Not a self-builder!

[HantsBob]

Hello.

 

Thought I ought to admit straight away that I've never self-built, nor do I have any intention of self-building in future.  Also, my currently proposed renovation is somewhat smaller in scope than many on this forum.

 

I am hoping to replace my current gas-boiler driven central heating and hot water system with an ASHP later this year and have been seeking quotes for this work.  Having googled many questions that have been perplexing me I keep finding useful information on this forum, hence my decision to join so that I can ask specific questions for which I've been unable to find answers.

 

So, if I'm not immediately evicted from the forum for fraudulent application I'd like to start off with some points about the Domestic Hot Water side of the problem.  My current hot water cylinder is over 40 years old, is relatively small and is probably heavily scaled internally.  I'm certain it needs replacing.  It may be possible, with difficulty, to fit a slimline modern cylinder into the airing cupboard where the current cylinder resides.  This is not desirable because we are used to using the airing cupboard as storage for bed linen, towels etc. and a slimline cylinder of reasonable capacity, together with associated elements (expansion vessels etc.) would take up virtually all the space.

 

The options appear to be:

1. A heat battery (e.g. Sunamp Thermino) in the airing cupboard
2. A modern cylinder located elsewhere, probably in the loft

 

The thermino has advantages such as low heat loss, not requiring annual maintenance and not requiring Legionella cycles.  The main disadvantages are initial cost and, if I understand correctly, the need to return to base for simple things like a faulty immersion.

 

The modern cylinder has advantages such as lower initial cost, easier repair of faulty components.  The disadvantages are higher heat loss (and being outside thermal envelope of house), requiring Temperature and Pressure Relief valve and associated pipework, annual maintenance.

 

The particular point that is puzzling me at the moment is the effectiveness of a heat pump at actually bringing the hot water cylinder up to temperature and the optimum settings.  For the sake of discussion I'd like to assume that the ASHP is a Vaillant Arotherm Plus 7kW (not yet decided, but it might simplify this discussion if we fix some items).

 

When heating the hot water cylinder the higher the flow temperature from the ASHP the quicker the cylinder will reach temperature, but:

1. The higher the flow temperature, the worse the CoP
2. Lower flow temperatures imply lower transfer of heat energy into the cylinder
3. If the heat energy transfer is lower than the ASHP can modulate down to then the ASHP will have to cycle on and off
4. As I understand, cycling is bad for the ASHP components and is also associated wtih waste of electricity.

 

The rate of heat energy transfer is connected to the flow temperature leaving the ASHP, the temperature of the cylinder water and the design of the coil.  In terms of the coil, different manufacturers provide different information.  For example, some provide coil surface area, some provide reheat time (in accordance with EN 12897, but I can't find the definition of this) and some provide a rating in kW (but I can't find the temperature difference that this is based on).  Some (e.g. Thermino) I can't actually find any firm data at all.

 

To summarise my main questions:

1. Am I correct to think that the main aim would be to ensure minimum cycling of the heat pump?
2. How low can a heat pump modulate down to at the sort of flow temperatures required for DHW and what is the associated CoP?
3. Is there any firm data in the public domain on heat energy transfer into a Thermino?

 

Apologies for the long introductory posting.  If I'm still a member of the forum for long enough for subsequent posts then I'll try and keep them substantially shorter!

[Big Jimbo]

Hello, and welcome. I can't answer any of your questions but, we do have some very smart people on here, who i'm sure will be along soon.

[SteamyTea]

Welcome.

 

First things first.

Have you done an energy audit of your house?

Do you currently have enough hot water for your needs?

 

The CoP of an ASHP is affected by more than just flow temperature.

 

[joe90]

Welcome, we like questions. What other work are you doing on your renovation 🤷‍♂️, I only ask because an ASHP really only works on a well insulated house. With regard the hot water tank an ASHP produces a lower temp than a gas boiler therefore it’s usual to have a larger DHW tank, mine is set to 43’ which is hot enough to wash dishes, shower bath etc BUT with no added cold water which you probably currently do with your gas boiler. It’s also a DHW tank with a larger coil specifically for an ASHP. As a heat pump produces lower temp water you also need bigger radiators for the central heating. 
although this a self build forum we also include renovations 👍

Edited May 8, 2023 by joe90

[Pocster]

57 minutes ago, Big Jimbo said:

Hello, and welcome. I can't answer any of your questions but, we do have some very smart people on here, who i'm sure will be along soon.

As @Big Jimbo basically tagged me …. I am a smart person who has ( is ) building their own abode . Equally I can’t answer any of your questions .

[HantsBob]

Thanks for the incredibly quick replies, even those that can't answer the questions😀.

 

To answer a few of the points raised:

 

Energy audit: I have done my own detailed heat loss calculation.  Before accepting any quote for the heat pump etc. I'll be looking in detail at the installer's own heat loss calculation and proposed solution and ensuring as far as possible that I understand what they are proposing and why it should work.  The questions I'm raising on this forum are part of the process of making sure that I'm as well prepared as possible for assessing proposals.

 

Current hot water situation: Yes we almost always have enough hot water - very occasionally when visitors stay it starts to run noticeably cooler.  The hot water is currently turned on (ie the gas boiler will heat the water whenever the thermostat calls for heat) from before we get up until about the time we go to bed.  In future I'd hope to be able to make sensible use of time of day electricity tariffs and excess PV production.

 

CoP: I had thought that the CoP was basically a function of flow temperature and ambient temperature.  I'm happy to be corrected.

 

Additional work: I expect much of the central heating pipework and the majority of the radiators to be replaced. 

 

Hot water temperature: good point that storing hot water at a lower temperature implies requiring a larger tank.

[Marvin]

Hi @HantsBob

 

Batteries. Check with battery specification but best temp 25°C?? Certinally our Renogy ones are. Very thick copper cable required between batteries and inverter, so the nearer the two the better.

 

Hot water tank. The last place I would put the hot water tank, is in loft because, as you say, the heat loss. Espically if you end up with a large one due to ASHP water temperature issues. 

 

Good luck

 

Marvin

 

 

[joe90]

4 minutes ago, Marvin said:

Hot water tank. The last place I would put the hot water tank, is in loft because, as you say, the heat loss. Espically if you end up with a large one due to ASHP water temperature issues. 

However if no other space in the house and a super insulated “room” built around it. Also Losses from a “cooler” tank are far lower than from a “hot” tank.

[Marvin]

38 minutes ago, joe90 said:

However if no other space in the house and a super insulated “room” built around it. Also Losses from a “cooler” tank are far lower than from a “hot” tank.

Yes... 

Loft -10°C hot water 50°C

Cupboard 20°C water 60°C

 

Volume of low temp tank?

 

Heat from loft tank escapes building through roof.

 

Heat from cupboard tank escapes through thermal envelope.

 

I am not convinced, unless as you say @joe90 in a very insulated room.

 

M

[Bozza]

Unless your house is, or will be, extremely well insulated, I would be wary about expectations from ASHP.  I have one and I am happy with it, but I wouldn’t put one in anything other that an extremely well insulated modern  house.   And certainly not one without oversized radiators, underfloor heating etc.

 

 

 

 

[Marvin]

7 minutes ago, Bozza said:

Unless your house is, or will be, extremely well insulated, I would be wary about expectations from ASHP.  I have one and I am happy with it, but I wouldn’t put one in anything other that an extremely well insulated modern  house.   And certainly not one without oversized radiators, underfloor heating etc.

 

 

 

 

And also important airtightness!

[joe90]

9 hours ago, Marvin said:

Loft -10°C

How often ..?

[Marvin]

11 minutes ago, joe90 said:

How often ..?

I get your point. 👍.

 

Based on 2 neighbours experiences I would still consider very hard the decision to put a hot water tank in the loft.

 

Because the water temperature required is low, its not just the tank but also the length of pipework that causes losses, both circulating from the ASHP and distance from tank to tap.

 

And yes each one of these is small but the accumulation times years of additional cost of both heating and water usage due to running tap to get the hot water add up.

 

Also, as I commented before, the heat loss from the tank not adding to the warmth in the house. I think the average temperature in the UK over the year is about 7°C.

 

 

 

 

[joe90]

9 minutes ago, Marvin said:

I get your point. 👍.

I was only pointing out the possibility if that’s the only place it could go restricted by the existing build of the house, a compromise worth paying maybe 🤷‍♂️.

[SteamyTea]

36 minutes ago, joe90 said:

How often ..?

If you look at the Met Offices' Central England Temperature data set (just happens to be the longest in the world), between 1900 and 2023, there has not been one full day when the temperature has been lower than -9°C.

The distribution starts at -8.7°C, and that has 1 instance, or one full day.

For temperatures below, for a full day, 0°C, there has been 1177 instances out of 44908.  That is 2.62% of the days.

Now if we look at the last 30 years 1993 up to 2023 then the lowest temperature is -7°C (1 full day), below 0°C is 175 days, or 1.59% of those days.

 

Now non of us can actually live at the CET location, but it shows that we remember exceptionally cold weather because it is exceptional, not because it is common.

[Marvin]

2 hours ago, joe90 said:

I was only pointing out the possibility if that’s the only place it could go restricted by the existing build of the house, a compromise worth paying maybe 🤷‍♂️.

Definitely better than no hot water 😂

[Alan Ambrose]

>>> Now non of us can actually live at the CET location

 

Well presumably at least one person can 

[HantsBob]

18 hours ago, Marvin said:

Hi @HantsBob

 

Batteries. Check with battery specification but best temp 25°C?? Certinally our Renogy ones are. Very thick copper cable required between batteries and inverter, so the nearer the two the better.

 

Hot water tank. The last place I would put the hot water tank, is in loft because, as you say, the heat loss. Espically if you end up with a large one due to ASHP water temperature issues. 

 

Good luck

 

Marvin

 

 

Thanks for taking the time to respond.  I didn't explain carefully enough about the Sunamp Thermino.  It is described as a heat battery but it does not store electrical energy.  It stores energy in the form of heat, so similar in function to a DHW cylinder.

 

However, you have answered a question that had occurred to me, but which I had not yet asked, regarding placement of electrical home batteries.  That may be next year's project.

 

As regards placing the hot water tank in the loft, it is certainly not my ideal solution but given our space contraints it is a potential solution that needs to be understood.  To this end, can anyone explain the, presumably, standard conditions under which heat loss is stated.  For example, I've just found the specification for a cylinder that states heat loss as 2.16 kWh/24 hrs.  I assume the critical information is the temperature difference assumed between the hot water and the air outside the cylinder.  But what difference will have been assumed in producing this heat loss figure.  Also, would I be correct in thinking that if the temperature difference were, say, 20% greater that the heat loss would also be 20% greater?

 

Thanks also to the others that have responded.  I am aware that ideally the house should be well insulated and air-tight.  We are doing some work in this area, perhaps we should consider more.  However, looking at gas usage over a few years gives a fairly good idea how much heat energy we need to keep the house warm.  Given suitably upgraded radiators and pipework a ASHP should have no difficulty in pumping a similar amount of energy in.

[SteamyTea]

18 minutes ago, Alan Ambrose said:

Well presumably at least one person can 

They can physically be at the same locations as the weather stations, at the same time, just to alive.

[Marvin]

Hi @HantsBob

 

Yes I missed that one....

 

This may help: with the Therminos

 

https://sunflowltd.co.uk/guide-videos/#manual_1602311761951-4c10cc86-18f9