Posted by: @tomhartley

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I’m in a new build with a Vaillant 7Kw. Rads up stairs and ufh downstairs. When up stairs wants demand downstairs struggles, but back to the hot water. We came home the other night and basically had a shower one after each other but by fluke, nearly 50 mins apart. So I used then son, etc etc. but after I used it, the heating stops to heat just hot water. So my heating was off for hours.

Given the very slow response of UFH, I'm surprised that it's noticeable if the heating goes off or an hour or so. And a new build, if insulated to current standards (perhaps a big "if"), shouldn't cool down noticeably in an hour.

Posted by: @jamespa

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Depending on your system you can also schedule multiple, shorter, DHW reheats during the day/night

This can be easily done using the Vaillant app. At the moment I've got my 7kW Arotherm+ to top up the DHW in the afternoon afternoon and before 5am when the house heating gets turned up slightly. If the DHW cools down between those periods then it waits until the next scheduled top-up period.

Returnng to the original problem. I've thought of two more work-arounds which wouldn't need heating the DHW to a very high temperature:

i) Persuade some occupants to shower in the evening and just do a quick hand wash in the mornings.

ii) Run the cold supply to the shower(s) via a tank which is at room temperature or warmed by the main heating system - you'll get much more water at 40C is the cold supply is warmer.

iii) Use a bath tub and take it in turns to take a dip in the same water (as was commonplace in an earlier era).

Posted by: @johnr

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Posted by: @tomhartley

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I’m in a new build with a Vaillant 7Kw. Rads up stairs and ufh downstairs. When up stairs wants demand downstairs struggles, but back to the hot water. We came home the other night and basically had a shower one after each other but by fluke, nearly 50 mins apart. So I used then son, etc etc. but after I used it, the heating stops to heat just hot water. So my heating was off for hours.

Given the very slow response of UFH, I'm surprised that it's noticeable if the heating goes off or an hour or so. And a new build, if insulated to current standards (perhaps a big "if"), shouldn't cool down noticeably in an hour.

Posted by: @jamespa

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Depending on your system you can also schedule multiple, shorter, DHW reheats during the day/night

This can be easily done using the Vaillant app. At the moment I've got my 7kW Arotherm+ to top up the DHW in the afternoon afternoon and before 5am when the house heating gets turned up slightly. If the DHW cools down between those periods then it waits until the next scheduled top-up period.

Returnng to the original problem. I've thought of two more work-arounds which wouldn't need heating the DHW to a very high temperature:

i) Persuade some occupants to shower in the evening and just do a quick hand wash in the mornings.

ii) Run the cold supply to the shower(s) via a tank which is at room temperature or warmed by the main heating system - you'll get much more water at 40C is the cold supply is warmer.

iii) Use a bath tub and take it in turns to take a dip in the same water (as was commonplace in an earlier era).

another is to fit waste water heat recovery

@jamespa hi, in the above where you state from 10 to 40 takes 30 mins, do you mean, 10degrees in the tank to get it up to 40c.  My system is on 110 mins max with a 30 minute break. From one of your earlier comments where you state, you have the dhw doing things at different times etc. my system has the hysteresis on 5, so once it drops, it starts to kick in straight away. Do you have this turned off? Thanks for the input.

Posted by: @tomhartley

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@jamespa hi, in the above where you state from 10 to 40 takes 30 mins, do you mean, 10degrees in the tank to get it up to 40c.  

Yes, sorry for any confusion.  10C is sometimes quoted as the temp of incoming mains (being, I presume, roughly the temp underground at the level the mains is typically buried)

Posted by: @tomhartley

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My system has the hysteresis on 5, so once it drops, it starts to kick in straight away. 

Depending on where the sensor is in the tank, leaving this feature on might well start the reheat 'automatically' sufficiently early in the 'run' of showers.  Its probably necessary to experiment to find out, although you could get a bit of an idea by estimating how far up the tank the temp sensor is.  The reheat should start when the cold water reaches this level.  Mine is about one third of the way up the (Vaillant) tank, so the reheat would start after a bit less than 2 showers.

@jamespa I have the Vaillant tank also. Is this why sometimes I can have a shower after a recharge and the temp doesn’t drop when I check the senso comfort. Next person gets in, then it reads like 35 when they get out. Sorry for side tracking the thread, the original topic at the start… why for what reason are we not just fitting 10kw or 12kw units, so the dhw heats as we’re using the shower at a higher speed and keep the same size cylinders?

Posted by: @tomhartley

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why for what reason are we not just fitting 10kw or 12kw units, so the dhw heats as we’re using the shower at a higher speed and keep the same size cylinders?

More expensive, larger, louder and won't modulate down so low and are thus less efficient for space heating if the house doesn't need it.  If you really need six showers in close succession there are other solutions.  You are of course also at liberty to decide your own trade offs.

Hi. 

Any chance that one of your bathrooms can have an electric shower installed. 

Gives you a little more flexibility with hot water reheat times.

Posted by: @tomhartley

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I have the Vaillant tank also. Is this why sometimes I can have a shower after a recharge and the temp doesn’t drop when I check the senso comfort. Next person gets in, then it reads like 35 when they get out. Sorry for side tracking the thread, the original topic at the start… why for what reason are we not just fitting 10kw or 12kw units, so the dhw heats as we’re using the shower at a higher speed and keep the same size cylinders?

Try reducing the DHW hysteresis setting and schedule the DHW to come on just before and during periods when the hot water is likely to be used. The hysteresis setting is there to stop excessive switching on the heat pump to top up the DHW (there's also the one hour default DHW anti-cycling setting which may need to be reduced). Each DHW heating cycle has a significant energy overhead in the summer months when the general heating is off and the primary circuit has to warm up from cold. This is less of a concern in the winter when the system is already warm.

One of the data files which can be downloaded using the Vaillant app contains hourly DHW temperature readings. You can scan through this to see how the system is behaving which provides useful guidance for setting the control paramenters.