re the 30 l/min- yes I measured it several times with a bucket and stopwatch and took an average figure. This was tested straight after the stopcock. Also did the same at the bath tap. Result was a bit less here as the water takes a pretty convoluted path to and from the old tank position before reaching the bath, but still showed 25l/m after travelling through upteen bends etc (I intend to cut out the dead legs and try and minimise the new pipe runs in preparation for the new system).

I agree that 10 mins is quite a long shower- I'm usually in and out in less than 5. However, the missus and the outlaws have webbed feet I think!

My main concern is to try and optimise the new system. I don't want to massively over spec the tank and spend 75% of the time heating water that's not used. Conversely, I don't want to under spec things either and be regularly running out of water, especially at this stage when the cost difference between for eg the unistor 210 and the larger capacity units is quite small. Having to upsize later on would be much more expensive and certainly frustrating!

Perhaps my fears are a little unfounded. Until you live with a system it's difficult to know exactly how it will perform and meet your needs.

Thanks again for your views

regards

jeff

Hi BB,

Thanks for your comments,

Re the glow-worm ultrapower- I have looked at that as a possibility- It seems a practical option where space is a bit limited. Space is not such an issue for us as the system will be in a separate utility room.

I had considered the idea of two combis- I wondered about the performance and efficiency of 2x small v 1 large. Kind of like the twin supercharger idea which seems to work in practice. I am certainly open minded about the suggestion and would welcome any other comments especially from anyone who is actually running such a set up.

regards

jeff

Hi there BigBurner I was wondering if you could help me. I just read your post about piping two combi's in to give a larger hot water demand. Is there any way that they can be piped in to the same heating circuit? I would use each one for a different zone but the customer who's house we are working on does not want to alter the existing pipework as this would be a lot more costly. The house is a 5 bed house and she will have three thermostatic showers in total which can be expected to be running simultaneously.

1) start a new post, this one is 4yrs old
2) you won't get any advice from BB(luckily) because he's been banned dozens of times under various alias's for giving bad, dangerous, abusive and misleading advice.