Boiler-Heat Pump Hybrids

[Roger440]

  On 24/03/2025 at 20:12, JohnMo said:

Pod - my summer house (heated all year)

 

CCT - close coupled tee

 

ASHP, can run two temperatures, set via a zero volt contact.

 

Shelly relay is used to switch between the two settings.

 

Shelly control is via home assistant, it monitors battery state of charge and PV output. So if battery state of charge is above 97% and PV is generating more than 600W, ASHP runs and stays running until battery drops to 95%. Have a similar setup for the immersion. Idea is I never let battery get to 100% charged and export little or no electricity.

 

 

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Thanks for the explanation.

 

Lots to think about, but i like the concept. 

 

You specify the conditions of the battery which make the ASHP run, but what if there no heat demand? ie, house up to temp and a tank full of hot water?

 

Id probably have to to have a long ponder how to avoid such things as home assistant in mine. I absolutely wouldnt want a computer or wifi involved in mine.

 

 

 

 

[JohnMo]

  On 24/03/2025 at 20:36, Roger440 said:

but what if there no heat demand

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I have the luxury of lots of concrete in the floor, so that will buffer loads of heat. Generally without over heating the house. Once heating is off, cooling generally goes on, so have to be little more careful with that. So will not be charging the battery fully overnight, so PV spends more time battery charging than anything else.

[Roger440]

  On 24/03/2025 at 20:53, JohnMo said:

I have the luxury of lots of concrete in the floor, so that will buffer loads of heat. Generally without over heating the house. Once heating is off, cooling generally goes on, so have to be little more careful with that. So will not be charging the battery fully overnight, so PV spends more time battery charging than anything else.

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Thanks. Makes sense.

 

I have lots of concrete too. This is shaping up for a "next year" project for me.

[Gus Potter]

  On 24/03/2025 at 09:06, JohnMo said:

Seems to work well

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Help ma boab John, some set up. Browsing though your spread sheet diagram there are some clever touches in there. The bit that interests me most is that box that I think is the plate heat exchange to keep the air pump separate from the gas boiler. I like your style and approach.. so + 1 from me and also I appreciate you sharing your diagrams.. I know that these don't get knocked up in five minutes.

 

In a funny way we are both trying to achieve the same end.. which is the most cost effective way of heating a house with say UF via say ASHP and Gas.. or anything else.. where we may differ is that I look at the cost and in particular the maintenance cost of a system over say 50 years.. with my commercial / professional hat on. With my hobby hat on (being mucking about with this stuff for at least 30 years) I love it!

 

I go for the simple stupid, affordable, hopefully mass market and you are exploring the forefront of control systems and innovation.

 

Is there a middle way that folk on BH can marry together what you are doing with my hard headed approach?

 

OK folks with my SE / raft design day job experience.. with underfloor heating hat chucked in...

 

Over the years I've adapted my raft designs to take account how you introduce under floor heating pipes into ground floor slabs. It took me along time to figure this out but I'm confident as an SE I have most of the answers. I also feel capable of explaining how, in laymans terms, this heating malarky is not as complex as it needs to be.

 

There are many ways to skin this cat SE wise.. but often the approach and one I take is to say..  how are you going to use what is often an open plan space. Do you want some rugs on the floor, do you have big areas of glass where we may want to close up the spacing of the UF pipes? What kind of controls do we need to suit your way of life.. are you out all day or work from home, young family, kids grow up and then I think about how I might want to lay out and form the slab below,  can I as an SE have my cake and eat it? sometimes yes..   But always..make it simple stupid and future proof.

 

For me what I've learnt over the last 30 years of doing this is that UF pipe layout and systems is it's  much more to do with buildability, ongoing maintenance cost and preserving your asset. l the size of the house qualitatively, the number of floors and look to see where the cold air comes from and design for that. That gives me a "feel" for what would work.

 

It is really is a craft rather than calculation.. the real craft is explaining to your Client how it works as a "craft" so the house performs the way they want.. the calculations are often there to convince BC!

 

 

 

 

 

 

Edited March 24 by Gus Potter

[JohnMo]

  On 24/03/2025 at 23:11, Gus Potter said:

most cost effective way of heating a house with say UF via say ASHP and Gas.. or anything else.. where we may differ is that I look at the cost and in particular the maintenance cost of a system over say 50 years

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Generally the cheapest overall would be plenty of solar, battery and heat pump and delete the boiler, no maintenance or standing charges for gas side. Both charges out way any of the saving made on the cold days even in NE Scotland.

 

Getting the diverter to ASHP to work outside home assistant, with standard stuff and no need for internet, would be good. But no idea how to do that. 

[SimC]

  On 24/03/2025 at 09:06, JohnMo said:

Sorry for the delay forgot all about the schematic.

 

Seems to work well

5 degs and below for at least 6 hrs, is needed to allow the boiler to run. All other times ASHP runs.  ASHP set up via Home Assistant to take excess PV when battery is above a set charge and generating enough PV to get a decent run time. Past few weeks nearly all heat pump running has been on excess PV - so zero cost, boiler had no need to start because house was warm enough.

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I am interested in how the specific PHE was chosen.

 

Was it based on kW rating, water flow rates required by the boiler and CHP, or some other arbitary parameter?

 

My experience in a work capacity is that the primary and secondary return temps are very closely matched at low flow rates, but when the flow rates increase there is a point when the primary return temp (to heat source) rises significantly which has a knock-on effect on the heat source efficiency.

[JohnMo]

  On 25/03/2025 at 14:20, SimC said:

am interested in how the specific PHE was chosen

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It was originally specified based on low temperature heat transfer. It's pretty huge Secespol LA34 40 plate. Used an online PHE sizing tool. It's getting an approach temp of a couple of degrees, with boiler UFH and similar when doing DHW heating.

 

Few trends below, first is boiler flow temp and cycle temp - using phe and heat pump cylinder. dT does decrease as cylinder gets hotter. But boiler seems happy enough.

 

Second is a typical run on a cold day, the bouncing about between 1 am and 3 am on second trend was heat pump also running. The spike is DHW heating.

 

Third trend is a start and run of just the boiler doing UFH 

 

[Wil]

I have a similar set up done slightly differently and nowhere near as neat. One question is- why separate the boiler and ASHP? Do they need to be hydraulically separate?

 

[JohnMo]

  On 25/03/2025 at 14:54, Wil said:

I have a similar set up done slightly differently and nowhere near as neat. One question is- why separate the boiler and ASHP? Do they need to be hydraulically separate?

 

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Mainly to do with house layout, Heat pump and boiler location. And existing pipe sizes and piping lengths. Cylinder has to have the additional pump as heat pump cannot do the distance because of existing piping size.  So additional pump that's a given, I already had the PHE, but could have done close coupled tees or tee in flow and return, but neither worked as neatly as the PHE.  But PHE also allows me to remove boiler at any time without breaking into the main heating system.

[Roger440]

  On 25/03/2025 at 07:53, JohnMo said:

Generally the cheapest overall would be plenty of solar, battery and heat pump and delete the boiler, no maintenance or standing charges for gas side. Both charges out way any of the saving made on the cold days even in NE Scotland.

 

Getting the diverter to ASHP to work outside home assistant, with standard stuff and no need for internet, would be good. But no idea how to do that. 

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Id probably being do the reverse, ie, when i reconfigure the heating ill put in ASHP provision, pending some (affordable) battery solution later. Im on oil, so no standing charges to worry about, so absolutely zero motivation to get rid of that heat source, especially with a flaky electrical supply.

 

Interesting on the home assistant, as i wouldnt be able to make it work with it! I suspect i would need to simplify further. No doubt at the expense of some efficiency.

 

Its something i need to ponder once ive got the current round or works out of the way.

 

All good stuff when someone doesnt just follow the herd and applies thought and logic to a problem