Adding DHW Storage to a Combi Boiler?

[rtcosic]

We are looking at a property with a Combi boiler. I have used showers supplied by these before and the achievable water flowrate is far too low compared to our system boiler fed mains pressure DHW system.

Is there any reason I cannot leave the Combi supplying kitchen and utility room, separate out the supply to the bathrooms and just add the usual Honeywell S Plan and storage tank to the 'central heating' function of the combi for them? The property is a bungalow so running plumbing in the loft should be easy enough.

This would also permit the using an immersion heater to dump surplus power if I added solar PV.

Edit: I haven't checked age and condition of the Combi, nor the rest of the system. I acknowledge it might be just as easy and cost-effective to junk the Combi and start again with a new system boiler, especially if other replacements or upgrades are needed. I would just like to have an indication of my options.

 

[gaz1]

whats the flow of the cold mains pressure ?

as the boiler flow rate would roughly be the same coming into it unless someones turned the pressure down on the boiler

hence the reason low flow rate

 

[julianf]

I don't know much about combis but I imagine the flow is reduced to maintain temperature.

There are solar setups that install a thermal store prior to the combi to bring the water temp up before it enters the combi.

If my assumption about flow reducing due to temperature is correct then that would probably be a fix.

You would still be limited to about 3 or 3.5 bar though, as most of thermal store coils are limited to this sort of pressure.

 

[Munkul]

Our combi gives fantastic shower pressure. Far better than the conventional boiler/storage system in the old house. If anything, it's too powerful... and too hot!

A lot of it has to do with mains incoming pressure I think.

 

[rtcosic]

A normal domestic Combi is limited to 37kW input (I think). This limits the amount of thermal energy that can be transferred. Incoming water pressure is limited to 3-3.5 bar by the pressure reducing valve - unless you are unfortunate to have a low mains supply pressure! I think some Combis might not even be capable of delivering 37kW.

Assuming the Combi can deliver 37kW then:

mass flow = Q/(delT *SH) = 37/(40*4.2) = 0.22kg/s = c. 13 litres per minute if my arithmetic is right.

My system delivers far more than that and it can supply 2 showers simultaneously.

First-time visitors have been known to complain that they have to turn down the shower flow rate because the jet hurts!

I'm spoiled - but I do like a GOOD shower.

 

[RonA]

Hi rtcosic,
When we had our combi fitted many years ago, we retained the hot water cylinder cold water header tank & three way valve.
The hot water cylinder acts like another radiator controlled by the three way valve. The DHW to the rest of the house is fed from the combi.
The cold water tank now feeds the hot water cylinder & then the outlet to one side of a double headed pump, the other inlet side of the pump is fed from the cold water tank. The double headed pump feeds a thermostatic shower mixer.
Result is you get a very powerful shower & the usual benefits of a combi. Only downside is your shower time is limited by the capacity of the hot water cylinder.
RonA

 

[Kayos]

It can be done but needs piping and the controls wiring correctly, get it wrong and you will damage the boiler and/or be at risk of legionella

 

[Gareth J]

If planning to future proof so you can store solar, you might as well plan to tie in all hot water, if not heating as well, in summer, 3-4kw of panels will do all of your DHW needs. (Well, most of)

 

[rtcosic]

Thanks all, especially @RonA for the real world example - seems just as I expected.

 

[Mikes]

A few thoughts that might assist :
Sometimes it's the shower head that is the problem and not the combi. The push to lower water usage has meant that many of the heads are very restricted (Eco Homes Green sh*t) (some even have a flow restrictor within them that naughty people remove after they have been inspected). I'm not a lover of combi boilers but some of the better makes do have very good performance but this normally means that the boiler is very much oversized for the heating load.
Obviously difficult to check on a house that you are interested in and not actually purchasing but before adding a cylinder, you need to establish if the heating side of the boiler is being run at suitable temps. To give you stored water at 60 C, your boiler flow temperature needs to be probably 70 C plus to get the necessary heat transfer and avoid legionella. Could be that its running at lower temps to get the benefit of condensing mode.
As very recently announced, the use of gas for a primary heat source is going to be a no no so it might be better to look at a cylinder purely heated by electricity from some PVs with a top-up immersion . The more of your free PV electric that you can use yourself, the better. You would need to check your water pressure but if suitable, you could have a mains fed cylinder. Most shower manufacturers have now made their units with much lower pressure loss so work with good flows even at relatively low pressure.
This would then leave you half way there when the green gestapo come knocking and insist that you have a nice electric heat pump.

 

[8ob]

As very recently announced, the use of gas for a primary heat source is going to be a no no so it might be better to look at a cylinder purely heated by electricity from some PVs with a top-up immersion . The more of your free PV electric that you can use yourself, the .

I read somewhere the government has done a uturn on that Mike.

Bob

 

[mtt.tr]

It can be done but needs piping and the controls wiring correctly, get it wrong and you will damage the boiler and/or be at risk of legionella

That legionella tastes wonderful on toast

 

[mtt.tr]

I read somewhere the government has done a uturn on that Mike.

Bob

Damn, i was up for burning witches

 

[Mikes]

I read somewhere the government has done a uturn on that Mike.

Bob

Think it's because they are being told that its difficult to provide alternatives that are effective. It is however noticeable that the major players in the domestic boiler market are nearly all promoting their ranges of domestic heat pumps now. Don't think gas boilers will disappear for replacements in the existing housing stock but can see it happening very soon for new-build to set the ball rolling. Just thinking that if you are going to the expense of adding a new cylinder and some PVs then it would be most cost effective to have a good demand on the system and will also hopefully give some degree of future proofing.

 

[Barking Mat]

It can be done but needs piping and the controls wiring correctly, get it wrong and you will damage the boiler and/or be at risk of legionella

Is that something related to Nigella Lawson?

Or do you mean Legionaires Disease?

 

[8ob]

Think it's because they are being told that its difficult to provide alternatives that are effective. It is however noticeable that the major players in the domestic boiler market are nearly all promoting their ranges of domestic heat pumps now. Don't think gas boilers will disappear for replacements in the existing housing stock but can see it happening very soon for new-build to set the ball rolling. Just thinking that if you are going to the expense of adding a new cylinder and some PVs then it would be most cost effective to have a good demand on the system and will also hopefully give some degree of future proofing.

With the way modern buildings are insulated I would imagine heating requirements will be minimal by the time gas boilers are phased out. Insulation has to be the way forward.

Bob

 

[brightspark]

With the way modern buildings are insulated I would imagine heating requirements will be minimal by the time gas boilers are phased out. Insulation has to be the way forward.

Bob

insulated to the hilt draught proofed then they stick air vents on to your windows to let draught and fresh air in

 

[Kayos]

Is that something related to Nigella Lawson?

Or do you mean Legionaires Disease?

Legionella is the bacteria that causes Legionnaires disease, so incorrectly heated or stored water allows the bacteria to breed


https://www.google.com/search?q=leg...msung-gs-rev1&sourceid=chrome-mobile&ie=UTF-8

 

[mtt.tr]

Legionella is the bacteria that causes Legionnaires disease, so incorrectly heated or stored water allows the bacteria to breed


https://www.google.com/search?q=leg...msung-gs-rev1&sourceid=chrome-mobile&ie=UTF-8

Does that need explaining. Geez

 

[Parm]

That legionella tastes wonderful on toast

only affects middle aged men so you’re ok