Using an indirect tank/cylinder as direct, i.e. without connecting coil

[piggoy]

Hi all,

We've just moved house and our new place has an old vented/gravity DHW system using a loft header tank + airing cupboard hot water tank with dual immersion heaters. The property is not on mains gas and is electric only. My main gripe with the system at the moment is the lack of hot water pressure due to the gravity fed header tank setup. In the near future we would like to upgrade the whole house to an air source heat pump with direct (mains pressure) unvented tank for DHW. However, due to lack of funds at present it will be a few years before we can do this upgrade.

So, at the moment I am considering our temporary options for improving the DHW pressure.

Option 1: Fit a shower pump. This would seem to be the cheapest temporary solution but I'm not convinced by the quality and reliability of the pump offerings (Salamander etc).

Option 2: Upgrade the tank to a direct unvented system. This is what I am most interested in exploring. However, our future ASHP upgrade would require an indirect tank with a coil fitted. My question: Would it be possible to fit an indirect tank (one with a coil which is suitable for an ASHP) and use it for a few years as a direct system (immersion heaters only) with the coil not connected? This option could turn out to be quite cost effective since we will need the indirect tank anyhow in the future when we do the full ASHP upgrade.

Thanks

 

[cross thread]

Yes you can use it without coils in use

 

[Exedon]

As above use without coil connected no problem.
Hot water boost pump look at stewart turner monson.
Fit no other my own at home over 20 years old zero problems.

 

[Madrab]

You may have to search hard for an indirect HW UV cylinder that has 2 immersion element unless it's really large. The immersion element in an indirect is usually for backup and they tend to only have one.

As suggested, ST for the pump is highly recommended.

 

[ChrisJP]

Can air source heat pumps produce water hot enough to circulate through the tank coil and heat the DHW adequately? I thought that houses that use ASHPs for their heating relied on electric immersion heaters for their hot water.

 

[Madrab]

Can air source heat pumps produce water hot enough to circulate through the tank coil and heat the DHW adequately? I thought that houses that use ASHPs for their heating relied on electric immersion heaters for their hot water.

In the main yes they can heat it hot enough to be usable (most domestic HW requirements are ~ 45deg) but they do have an electric boost to allow the HW to be heated to ~60deg once a week for sterlisation

 

[ChrisJP]

Ahh...I see.
Still, 45deg water wouldn't suit me and, because you'd use this undiluted with cold water, you'd use the contents of the HW tank much more quickly wouldn't you?
I guess the weekly 60deg heat up using a traditional electrical resistance heating element is mainly to guard against legionairres disease bacteria building up, and I have to say I wouldn't be confident about a once per week heat up being adequate to guarantee no such buildup.
Seems to me if you want to use an ASHP you do really need to use an electric immersion heater for your hot water.

 

[JohnD]

It does not grow at 45C or above.

It is killed at 50C and above, the hotter the water, the faster it dies.

The 60C recommendation gives an extra margin of safety.

If the tank is fed from the watermain, how will contamination get in?

 

[piggoy]

Thanks all for your replies.

I think I need to do some more research on the cost effectiveness of using an ASHP to heat DHW. 45degC doesn't sound great, especially if you've got to run an immersion cycle one a week anyhow to kill off bugs. Furthermore, from what I understand the efficiency of the ASHP is a lot lower when heating water to the highest possible temperatures.

Perhaps the better solution afterall is to install a brand new direct dual immersion cylinder and have the future ASHP install do the central heating only.

 

[Madrab]

Most if not all bacteria is destroyed at the treatment plant before the (mains) potable water gets to you. Of course, sometimes not all bacteria is destroyed and when there are instances where the water is allowed to stand at an optimum temperature for longer periods of time that can then promote bacteria growth. A HW cylinder at too low a temp can be a premium breeding ground

Taking stored hot water up above 50degC is a safety procedures just to ensure that if there are any bacteria that they are then destroyed. That is the temp that bacteria start to die. 60 degC is considered the ideal as that can then deliver >50deg at tap, therefore running the hot water to all outlets, even ones that are rarely used will then sterlise the whole circuit. Key areas are showers, longer HW runs, bathrooms that are rarely used and longer lengths of pipes that have been capped off (dead legs).

A direct cylinder can be an expensive way of heating water is peak time use is high but that can be offset if PV panels are utilised or a large cylinder with E7/E10(rare) tariffs.

 

[robt100]

Apologies for resurrecting and older thread, but my question was on similar lines so thought it best to carry on here...

We currently have a (3 year old) unvented indirect hot water cylinder, with the heater coil fed by the oil CH system. Our oil tank is beyond repair, so we will be switching over to either ASHP or Air to Air. Quotes for ASHP so far all include a new cylinder, so I'm guessing the current one cant be reused? (Joule Invacyl 150L). Would it still be ok to just disconnect the coil and run direct-only, or as it has already been used indirect, is there hidden issues I may not be foreseeing? Also, I assume you just leave the ends open and take the hit of the heat loss rather than capping them as you wont want the risk of pressure buildup etc?

 

[polesapart]

No problem whatsoever with draining the heating coil of an indirect cylinder and leaving it empty and unused..

 

[Rodders53]

AIUI the main reason why the existing unvented would need replacing is the heat up time for its coil with the heat pump... the ones designed for heat pumps have greater surface area in the HW cylinder:
Yours https://jouleuk.co.uk/products/unvented-cylinders/invacyl has a surface area of 0.59 sq m and a 10kW rating (likely at 70 C).

A quick look-see suggests their Kodiak range is used with their Samsung ASHP packages... with a coil surface area of 1.27 sq m (double yours) and a rating of 27.3 kW.

So must be worth challenging/discussing with the potential suppliers whether it's possible to use it (or if the Govt Grant you may be eligible for specifically forbids that option).

NOTE a single immersion HW alone will leave you with no HW when (not if) it fails... it's a 'backup' for a reason!

Check out the Moneysaving Expert ASHP forum for a similar query... it's likely to have come up before and there are some really knowledgeable types on there, as well as here.


Also check the cost of a replacement oil tank to the latest standards if the boiler is not also on its last legs. {With all the appropriate fire safety precautions}. Just to cover all bases.

 

[Madrab]

The latest ASHP's with their newer refrigerants can now obtain much higher flow temps, up to 70deg, which the industry has now designed, removing the need to incorporate new upgraded radiators and HW cylinders into the install, as it was with the older, cooler HP's and allow a much easier retrofit/transition.

The only reason I could see why any existing cylinder wouldn't be suitable would be the coil size and it wasn't a fast recovery but looking at the spec, a 35min reheat time is certainly a high efficiency/recovery cylinder especially with the higher achievable system temps for the new ASHP's. It wouldn't come with an ASHP interlock, controller, and be pre-plumbed with pump, zone valves etc either though, not sure that would all be essential TBF.