Hybrid system layout to ponder

[Beelbeebub]

Was chatting with heating engineer brother in law at a family get together.

 

He's shortly going to be upgrading his home system.

 

He currently has a Vaillant combi boiler rigged up as a system boiler (ie. the DHW part isn't connected)

 

His heating is via rads and DHW is via an unvented cylinder with a fairly large coil.  It also has a secondary coil for a solar system that provides almost all their DHW in the summer.

 

The upgrade involves putting UFH throughout the ground floor.  The upstairs will remain on rads.  They are all fairly oversized k2 units, with one exception which can be upgraded.

 

The plan or the heating would be to have 2 zones, upstairs and downstairs both running at the same low temps ie, less than 40C

 

He was pondering sticking a heatpump in parallel with the existing boiler so it could be run when it was most cost effective (i.e. when the cop is greater than 3.5ish making the p/kwh lower than gas)

 

to that end the HP wouldn't have to be large one, possibly a 5kw Vaillant.

 

It probably wouldn't be called on to do DHW as that is almost always cheaper by gas given the temperatures involved, though if he gets solar PV that may change.

 

So we came up with this very simple layout

 

 

Sources 1 & 2 are the Heat pump and Boiler respectively and both have their own pumps

 

There are 3 zones.

Z0 is the DHW and will require a flow of around 65

Z1 & Z2 are the UFH and Rads.  They are lower temp, weather compensated.

 

there are 6 x 2 port zone valves

 

All the zones link back to a common return line which feeds both heat sources

 

This should allow him to chose which heat source heats which zone.  It can allow for simultaneous heating and hot water from both sources.  He can heat the rads with gas and the UFH with HP.  Or the entire heating system with gas.  Eventually, when the gas boiler keels over, he might replace it with a second heatpump.  This would give him really high modulation using just one HP in low demand periods and both together when he needs the power.

 

Anyway, it was all a bit of idle speculation over mince pies - he is not experienced with HPs, though he has lots of experience with gas/oil/lpg systems

 

I though i would throw it open to comment, see if anyone can spot any obvious flaws or suggest improvements etc.

 

Merry Christmas all.

 

[JohnMo]

Have had mine running hybrid mode when we had cold spell.

 

Can see two issues

 

Two pumps running without hydraulic separation. I would choose which you want as master so the one that runs most the time. The other introduce via close couple tees.  If you are running WC the circulation pump will run all the time and circulate through the system when either heat source runs.

 

The next issue is when one heat source is running the other never gets permission to start as the return temp is always to high. So you have to be careful with settings.

 

[RobLe]

There might be an issue with a heatpump being force fed super hot water into its heat exchangers, which a gas boiler could do.  Doubt an issue in practice, but let’s say you pushed in water at 90C then the refrigerant pressure might end up above the heat exchanger design pressure.  Maybe.

 

We ran with a heatpump and a gas boiler for a year and a half, until Mrs RobL was convinced the new fangled (diy) heatpump was ok.  The electrics totally switched one or the other in with a 4 way rotary switch, the water flow was magnetic filter->gas boiler->heatpump, and the heatpump flow & return could be closed off and bypassed.

The gas boiler was eventually taken out, as it was never used, gas meter gone, no standing charge.

[Iceverge]

One or the other would be my vote. 

 

The cheapest thing would be keep the rads and boiler. Spend the savings on dMVHR, airtighess and insulation. 

 

 

[JohnMo]

17 hours ago, Beelbeebub said:

Was chatting with heating engineer brother in law at a family get together.

 

He's shortly going to be upgrading his home system.

 

He currently has a Vaillant combi boiler rigged up as a system boiler (ie. the DHW part isn't connected

Hooking things together is pretty easy, if you have two pumps hydraulic separation is needed.

 

The other bit that occurred to me was how has he rigged the current setup? If S or Y plan he will need to rearrange to something like X or W, a PDHW plan. Or convert the combi to a 4 pipe boiler. So the existing DHW side of the boiler directly connected to cylinder coils, with pump (call for heat starts pump), expansion vessel, pressure relief filter. You can then easily run independent flow temps or WC on the CH side and fixed flow temp for cylinder.

 

The other way to connect boiler and HP is in series. Run both with the same WC curve. HP curves stops at say 4 degs outside and the boiler curve continues to -10. Boiler would automatically pick up heating duty as HP flow temp flat lined. So you have a temp range where both hp and ASHP run together, then the ASHP would naturally switch off. Complication is DHW.

 

Control

My hp has the controller built-in for hybrid, but found it wasn't really doing what I wanted, so switched that side off. So heat pump is not aware it's running hybrid mode - used home assistant instead. Have 2x generic thermostats (home assistant), one for boiler and one for heat pump, and a schedule (home assistant). Rigged up an outside temp sensor within HA. So optimised based on likely running costs based on OAT and E7. Then set up decisions based on OAT and tariff to make either HP or boiler run. DHW is done in pure hybrid mode with both boiler and HP running, this is just hard wired and a dumb relay.

 

If internet falls over have a normal thermostat to pick things up on ASHP only, just need to move setpoint (20) up to 20.5 and normal service is resumed. Boiler is then off for heating, but stays hybrid for DHW (all hard wired).

 

You can thermostats that do all the above in the USA, but the same thermostat in UK is designed for S plan, so doesn't do much for you.

[Beelbeebub]

Hope you all had a nice Christmas! 🎁

 

Anyway, thanks for the comments.

 

Hydraulic separation:

 

It's unlikely (as in it could be locked out in the control strategy) that two sources (HP & Boiler) would ever be connected to the same zone so the pumps would not be interacting anywhere except the return line.  As long as there is a close coupled tee/LLH or even buffer in the return at the split to each source I don't think there would be any issues?

 

Return temps :

 

This might be an issue if the boiler is running high temps and flow to the DHW and the coil isn't big enough to dump the energy so the return temp is higher than the HP wants and it never starts (as you said). However, in this case wouldn't the warmer water just flow through and heat the heating zone anyway. This would be a slight ineffiicency as you would effectively be using the gas boiler to heat the house when you could have been using the heat pump. On the other hand, you'd probably get a bit more efficency from the boiler as the rerun temp would be lower?  If this situation was a major concern it could be avoided by not running the HP when the boiler is running or at least when the return temp is too high.  This would also be used to prevent too hot water ever reaching the HP and any concerns about refrigerant over heating/pressure.

 

Current set up.

 

He has a Y plan I believe, but it's not a major issue as the current plant room will have to be moved and reconfigured during the upgrade (they're extending their kitchen into the current utility room) and he'll be repiping to the upstairs and of course the new UFH.

 

The series layout is intriguing but the idea is the system would initially be just the boiler but piped out so adding a HP later would be trivial. I can see there would be a slight efficiency advantage when there is overlap but, as you say, the DHW might be problematic.

 

Control

 

This is the heart of the issue. He'd need to find a control system to do all of this. I'm not sure if the vaillant controllers can and if they can't, can they play nice with a 3rd party system that can?

[joth]

Couple thoughts

 

Running Cost savings 

 

Forget timing ASHP usage for when COP is above 3.5.  You won't know when this is without firing it up and measuring and then if it's not at the target you've wasted electricity finding out.

Instead get a flexible or dynamic import tariff, e.g. octopus Go or eon Next which is 7p/kWh for 7 hours. This gives 2-3p per unit of delivered heat for almost a third of the day: not just cheaper than gas, but cheaper than gas heating has ever been.

In a well built house this can provide 99.99% of the heating requirement.

 

Controls

 

Having setup a few complex systems I can't imagine doing it with anything other than Loxone or an equivalent customisable and programmable system. This integrates with everything and can fully automate selecting the most cost effective solution at the time. Also integrates well as an Aircon control if that's ever ok the cards 

Not cheap and a serious learning curve (or more cost in having someone else set it up). But a mini server compact, modbus extension and 32 waveshare relay could be done for under £400.

 

 

 

Edited Thursday at 09:14 by joth

[JohnMo]

11 minutes ago, joth said:

£400

I have two shelly relays and a single automation in Home Assistant so doesn't need to be costly.

 

Would prefer a control system that was totally offline through.

 

 

[joth]

Totally agree I avoid anything with public internet dependency. Where possible I avoid wireless and network dependency of any sort for critical functions like heating and lighting, which tends to steer towards a hardwired PLC like Loxone for those things rather than Shelly / HA.

 I have a growing army of shelly devices and been running Home Assistant for 8+ years, it's great for a lot of stuff but find it takes much more hands-on tinkering to keep working, ymmv

[SteamyTea]

15 minutes ago, joth said:

I avoid anything with public internet dependency. Where possible I avoid wireless and network dependency of any sort for critical functions

Very wise.

 

Don't think there are any cars or aeroplanes that use wireless technology for critical functions.

Maybe @TerryE knows different.

[JohnMo]

54 minutes ago, joth said:

Where possible I avoid wireless and network dependency of any sort for critical functions

I am starting to steer towards.

 

One thermostat (with opentherm) for boiler, such as a Tado (Euro model). Just time to boiler run when HP is expensive. Then another for HP. HP one would do heat and cool.

 

@Beelbeebub could then just configure the existing boiler to do DHW cylinder heating via the existing and not used cold and hot DHW outlets. Add a pump, filter expansion and pressure relief. Then zero interference with CH system.

[Iceverge]

I absolutely love tinkering, inventing, thinking, experimenting. 

 

However the above setups are barmy. 

 

Internet connected, shelly controlled,  bespoke hybrid, Loxone, mini servers, mod box extensions.  If you die, get infirmed or sell you house NOBODY WILL FIX IT. 

 

There was a tale on here of one of our members very sadly widowed. Her late husband's similarly complex and bespoke setup broke and she was left with no heating. Nobody local could mend it. Very kindly a couple of good souls , I think @PeterW and @Nickfromwales drove to Scotland to fix it up. 

 

Keep the core mechanics of a house absolutely as simple as possible. The least tech savvy house member should be able to operate them with ease after 3 bottles of wine.

 

If you must stray into complexity make sure that it is an off the shelf variant that there's enough public knowledge available hat Google will allow you to repair it easily. 

 

 

 

 

 

[JamesPa]

59 minutes ago, Iceverge said:

I absolutely love tinkering, inventing, thinking, experimenting. 

 

However the above setups are barmy. 

 

Internet connected, shelly controlled,  bespoke hybrid, Loxone, mini servers, mod box extensions.  If you die, get infirmed or sell you house NOBODY WILL FIX IT. 

 

There was a tale on here of one of our members very sadly widowed. Her late husband's similarly complex and bespoke setup broke and she was left with no heating. Nobody local could mend it. Very kindly a couple of good souls , I think @PeterW and @Nickfromwales drove to Scotland to fix it up. 

 

Keep the core mechanics of a house absolutely as simple as possible. The least tech savvy house member should be able to operate them with ease after 3 bottles of wine.

 

If you must stray into complexity make sure that it is an off the shelf variant that there's enough public knowledge available hat Google will allow you to repair it easily. 

 

 

 

 

 

I agree with this view (I too love tinkering etc).  Complex stuff that nobody else understands is fine if its not 'mission critical' - heating is definitely mission critical to a house.  Therefore the least techy person in the house needs to be able to operate it and the local plumber, or at worst the authorised agent, must be able to fix it.  There is nothing wrong with add-on complexity for monitoring, but having basic functionality which relies on such complexity is inviting problems IMHO.  To my mind these considerations come close to ruling out hybrid systems altogether, although a hybrid where the boiler is controlled by the heat pump controller is probably OK or @JohnMo s suggestion (but with some notes left for a repairer in an accessible and visible place (eg taped to the boiler and heat pump) so they know how its intended to work - which otherwise may not be obvious).

 

At a much less mission critical level, we rip all our music to a NAS so we can enjoy it from several places in the house and control it with various devices.  My wife can operate this (and loves it) but she couldn't maintain it.    We therefore keep the CDs and a CD player so that there is a fallback should the NAS (or more likely me) become unavailable (it also serves to prove we have the rights to use the music under copyright law).

 

Obviously its all a personal choice but perhaps one that should be made thinking of the others in the household.

Edited Thursday at 14:30 by JamesPa

[SteamyTea]

7 minutes ago, JamesPa said:

we rip all our music to a NAS

Why.

Only one song is needed to remind us all about how tragic life is.

 

 

[JohnMo]

Got to agree with everything above. Mine is a pure tinker because I still have the boiler. Doing things in home assistant is pretty nuts, 99% of the population no clue. You can buy just about any good Honeywell thermostat (in the States) that does this stuff out the box. But they just don't work in the UK because everyone drummed down to S and Y plan compatibility.

[Beelbeebub]

I have to say, I would want the core heating control in the house to be independent of Internet, wireless etc. I'd also want the interfaces between the major parts to be a simple as possible. In 10/20 years time I dint want to be tracking down an obsolete part just to avoid ripping out the entire system.

 

To be honest the simplest interface is a volt free contact at one end and 240ac or 24/12v dc circuit.

 

Ideally a simple switch/relay to indicate DHW demand for each source. The flow temp being an inbuilt setpoint function of the source and a separate switch/relay to indicate heating demand  the setpoint ideally being an inbuilt WC function from the source.

 

For user controls the DHW just needs time to be active (a simple timer) and a setpoint temp. Possibly a disinfectant cycle on a timer as well.

 

For heating, if using WC a simple "hotter/colder" dial for the user thar adjusts the WC curve would be easiest.

 

No touch screens, no menus, no Internet or server's.

Edited Thursday at 15:38 by Beelbeebub

[JamesPa]

53 minutes ago, Beelbeebub said:

For heating, if using WC a simple "hotter/colder" dial for the user thar adjusts the WC curve would be easiest.

 

Interestingly this is pretty much what Vaillant implements, albeit with a touch screen either wirelessly (but independent of the internet) or physically connected. The way they have done it has the effect, IMHO, of making WC accessible to the ordinary person.  You think you are turning the room temperature up and down, which you are, but actually by adjusting the WC curve.  Others may of course do similar but it seems to me to be a major advance on the basic 4 point adjustment some machines feature.

[joth]

2 hours ago, Iceverge said:

Internet connected, shelly controlled,  bespoke hybrid, Loxone, mini servers, mod box extensions.  If you die, get infirmed or sell you house NOBODY WILL FIX IT. 

 

 

Well that part I've emphasised is not true: keep with a Loxone install (per my original suggestion) and there's hundreds of certified partners in the UK that will maintain it after you're dead and gone. They are more concentrated in the affluent south so definitely avoid it if in the North, for now. OTOH if you're in Netherlands you can't leave your front door without tripping over one lol

(I don't know what a mod box is, but the modbus extension is a standard Loxone part any competent installer will know their way around)

 

To the wider point, totally agree KISS is much preferred to complex controls, but that ship sailed long ago in this post: adding ASHP to a existing dual fuel Gas + Solar thermal system is anything but simple. Hence the suggestion to use one of the more (the most?) established control systems for more complex domestic HVAC scenarios.

 

[JohnMo]

Just done some simplification to mine - touched nothing more than a keyboard and the existing thermostat for the heat pump.

 

The only thermostat in the house - this connected to ASHP, timings changed slightly to batch charge during E7. Outside E7 times thermostat simply set to 20.

 

Home Assistant thermostat - this is connected to boiler (may wire a spare normal thermostat in at some point) allows boiler to fire if E7 is in peak period and house drops below 20.6. If internet goes down, heat pump kicks back in at 20 degs as a backstop. So no one is going to freeze.

 

Due to hydraulic separation via a PHE, boiler could be removed tomorrow and have zero effect on the heat pump running. Could even be removed while heat pump is online.

 

So you don't need anything complex really, mine is in fact just two thermostats operating at different times. Ideal is one thermostat with two outputs that can be operated independently of each other.

 

With no one looking after HA, worst case is the thermostat times are set continuous on the heat pump. Boiler never starts. House would be warm.

[Iceverge]

Ok rereading the original post it seems to be more of a "could" it be done rather than "should" it be done. 

 

ASHPs are really good in 3 ways I think. 

 

1. You can rid yourself of a gas line and the associated standing charges/storage tank. 

2. Heating larger volumes of water by a small amount. 

3. Cooling. 

 

They are bad at:

 

1. Heating water by a large amount of to high temperatures. 

 

Gas boilers on the other hand are:

1. good at heating stuff to high temps.

2. High power outputs 

 

 

In the ideal setup I would say you would preferentially take care of all low temp heating by the ASHP. 

 

Without working out the details maybe a very tall thin high volume thermal store. Feed the ASHP in the bottom. The gas above it. Take the heating for the UFH from lower down than the rads. 

 

Extract your DHW from a coil near the top. 

 

 

[JohnMo]

3 hours ago, Iceverge said:

They are bad at:

Plus - Having to defrost in humid areas. Takes a hit on CoP and heat delivery.

[JohnMo]

Just been thinking and in it's simplest form, you would run both the boiler and heat pump on the same WC curve. All could be hard wired.

 

Time based switching (cost) - £60

Then use a simple single channel time switch to deliver a run permission to either the HP or boiler. HP connected to the NC connection and boiler NO connection, with the other zero volt switching wire to common. 

 

Temperature based switching (heat pump capacity limited) - £100

Same as above but replace time switch for external thermostat.

 

Time and temperature switching (combination of cost and capacity limit) - £160. combine the two above.

 

 

[Beelbeebub]

2 hours ago, JohnMo said:

Just been thinking and in it's simplest form, you would run both the boiler and heat pump on the same WC curve. All could be hard wired.

 

Time based switching (cost) - £60

Then use a simple single channel time switch to deliver a run permission to either the HP or boiler. HP connected to the NC connection and boiler NO connection, with the other zero volt switching wire to common. 

 

Temperature based switching (heat pump capacity limited) - £100

Same as above but replace time switch for external thermostat.

 

Time and temperature switching (combination of cost and capacity limit) - £160. combine the two above.

 

 

Some thing like this would be preferable. That way any of the components can be swapped out simply.