aroTHERM Plus DHW Flow Temperature

[Beelbeebub]

10 minutes ago, dpmiller said:

then add a drain HE too

What sort of kw heat transfer can they support?

 

 

 

 

[sharpener]

This discussion needs to include the benefits of heating water using off-peak electricity and storing it for daytime use. As this is typically half price or less the losses are dwarfed by the savings leaving a big net gain whether or not a HP is part of the process.

 

Power showers and "rain showers" are fashionable but use dreadful quantities of HW. Will perhaps try one of the aerated shower heads. (Saving water is also a consideration though the rainwater tank is unseasonally full just now.)

 

But so far I haven't read anything that suggests I will be better off with anything more complicated than keeping the existing conventional small-coil UVC, heated to 55C with a HT HP*. At the same (off-peak) times I plan to heat a separate 200l TS for the radiator circuit, that alone will halve the cost of some 360kWh(e)/yr and should act as a big parallel load so the HP will be happy.

 

Except that my prospective installers do not understand and/or like the idea.

 

 

*This is the temp the PV diverter immersion is currently set to. Anything higher gives a green deposit in the shower tray which is to be avoided for obvious reasons.

 

[Beelbeebub]

A typical shower uses 10lpm. With low flow aerating head we can get that down to 9lpm fairly easily.

 

A reasonable sized heat pump can punt out 9kw at 45C using 3kw of power.  If that flows into the "buffer/TS" vessel it should be able to heat the incoming mains by 14C.

 

A 10kw instantaneous heater on top of that will raise another 15C for a total rise of 29C

 

That should be enough to raise 9lpm by from 10C (minimum) to 39C, pretty much where you need for a shower.  

 

Reduce the flow down to 8 and the total rise becomes 33C for a 43C shower (and that assumes a fairly cold incoming main).  

 

In the summer the HP can kick out 10 or even 11kw.

 

And a 9 kw HP, especially with a 50-100l buffer vessel should be small enough for all but the smallest properties.

 

A rectangular 50l buffer vessel with a 10kw instantaneous heater could be packaged into a 85l box about 0.7x0.4x0.3 ie about the size of a boiler.

 

So you have a sub 1mx1mx0.4m monoblock outside connected to a "boiler" in the same place as the combi boiler.  The "boiler" is fed by a 40A spur (shower) and the HP by a 20A spur.

 

You get infinite shower water at 8lpm and run your CH at whatever flow temp you like (eg use weather compensation), though higher flow temps would give better DHW performance at the expense of efficiency.

 

The only issue would be your HP hitting defrost at low temps.  In this case your would still get hot water but the flow rate might have to drop whilst the 9kw from the HP is removed.  If the defrost cycle was short enough and didn't exhaust the buffer, you would still be able to operate, just that the flow rate would be limited by temp rise achievable by 10kw plus whatever the average output of the HP was over the charge/defrost cycle.

 

Of course a bigger buffer/TS would help but that would be used where a large cylinder wasn't a problem, the small buffer would be specifically for "like for like" Combi replacements 

 

[JohnMo]

Alpha boilers make/made such an animal, but for preheat to a gas boiler.

Alpha Flowsmart 25 and 50. Designed to go in the heating circuit and has a DHW coil.

 

 

[dpmiller]

I reckon we're all a bit spoilt these days ref. showers. Our first one was 4.5kW and do you know what? It worked...

[JohnMo]

53 minutes ago, dpmiller said:

4.5kW and do you know what?

So was mine. The next house had 120L uninsulated vented cylinder and that was in a four bed house, that was fine also, didn't know any different then. If we ran out of water, the immersion was flicked on for 1/2 hour.

[JamesPa]

@Beelbeebub shows that instant DHW is possible.  Maybe we can't expect to fill two baths or have two showers simultaneously, but really is this necessary?  Good work!

 

I'm now pretty much convinced that the best dhw system for a hp retrofit where there is already a cylinder is the one you already have (with a retrofit PHE if the hp cant get to 70C).  It's clear that the 'perfect' system hasn't been invented so why change to an alternative set of imperfections?

 

Nevertheless I have been trying in parallel to reimagine the vented cylinder.  I'm thinking 

 

Rectangular footprint so it fits in a cupboard 

Fully sealed except the overflow 

Pumped hot water (with option for matched cold water to deal with combining outlets).  Pump would need a small accumulator so that the tank could be below the outlets.

No header tank

Overflow to convenient drain or outside if available at user option

 

So basically an insulated cistern.

 

I'm struggling with:

 

a. the fill mechanism 

b. keeping the vent hygienic to avoid the 'rats' objection

 

Basically the full needs to be like a cistern, but with bottom entry for stratification.  I'm not sure if there is a mechanical valve that does this and will work in a hot environment, it could perhaps be done with a level sensor and electric valve but mechanical would be better.  Maybe a metal bodied torbek valve with a tube from outlet to bottom?

 

Keeping the vent hygienic might be just making it small enough and putting a stainless steel mesh on it.  Certainly that is better than an open, or even lidded, header tank in the loft.

 

 

 

Edited August 8, 2023 by JamesPa

[JohnMo]

47 minutes ago, JamesPa said:

Nevertheless I have been trying in parallel to reimagine the vented cylinder.  I'm thinking 

 

Rectangular footprint so it fits in a cupboard 

Fully sealed except the overflow 

Pumped hot water (with option for matched cold water to deal with combining outlets).  Pump would need a small accumulator so that the tank could be below the outlets.

No header tank

Overflow to convenient drain or outside if available at user option

 

Is this one they inverted earlier - but round. Available in direct and indirect. Filling controlled by ballcock.

 

https://www.screwfix.com/p/rm-cylinders-indirect-vented-combination-cylinder-85ltr-900-x-450mm/58056?tc=TA5&ds_rl=1249404&gclid=Cj0KCQjwz8emBhDrARIsANNJjS5MEl-8asvGrJRypWI7b_-_BttGe55t0lnawxdP2fAMuuTmHtrhWFgaAgM3EALw_wcB&gclsrc=aw.ds

[JamesPa]

8 minutes ago, JohnMo said:

 

Is this one they inverted earlier - but round. Available in direct and indirect. Filling controlled by ballcock.

 

https://www.screwfix.com/p/rm-cylinders-indirect-vented-combination-cylinder-85ltr-900-x-450mm/58056?tc=TA5&ds_rl=1249404&gclid=Cj0KCQjwz8emBhDrARIsANNJjS5MEl-8asvGrJRypWI7b_-_BttGe55t0lnawxdP2fAMuuTmHtrhWFgaAgM3EALw_wcB&gclsrc=aw.ds

Not quite, this has a cold header tank above.

[JohnMo]

Stubbled across this MCS cylinder sizing guide 

Domestic_HW_cyl_selection_guide.pdf

[JohnMo]

1 hour ago, JamesPa said:

Rectangular footprint so it fits in a cupboard - easily doable

Fully sealed except the overflow - isn't that an UVC 

Pumped hot water (with option for matched cold water to deal with combining outlets).  Pump would need a small accumulator so that the tank could be below the outlets. Starting to get complicated and more expensive 

No header tank - cannot see any alternative to some form of heater to allow building regs compliance 

Overflow to convenient drain or outside if available at user option - easy done

 

So basically an insulated cistern. That's heated without anywhere to expand into

 

I'm struggling with:

 

a. the fill mechanism - simple and tried and tested is a ballcock

b. keeping the vent hygienic to avoid the 'rats' objection

 

Basically the full needs to be like a cistern, but with bottom entry for stratification.  I'm not sure if there is a mechanical valve that does this and will work in a hot environment, it could perhaps be done with a level sensor and electric valve but mechanical would be better.  Maybe a metal bodied torbek valve with a tube from outlet to bottom?

 

Keeping the vent hygienic might be just making it small enough and putting a stainless steel mesh on it.  Certainly that is better than an open, or even lidded, header tank in the loft

Some comments in bold above

[Beelbeebub]

4 minutes ago, JohnMo said:

Some comments in bold above

I think @JamesPa means, is 

 

"Fully sealed" - a tight fitting lid for hygiene rather than pressure sealed.

 

The pumps are commodity items, they have a small accumulator and check valve built in. Fit and forget. If it breaks, swap out.

 

"No header tank" - i think he means not the traditional large F&E tank in the loft.  If you have a pumped system you don't need the F&E tank in the loft anymore.

 

It doesn't need to be as big as an 8l capacity would be ample to account for expansion.  That could be easily integrated into the top of the tank even if it's nitnthe highest point in the system.

 

The ballcock would probably do, but you'd probably want something a bit more advanced, both to start and stop quicky and to deliver high flows quietly.  Maybe something more like modern toilet fill valves (also more compact).

[Beelbeebub]

This is just for small volume (sub 50L) "boiler" wall box.  To provide as close to a "combi" experience as possible.

 

If you have space for a tank in the airing cupboard, then a bigger version with or without the electric heater is possible.

 

If we ditched the requirement for "unlimited hot water at high flow rates" it makes the whole heatpump installation a lot easier.  No diverter valves, no G3, no overflows, no tank primaries just a 10kw box under the sink. This would also make air to air heatpumps very competitive.

 

One interesting possibility would be if home batteries become more of a thing, a 10kwh battery could supplement the 10kw mains for 1 hour and provide enough electrical power for a 20kw instantaneous water heater.  The 10kwh battery would also  work with the home electrical system to provide load shedding, uninterruptible power, PV storage and all the good things home batteries can provide.

[sharpener]

2 hours ago, JamesPa said:

Basically the full needs to be like a cistern, but with bottom entry for stratification.

 

This was definitely a feature on the old Fortic tank (very similar in principle to the RM one from screwfix upthread). There was a cold fill pipe which went from the bottom of the cold tank to the bottom of the hot tank like @Beelbeebub has just drawn, and also a vent pipe from the top of the hot tank to the top of the cold tank above the water line.

 

I imagine the RM tank will have a similar arrangement but you can't see it because of the integral lagging - which the Fortic in my flat did not have, only a loose jacket.

 

It seems cheap of them not to fit a ball cock and overflow arrangement. I suppose it allows you to choose the orientation but they should at least supply compatible parts IMO.

 

I have seen the table on the MCS guide somewhere before, I think all the material on p1 under the heading "Cylinder Sizing Example: a manufacturer produces a range of 90 to 300 litre indirect domestic cylinders" is taken, presumably with permission, from one of the tank mfrs literature, maybe Gledhill or Tempest. The rather long-winded calcs were new to me.

 

I am not sure why they think  "Mixing rather than stratification is typically realised with an indirect cylinder". Maybe this is the case for HP cylinders which have long coils to get enough surface area. It doesn't happen with my old OSO 210l cylinder. The coil is right at the bottom and we can get 4 good  showers out of it before it runs cold.

 

Another reason not to replace it.

[Beelbeebub]

23 minutes ago, sharpener said:

also a vent pipe from the top of the hot tank to the top of the cold tank above the water line.

Damn! Meant to draw one but forgot! Yes a vent pipe from top of tank back into cistern.

[JamesPa]

1 hour ago, Beelbeebub said:

This is just for small volume (sub 50L) "boiler" wall box.  To provide as close to a "combi" experience as possible.

 

If you have space for a tank in the airing cupboard, then a bigger version with or without the electric heater is possible.

 

If we ditched the requirement for "unlimited hot water at high flow rates" it makes the whole heatpump installation a lot easier.  No diverter valves, no G3, no overflows, no tank primaries just a 10kw box under the sink. This would also make air to air heatpumps very competitive.

 

One interesting possibility would be if home batteries become more of a thing, a 10kwh battery could supplement the 10kw mains for 1 hour and provide enough electrical power for a 20kw instantaneous water heater.  The 10kwh battery would also  work with the home electrical system to provide load shedding, uninterruptible power, PV storage and all the good things home batteries can provide.

That plus the overflow pipe you mention later pretty much nails it.  No rats or other detritus from the loft to contaminate the tank, good pressure, no G3 and rectangular so fits with the shape of houses.  Small & wall hung to replace a combi, or larger without the 10kW electric heater where there is space.  Made of plastic so it can have mouldings to take fittings within the rectangular envelope.

 

Still better to retain existing, industry permitting.  But when existing fails I think I'd prefer this to unvented, and for the landlord use case either this or the upthread instantaneous concept by @Beelbeebubdefinitely better than unvented, unless of course there is a continental European practice that is better still.

 

Eventually I think industry has to permit the retention of existing, the current situation where essentially the government pays for the wholly unnecessary replacement of a functioning DHW system, plus some MCS overhead, is wholly incompatible with mass heat pump roll out!

Edited August 8, 2023 by JamesPa

[JohnMo]

Not really seeing anything different from the link provided to that drawn above. Except the expansion tank is very small.

 

Although similar to my previous link, this is smaller could be positioned where a combi boiler came from (900x400mm), on a simple shelf.  It has a heating coil, and everything after that is customisation friendly, maybe even get a bigger coil if you asked and if there enough space for it.

 

https://newarkcylinders.co.uk/product/newark-indirect-combination-cylinder/

[Beelbeebub]

32 minutes ago, JohnMo said:

Not really seeing anything different from the link provided to that drawn above. Except the expansion tank is very small.

 

Although similar to my previous link, this is smaller could be positioned where a combi boiler came from (900x400mm), on a simple shelf.  It has a heating coil, and everything after that is customisation friendly, maybe even get a bigger coil if you asked and if there enough space for it.

 

https://newarkcylinders.co.uk/product/newark-indirect-combination-cylinder/

Yeah, not much difference, except packaging.

 

Key bit would be that it was a "box on the wall" hanging where the boiler was. The pipes from outside would come in via the flue (possibly in an insulated duct, so looking exactly like a flue)

 

Flow and return would come out the bottom, cold feed in and DHW out would also come out the bottom.  The drain would go where the condensate line goes.  Plus a bigger 10kw freed from the consumer unit.

 

This is for the "direct swap with a combi" cases.  Gives near as possible to combi like performance and packaging.

 

People existing tanks or space for a new tank could have a UVC or VC as they desire.

[JohnMo]

The main I would question should you have DHW within the cylinder? Or a small body of heating system water?

 

The issue may be legionella if you have the body DHW floating on heating temperature via WC. This could be at the least ideal temp for period of time.

 

For it to work all year you need it to heated in the summer also. So you really need 3 way valve and thermostat, if you intend taking any heat from the heat pump. Then the volume you are taking will mean short cycling if not managed well with a 8 to 10kW heat pump.

 

Also don't assume all flue come out through the wall, mine is in the roof for example.

[Beelbeebub]

58 minutes ago, JohnMo said:

The main I would question should you have DHW within the cylinder? Or a small body of heating system water?

 

The issue may be legionella if you have the body DHW floating on heating temperature via WC. This could be at the least ideal temp for period of time.

 

For it to work all year you need it to heated in the summer also. So you really need 3 way valve and thermostat, if you intend taking any heat from the heat pump. Then the volume you are taking will mean short cycling if not managed well with a 8 to 10kW heat pump.

 

Also don't assume all flue come out through the wall, mine is in the roof for example.

All valid points,

 

But all I've described is a standard vented cylinder with an "afterburner" to boost the temp

 

So all the objections are the same as for a standard UVC set up etc. They are not differential objections.

 

As for the flue, in cases where the flue isn't in a convenient location,.you just block it up and drill a convenient hole in the wall, like you do now