Am I being dumb or is this a thing?

[Bazbazbaz]

I’m not a heating engineer but I’m fairly knowledgeable on the way systems work. But I have come across an issue which I’m wondering is common.

Earlier this year I had a fully licensed engineer install a new baxi combi in a property I’m doing some work on. Great job done without any problems.

Last week the owner mentioned that the shower (which is a aqualisa with a conversion box fitted to work with the new combi boiler) wasn’t getting hot.

I thought it was “user error” but having spent some time their today I cannot get it to run hot. I thought it might be the conversion box from aqualisa but the other shower which is a standard mixer unit is the same.

When I investigated further, I can get hot water out of the sink taps but as soon as I put the bath on full, the hot water drops to “quite hot”. The actual temperature that the boiler shows is 60 degrees when the sink taps are on but only 51 when the bath or shower taps are fully on.

It would seem that the boiler isn’t strong enough to heat a large enough volume of water to feed the bath or shower if the water temperature coming in from outside is too cold. And with current outdoor temperatures about freezing, this is a problem.

Or as I said, am I being dumb?

 

[ShaunCorbs]

Is the box rated for high pressure/ combi system ?

Bath taps are full bore especially if there older style and they won’t get 60 dc hot water out due to need a 40 kw plus boiler

 

[Bazbazbaz]

Yes - the box is rated for combi/high pressure.

So the boiler isn’t powerful enough?

 

[ShaunCorbs]

What’s the kw of the boiler now ?

 

[Daz_jenkins]

Whats the kw of the boiler?, and how many rads are in the property/bathrooms.

Could well be the boiler, but the engineer should of fitted something that would be appropriate.

 

[Bazbazbaz]

Whats the kw of the boiler?, and how many rads are in the property/bathrooms.

Could well be the boiler, but the engineer should of fitted something that would be appropriate.

It’s a baxi 428 which I assume is 28kw

 

[Bazbazbaz]

think there are 11 radiators

 

[Daz_jenkins]

Hmm boiler should be fine then imo mate, maybe get them to come back. They should of checked everything to makesure it was working before leaving the job.

 

[Bazbazbaz]

It heats the house fine and before this spell of cold weather the hot water was fine too it seems that the system struggles to heat up very cold incoming mains pressure water and push it through at a high temperature.

Is this a common thing?

 

[Daz_jenkins]

Not that i have come across mate , but this is defo something you want to get sorted sooner rather than later, as the weather will be getting colder. Sorry i couldnt help more

 

[ShaunCorbs]

Yes common as the incoming water temp drops your rise won’t be as much eg in summer you will have hotter water than in winter

 

[Ric2013]

Yes it's a thing. Combi boilers are a compromise. You have hot water on demand, you don't use energy to heat and maintain a cylinder, and you save space, but the faster you run your hot water the less hot it will get.

Your penultimate paragraph shows you have understood the issue perfectly.

 

[Bazbazbaz]

Thanks all…. So next question….

Is there a fix?

Something like a small pressurised cylinder that can sit in loft just before boiler? Perhaps with a small element that takes the chill off the water inside so the boiler doesn’t have to work so hard?

 

[Ric2013]

Of course there is a fix. There is absolutely no reason you can't have a cylinder of whatever kind you like (with or without electric backup) as well as a combi boiler. You could even run some hot taps off the 'combi feature' of the boiler and others off the cylinder. Or you can just restrict the maximum flow to the bath or shower.

In practice, you won't want a bath or shower above 40°C, so it may not be a massive problem so long as your hot water temperature stays at 50°C or above. It's worth noting many thermostatic mixers will always deliver water that is no warmer than 10°C less than whatever your hot water temperature may be, but you can find thermostatic mixers (Grohe makes one, for example) that will deliver water to within 2°C of the hot water temperature. So, with such a kind of mixer, you could still have a 38°C shower (the normal recommended maximum safe temperature) when the boiler is outputting hot water as low as 40°C.

I must say I have toyed with the idea this kind of setup in a personal capacity as I have a bath that is only used once a week or so but could easily live with a combi's hot water output the rest of the time.

 

[DrMaybe]

Not sure if this helps, but I had a combi in my last house and it had a pump connected to a temperature controlled head (electronic), it seemed able to pull the water through at a decent temperature.

 

[Sparkgap]

It heats the house fine and before this spell of cold weather the hot water was fine too it seems that the system struggles to heat up very cold incoming mains pressure water and push it through at a high temperature.

Is this a common thing?

Think on it: you have 28kW which should be plenty for heating a medium/large house. However when heating an incoming water supply you need a lot more: assuming heating from 10C to 43C 28kW will give you about 0.2 litres/sec. A domestic bath is normally assumed to be about 0.4 l/s so no chance of getting high flows and temps unless you have a stupidly large boiler! Suggest you look at an unvented cylinder.

 

[Bazbazbaz]

All makes sense gents. Thanks for your help.

For the time being I’ve reduced the flow from the hot into the aqualisa system. Not by much but just enough so that when it calls for hot it allows the flow to be hot enough to shower under. It makes little difference to the actual power of the shower.

Longer term and invented cylinder is definitely the answer.

 

[Ric2013]

Reducing flow is not a bad idea. You can even put flow regulators into the pipework eg these ones (BES.co.uk | Next Day Delivery On 15,000 + Plumbing Supplies - https://www.bes.co.uk/flow-regulator-ball-valve-cp961-15mm-17527/) if you want to control the maximum flow to certain taps to avoid them 'robbing' others.