Microwave Boiler

[lpm]

This could be an interesting solution? A boiler, like a gas combi boiler, that heats the water via microwaves.

(Technically the microwaves heat some other fluid, with the heat then transferred to water via a heat exchanger.)

Advantages: would simply slot in where the current gas boiler is. No new plumbing. And should be more efficient.

Disadvantages: no COP, just a 1 to 1 use of electricity. Currently UK subsidised electricity 34p per kWH vs subsidised gas at 10p. That gap has to close significantly.

Prototype is called Heat Wayv, which suggests they might be weak at spelling.

https://www.theecoexperts.co.uk/boilers ... ve-boilers

[dyqik]

This could be an interesting solution? A boiler, like a gas combi boiler, that heats the water via microwaves.

(Technically the microwaves heat some other fluid, with the heat then transferred to water via a heat exchanger.)

Advantages: would simply slot in where the current gas boiler is. No new plumbing. And should be more efficient.

Disadvantages: no COP, just a 1 to 1 use of electricity. Currently UK subsidised electricity 34p per kWH vs subsidised gas at 10p. That gap has to close significantly.

Prototype is called Heat Wayv, which suggests they might be weak at spelling.

https://www.theecoexperts.co.uk/boilers ... ve-boilers

An electric immersion element would be cheaper and more efficient. 1:1 electric heaters for fluids already exist, in things like your kettle. Insulating them properly will make them more or as efficient as this system can ever be.

If there are advantages to this system, it's in having a thermal mass of hot fluid available to transfer heat from to the water on start up. Nothing about that requires microwaves.

Just about the only actual advantage over an electric resistive boiler listed there is smart app control. Which has nothing to do with microwave heating.

[lpm]

But the UK is heavily into combi boilers. 55% of households, according to google.

You turn the tap on, the gas boiler fires up, and you get hot water very quickly. (While wasting efficiency in that system start up.) So the UK needs solutions that swap out gas combi boilers, without needing big investment in the home's entire plumbing infrastructure. Most of these homes no longer have or never had space for a hot water cylinder.

Electric immersion elements are more suitable for heating a tankful during the night at a low electricity tariff. They can't run you a bath.

[dyqik]

But the UK is heavily into combi boilers. 55% of households, according to google.

You turn the tap on, the gas boiler fires up, and you get hot water very quickly. (While wasting efficiency in that system start up.) So the UK needs solutions that swap out gas combi boilers, without needing big investment in the home's entire plumbing infrastructure. Most of these homes no longer have or never had space for a hot water cylinder.

Electric immersion elements are more suitable for heating a tankful during the night at a low electricity tariff. They can't run you a bath.

A microwave combi boiler can only operate exactly like an electric immersion combi boiler, but with a more expensive heating element. Running the microwave generator needs slightly more electric current than running an equivalent immersion heating element. This is a fundamental consequence of the first law of thermodynamics: useful heat output <= electrical power in.

You can make a cheaper and more reliable version of this boiler by replacing the microwave heater with a kettle element.

To run any kind of electric combi boiler, you likely need to upgrade the electrical service to houses using it.

None of this requires a large hot water tank - that system only existed because of cheap rate tariffs and electric supply lines not being beefy enough to run combi boiler type systems.

[lpm]

But surely I couldn't run 30 amps / 7kW through an immersion element? Hence it can't make water hot quickly, it has to do it slowly? With a hot water tank that's fine, just run in for 4 hours overnight.

[philip]

Electric showers typically run 8-10kW through the heating element.

[dyqik]

But surely I couldn't run 30 amps / 7kW through an immersion element? Hence it can't make water hot quickly, it has to do it slowly? With a hot water tank that's fine, just run in for 4 hours overnight.

I run 26A @240 volt through the 5.5 kW immersion element in my home brewery, using an entirely home made control system.

It's trivial to add a second 4.5 kW element in parallel, running both off a standard (US) 50A 240V domestic circuit.

One other concern with a microwave heating system is that the power factor is probably less than 1.0, meaning that it's an inductive or capacitive load. This means that it needs a higher current rated circuit than a resistive heater, and switching it can cause voltage spikes. This isn't impossible to deal with, but it's added cost and complexity.

[lpm]

Here's the Heat Wayv guy saying why his product is so good:

https://www.hvpmag.co.uk/HeatWayv-expla ... ture/12668

A bit of an alarm bell with his wild inflation of heat pump costs...

So are they wasting their time and money? Using the same electricity supply would get the same/better results if simple immersion elements are used?

[dyqik]

An average UK gas combi boiler is 35 kW. Replacing that like for like would need a 35 kW electric combi boiler. Standard electric combi boilers are 15 kW.

Obviously they can't be a like for like replacement. What can be done is to add a small amount of storage to the combi boiler for near instant hot water for small volume uses. The other thing is to install a heat pump warm water storage system somewhere in the property, to provide a source of water preheated to an intermediate temperature, so that less power is needed to heat it to the typical 50-55C hot water temperature.

[dyqik]

Here's the Heat Wayv guy saying why his product is so good:

https://www.hvpmag.co.uk/HeatWayv-expla ... ture/12668

A bit of an alarm bell with his wild inflation of heat pump costs...

So are they wasting their time and money? Using the same electricity supply would get the same/better results if simple immersion elements are used?

Likely for marketing and VC funding reasons. A microwave boiler sounds more exciting, and so makes it easy to get the cash to develop the boring bits of the product, like the smart control and fluid heat exchange system. And it gives a way to sell them at a premium.

[Imrael]

Having recently switched to a heat pump, I'm wondering about a "last 10 degrees" booster using this idea. One of the main expenses of heat pump conversion isnt the pump itself but adapting to lower output temperature. Its typically only 10 C, but thats enough to necessitate bigger radiators etc.

We're sort of 2/3 of the way through, and the result is that upstairs is fine without changing rads due to extra insulation work, but some rad changes are going to be needed downstairs. A booster availaible for cold days would mitigate this.

If I've understood it right (not guaranteed) the system already has something in place to electrically boost-heat the water - used once a week as a "purge" to prevent build-up of bugs.

[dyqik]

Having recently switched to a heat pump, I'm wondering about a "last 10 degrees" booster using this idea. One of the main expenses of heat pump conversion isnt the pump itself but adapting to lower output temperature. Its typically only 10 C, but thats enough to necessitate bigger radiators etc.

We're sort of 2/3 of the way through, and the result is that upstairs is fine without changing rads due to extra insulation work, but some rad changes are going to be needed downstairs. A booster availaible for cold days would mitigate this.

If I've understood it right (not guaranteed) the system already has something in place to electrically boost-heat the water - used once a week as a "purge" to prevent build-up of bugs.

Yes, this may be "only" a control system issue. If the booster can be used on demand rather than time based, then it may have enough power to make the downstairs warm enough.

[dyqik]

It may be that the upfront price differential between a microwave heating element and a resistive heating element isn't that big, compared to the overall cost of a boiler.

OTOH, replacing immersion heating elements is an absolutely standard plumbing job, that pretty much every plumber can do.

Don't underestimate the business power of tying your customers in for expensive specialist maintenance services.

[lpm]

Having recently switched to a heat pump, I'm wondering about a "last 10 degrees" booster using this idea. One of the main expenses of heat pump conversion isnt the pump itself but adapting to lower output temperature. Its typically only 10 C, but thats enough to necessitate bigger radiators etc.

We're sort of 2/3 of the way through, and the result is that upstairs is fine without changing rads due to extra insulation work, but some rad changes are going to be needed downstairs. A booster availaible for cold days would mitigate this.

If I've understood it right (not guaranteed) the system already has something in place to electrically boost-heat the water - used once a week as a "purge" to prevent build-up of bugs.

Hmm. Not convinced by this idea. If you're adding 1:1 electrical heating to the system, why not just do it via a £20 fan heater or a £50 electric radiator? A flexible top up for the few weeks like now when daytime temperatures are below freezing, rather than building it into the system?

[bjn]

Slight aside, people are now putting batteries into things like ovens and so on. This allows you to deliver peak power well above a normal high amperage connection, but still run it off a normal 13A plug. It works because you aren't running the thing at XXX kW all day. For on demand hot water you could do something similar. Sucks for central heating though. Use a heat pump for that.

[Martin Y]

One reason your microwave oven has a fan is to cool the magnetron. That's heat energy wasted rather than turned into microwaves to heat the food. A quick google finds this experimental result showing roughly 50% efficiency: https://www.physicsforums.com/threads/h ... ven.96192/

(I'm taken aback by how poor that efficiency is, to the extent that I'm tempted to try repeating the same experiment myself.)

A kettle with a heating element turns virtually all of the electricity used into heat in the element which in turn almost all heats the water rather than the surroundings.

[dyqik]

Having recently switched to a heat pump, I'm wondering about a "last 10 degrees" booster using this idea. One of the main expenses of heat pump conversion isnt the pump itself but adapting to lower output temperature. Its typically only 10 C, but thats enough to necessitate bigger radiators etc.

We're sort of 2/3 of the way through, and the result is that upstairs is fine without changing rads due to extra insulation work, but some rad changes are going to be needed downstairs. A booster availaible for cold days would mitigate this.

If I've understood it right (not guaranteed) the system already has something in place to electrically boost-heat the water - used once a week as a "purge" to prevent build-up of bugs.

Hmm. Not convinced by this idea. If you're adding 1:1 electrical heating to the system, why not just do it via a £20 fan heater or a £50 electric radiator? A flexible top up for the few weeks like now when daytime temperatures are below freezing, rather than building it into the system?

In this case, the system has the boost heater already in it to pasteurize the system. It may be as cheap as programming it to turn on in response to heating demand.

OTOH, I do wonder if you can avoid the need to upgrade the radiators by adding some forced convection to them with small fans - or with ceiling fans in the rooms, which are nice in hot weather as well.

More air flow means more heat transfer for a given size of radiator, and the reason they need upgrading for lower operating temps is that the passive convection generated at the lower temperature isn't high enough given the size of the radiator.

[WFJ]

One reason your microwave oven has a fan is to cool the magnetron. That's heat energy wasted rather than turned into microwaves to heat the food. A quick google finds this experimental result showing roughly 50% efficiency: https://www.physicsforums.com/threads/h ... ven.96192/

(I'm taken aback by how poor that efficiency is, to the extent that I'm tempted to try repeating the same experiment myself.)

A kettle with a heating element turns virtually all of the electricity used into heat in the element which in turn almost all heats the water rather than the surroundings.

That's only true when your aim is heating water, which is not a common use case for microwaves. "Boiling" vegetables in a microwave, for instance, is a lot more energy efficient than using an element to heat a pan of water to slowly cook submerged vegetables while losing huge amounts of heat to the environment.

[lpm]

Having recently switched to a heat pump, I'm wondering about a "last 10 degrees" booster using this idea. One of the main expenses of heat pump conversion isnt the pump itself but adapting to lower output temperature. Its typically only 10 C, but thats enough to necessitate bigger radiators etc.

We're sort of 2/3 of the way through, and the result is that upstairs is fine without changing rads due to extra insulation work, but some rad changes are going to be needed downstairs. A booster availaible for cold days would mitigate this.

If I've understood it right (not guaranteed) the system already has something in place to electrically boost-heat the water - used once a week as a "purge" to prevent build-up of bugs.

Hmm. Not convinced by this idea. If you're adding 1:1 electrical heating to the system, why not just do it via a £20 fan heater or a £50 electric radiator? A flexible top up for the few weeks like now when daytime temperatures are below freezing, rather than building it into the system?

In this case, the system has the boost heater already in it to pasteurize the system. It may be as cheap as programming it to turn on in response to heating demand.

OTOH, I do wonder if you can avoid the need to upgrade the radiators by adding some forced convection to them with small fans - or with ceiling fans in the rooms, which are nice in hot weather as well.

More air flow means more heat transfer for a given size of radiator, and the reason they need upgrading for lower operating temps is that the passive convection generated at the lower temperature isn't high enough given the size of the radiator.

I'm not sure I understand this. Where does the energy go, if it doesn't stay in the house somewhere?

I thought it all depended on the heat loss calculation. So in the current UK freezing weather, -5C at night and 0C day, means the differential to a 20C indoor temperature can't be maintained with radiators at 45C.

Or is it that the heat pump doesn't deliver enough kWH of heat from the outside of the house into the inside? Are you are saying a fan will mean more heat will flow out of the radiator, leaving more "space" in the radiator to be refilled by the heat pump?

[Martin Y]

That's only true when your aim is heating water, which is not a common use case for microwaves.

Though it is the use proposed in the case in the OP; a microwave heated domestic hot water supply. I tend to think the reason it might be uncommon is because it's overelaborate and less efficient than resistive heating.

[lpm]

They did say the microwaves heated some other stuff, not the water directly. They don't reveal what that stuff is, but presumably something that fits nicely with microwaves?

[Martin Y]

Well that doesn't sounds much different from a traditional gas boiler, which heats the radiator water in a closed circuit, and that hot water also heats the hot tap water in a heat exchanger (traditionally a hot water tank). Microwaves are generally run at a frequency which is strongly absorbed by water so it makes sense that they'd use the same system: microwaves heat water in a closed circuit which either goes around radiators or heats hot tap water (or both).

I see they also claim a benefit is being able to control the microwave system with a phone app. Surely any type of new-install domestic heating system could do that, couldn't it?

[dyqik]

Well that doesn't sounds much different from a traditional gas boiler, which heats the radiator water in a closed circuit, and that hot water also heats the hot tap water in a heat exchanger (traditionally a hot water tank). Microwaves are generally run at a frequency which is strongly absorbed by water so it makes sense that they'd use the same system: microwaves heat water in a closed circuit which either goes around radiators or heats hot tap water (or both).

I see they also claim a benefit is being able to control the microwave system with a phone app. Surely any type of new-install domestic heating system could do that, couldn't it?

My resistive immersion boiler is controlled by a raspberry pi running CraftBeerPi 3, and with a web interface.

[dyqik]

Hmm. Not convinced by this idea. If you're adding 1:1 electrical heating to the system, why not just do it via a £20 fan heater or a £50 electric radiator? A flexible top up for the few weeks like now when daytime temperatures are below freezing, rather than building it into the system?

In this case, the system has the boost heater already in it to pasteurize the system. It may be as cheap as programming it to turn on in response to heating demand.

OTOH, I do wonder if you can avoid the need to upgrade the radiators by adding some forced convection to them with small fans - or with ceiling fans in the rooms, which are nice in hot weather as well.

More air flow means more heat transfer for a given size of radiator, and the reason they need upgrading for lower operating temps is that the passive convection generated at the lower temperature isn't high enough given the size of the radiator.

I'm not sure I understand this. Where does the energy go, if it doesn't stay in the house somewhere?

I thought it all depended on the heat loss calculation. So in the current UK freezing weather, -5C at night and 0C day, means the differential to a 20C indoor temperature can't be maintained with radiators at 45C.

Or is it that the heat pump doesn't deliver enough kWH of heat from the outside of the house into the inside? Are you are saying a fan will mean more heat will flow out of the radiator, leaving more "space" in the radiator to be refilled by the heat pump?

The problem stated by Imrael is that the water produced by the heat pump is not hot enough to make the heat output from radiators in downstairs rooms high enough to overcome the losses. The heat pump can supply enough heat via the hot water, but the radiators can't transfer it to the air quickly enough.

You can increase the heat output from a radiator* by increasing the airflow across it. With standard domestic radiators, the airflow is provided by convection of air past the radiator, driven by heating the air close to the radiator. With cooler radiators, the air is not heated as much, and so the convection air currents are weaker, reducing the heat transfer to the room. You can correct this by either running the radiators hotter, increasing the surface area of the radiators, or by supplying forced air flow.

*"Radiators" are in fact water to air heat exchangers driven by convection.

[Sciolus]

I see they also claim a benefit is being able to control the microwave system with a phone app. Surely any type of new-install domestic heating system could do that, couldn't it?

Yup. (Or you can have controls that don't depend on some random website run by someone who will go out of business next year when people realise their product is sh.t.)