Self built hybrid heating and hot water system

[JohnMo]

Plenty of information on ASHP is available on this site and very little on hybrid heating systems, i.e. boiler and ASHP. A few manufacturer designed hybrid systems are also available.

 

When we built our house it was with an Atag gas combi boiler. We then installed an ASHP mostly for cooling, but it also heated our house well last winter and provided hot water via an UVC.

 

The system below could easily be replicated by anyone wanting to install a hybrid heating system meaning little or no changes to heating system such as radiators.

 

So the Atag boiler only 2 years old has sat idle for the last year.

 

For periods where the CoP of the heat pump, means gas is cheaper why not marry the boiler and heat pump together. Most of the off shelf solutions, rely on the gas boiler doing the hot water, not what I wanted. Most the time the ASHP is way cheaper than gas (especially with solar). So makes sense to do hot water via the ASHP most of the time.

 

Most ASHP already have all the control logic for hybrid mode built-in - mine does. I have a few location issues which makes the implementation a little more complex than I could be.  Boiler and UVC are about 20m plus away from the ASHP, with UFH in the middle.

 

The solution I chose to use has the boiler on a separate circuit to the ASHP. Combi DHW tails are not connected to anything and the isolation valves closed.

 

Phase one 

Get hybrid hot water heating. Had a 40 plate PHE sitting doing nothing, so inserted in the flow line from the ASHP close to the UVC, the other side to combi boiler heating circuit. Set max flow temp to 60 on the gas boiler. I can change the rate at which the boiler adds heat (gradient), set to a max of 2 degs every minute. Initial run was all manually done to check concept. 

 

With ASHP only the reheat time of the UVC is around 40 to 45 mins, with boiler engaged about 20 to 25 mins. During test, I monitoring the ASHP and gas boiler and both work well together, neither running away temperature wise.

 

Now how to automate. All control logic is in the ASHP, to use a boiler as a secondary heat source, you can set different outside temperatures for the following control schemes -

1. ASHP only,

2. ASHP leading, boiler following,

3. boiler leading ASHP following

4. boiler only.

 

ASHP controller supplies 230v as a control signal to the boiler. But boiler is 20m plus away from the ASHP. So used a Shelly relay at the ASHP end and a remotely controlled Shelly relay at the boiler end. Set up a scene in the shelly app, switched live to ASHP end Shelly energised the boiler end Shelly relay. All worked fine. Set the hybrid mode (ASHP leading) to start at 7 degs and a delayed start of 10 minutes for the boiler, after cylinder heating starts with the ASHP.

 

PHE, boiler on left side ASHP on the right.

 

Phase 2 UFH - coming next.

 

Edited June 27 by JohnMo

[JohnMo]

Phase 2, part 1

 

One issue that had me scratching my head was, how do I get the boiler, using only the central heating connections, to flow at two different temperatures one for DHW heating and the other for UFH. DHW set to 60 and UFH set to a max of 35. So using it priority hot water demand mode (PHWD).

 

Reading the installer manual for the boiler, it states weather compensation is only active when an outside sensor is attached. Meaning a set flow temp when not attached. So tried a resister (simulating 7 degs outside) on the outside temperature sensor connector. Connected boiler thinks it's 7 degs outside and disconnected nothing shows. So switching a resister in and out of circuit should give two different flow temperatures, by selecting an appropriate WC curve I should be able to flow at 35 degs for UFH and then 60 degs for DHW. So PHWD can be activated by switch in a resister (or outside sensor)

 

Controlling the resister switching in and out.

Shelly relay and a Shelly relay add on with temperature sensors was used. So the logic is quite simple.

 

Hybrid relay on and flow temp below 40 - the resistor is in line for low flow temperature. Hybrid relay on and above 40 degs - the resistor is taken out of circuit to enable high temperature flow for DHW heating.

 

Plumbing coming next

[JohnMo]

Phase 2, part 2,

Basic requirements are

1. single zone house and summer house, 

2. no buffer

3. keep as simple as possible 

4. no mixer or pump on UFH.

 

Original plumbing had a return booster pump from the UVC as piping was too small and long for the heat pump circulation pump alone.

This is the plumbing at the UFH. ASHP supplies hot water from the left to the CCT and pump and to a 3 port diverter.

 

Normal operation mode ASHP and hybrid mode for cylinder heating is exactly the same. DHW heating, ASHP puts a 230v signal out to move a three port valve. This supply is used to start CCT pump also. 3 port valve when energised opens lower port and closing upper port. This flow goes to the return back to ASHP teeing into the UFH return pipe. The CCT pump running takes hot water at the first tee and pushes it to the cylinder coil, via the PHE and cooled water comes back to the second tee, goes to the 3 port diverter and back to ASHP.

 

At the cylinder there is a second 3 port diverter. This is controlled by the cylinder thermostat. This diverter will be used in hybrid UFH mode.

 

UFH on, flow goes to 3 port valve (not energised), up and to top UFH rail, then out of the UFH to bottom rail and back to ASHP. No zone valves, pumps or mixers. The summer house is close to the ASHP and the flow to that is simply teed into the flow and return with 15mm Hep2O pipe.

 

Next

Phase 2 part 3 hybrid UFH 

[JohnMo]

Phase 2, part 3, UFH hybrid mode

 

The following differences occur when doing hybrid UFH.

 

CCT pump is started, warm water passes through PHE and is heated via boiler. The water then goes to the cylinder diverter (not energised) so by passes the cylinder and goes to the downstream tee at the CCT. This water flows through the UFH loops as normal just a bit warmer.

 

A few things needed to be added to make the above work

 

The 3 port diverter at the UFH and CCT pump needed to be electrical separated and controlled together and seperately. So on DHW heating both are energised, for hybrid UFH mode only the CCT is energised.

 

So power supply from ASHP now goes to a standard 8 pin relay. When energised the 3 port valve is energised and the CCT starts via separate circuits. A Shelly relay is energised when ever the ASHP hybrid Shelly is energised. This voltage by-passes the 8 pin relay powering the CCT pump directly.

 

Next a summary of the additional bits added and the control logic.

 

[JohnMo]

Parts used

1. A large 40 plate PHE, this could of easily been a close coupled tee. Already had this cost zero

2. 8 pin 230 volt relay. Already this cost zero

3. 4x Shelly plus 1 relays and

4. 2x Shelly plus 1 add-on and 5 temp probes (using for commissioning and one for control) items 3 and 4 £120

5. Plumbing bits and pipe. Around £100 Everything else was already in the system.

6. A resistor already has a box of 1000.

 

Costs circa £220.

 

Control logic

Settings within heat pump sensor out a 230v signal.

 

Shelly relay energised by the ASHP. Then via a Shelly scene, both boiler relay, CCT relay energised.

 

A further relay used to select two boiler flow temperatures. A scene again controls this - hybrid relay on and ASHP flow temp below 40 low temp mode, above 40 high temperature mode. 

 

Need to wait for next heating season to test UFH hybrid mode.

 

Any questions fill your boots and ask away.

[JohnMo]

Just completed a proof of concept on the boiler flow temp switching.

 

Started boiler via the hybrid relay manually, as a DHW heating cycle started. Boiler temperature is held back in line with the temperature curve for WC, boiler thinks it is 7 degrees. Once flow from ASHP gets to 40, WC switches off and boiler ramps up. When heating cycle is complete, WC relay remained energised, (wrong logic) supposed to be de-energised until hybrid mode re-calls for the boiler start. Found I had an OR instead AND in the switching logic. Quick correction, all good.

 

Boiler seems happy having WC on one minute off the next and back again.

 

So now have a combi boiler converted to PDHW (or X plan), for the cost of a relay.

[JohnMo]

Layout and control schematic

 

 

Heating Layout and Control.pdf

Edited June 29 by JohnMo
Removed check valve from schematic as not required or fitted

[MikeSharp01]

Interesting stuff @JohnMo - great posts. Not for us here as we won't have a boiler in our system anywhere although we did fit the trackpipe at the start and are leaving it in in case any future residents want it - not that gas will be available to them by then I hope. 

[JohnMo]

47 minutes ago, MikeSharp01 said:

Interesting stuff @JohnMo - great posts. Not for us here as we won't have a boiler in our system anywhere although we did fit the trackpipe at the start and are leaving it in in case any future residents want it - not that gas will be available to them by then I hope. 

I am really doing it because I have a nearly new boiler sitting there. Will get pennies to pound if I sell the boiler.

 

Did it as an experiment. But also to show it's easy enough and provided all the information for someone to replicate, especially with a radiator system as a retro fit. 

 

Around 80% reduction in CO2 emissions compared to a boiler alone. Re utilisation of a combi boiler as a system boiler or keep the combi part in service if you don't have the space for cylinder.

 

Think the statics are the average house requires 6kW to heat and 80% are below 8kW, but the average size of boiler sold is 30kW.  So you only need the smallest heat pump to make a real difference to CO2 output. At 7 degs the house heat demand is also half of the -3 heat demand. So most houses only need 3kW. But have the average sized boiler of 30kW, which doesn't turn down enough, not to short cycle when it above zero.

[marshian]

On 29/06/2024 at 11:05, JohnMo said:

I am really doing it because I have a nearly new boiler sitting there. Will get pennies to pound if I sell the boiler.

 

Did it as an experiment. But also to show it's easy enough and provided all the information for someone to replicate, especially with a radiator system as a retro fit. 

 

Around 80% reduction in CO2 emissions compared to a boiler alone. Re utilisation of a combi boiler as a system boiler or keep the combi part in service if you don't have the space for cylinder.

 

Think the statics are the average house requires 6kW to heat and 80% are below 8kW, but the average size of boiler sold is 30kW.  So you only need the smallest heat pump to make a real difference to CO2 output. At 7 degs the house heat demand is also half of the -3 heat demand. So most houses only need 3kW. But have the average sized boiler of 30kW, which doesn't turn down enough, not to short cycle when it above zero.

Very interesting - well documented - I'm not yet ready to go ASHP but I do need to replace the boiler because it's oversize 24kW for the house demand and the short cycle issues waste gas and annoy me even though I've range rated it down to 12kW. The new one will solve a lot of the short cycle issues but spring and autumn will be a challenge when heating required will be less than it's minimum

We also prefer to cook with gas so going ASHP (with a grant means we lose that)

 

 

[JohnMo]

1 hour ago, marshian said:

need to replace the boiler because it's oversize 24kW

Out of interest what boiler are you going for?

 

There are ways around an oversized boiler, run everything heating wise fully open, add a volumiser. Or with a gas boiler just use a thermal store. Then size really doesn't matter.

 

If you do have a system boiler do priority demand hot water, with weather compensation or/and room compensation for heating.

[marshian]

2 hours ago, JohnMo said:

Out of interest what boiler are you going for?

 

There are ways around an oversized boiler, run everything heating wise fully open, add a volumiser. Or with a gas boiler just use a thermal store. Then size really doesn't matter.

 

If you do have a system boiler do priority demand hot water, with weather compensation or/and room compensation for heating.

 

Viessmann Heat Only 16kW with weather comp and DHWP I did consider the 11kW version but we heat only when needed rather than 24/7 so need a reasonable uplift in output to bring the house up to temp quickly

 

Currently boiler can't do either - I did consider adding a volumiser but I was always going to have issues with short cycling