Losing temp from the DHW cylinder when the ASHP runs for central heating

[kron77]

Hello, I have  a mitsubishi ecodan R32 with a 200 litre unvented tank (mcdonald not ecodan). I got the cylinder installed over the summer (an issue with the old one) and no problems. When winter kicked in and I put on the central heating I have noticed that the cylinder is losing temp. It's quite significant (20 degrees sometimes). The old cylinder didn't do that (also mcdonald but only rated to 1.5bar). 

 

What has my installer done wrong? They have gone bust so i'd like to know the answer before I approach a new company. 

 

 

[Nick Thomas]

If it's plumbed in an "S plan" and you have both heating and hot water on at the same time, heat will be robbed from the cylinder to power the central heating. Mine is plumbed like this, and it's quite annoying - but apparently it's not "wrong", just not ideal. The solution when it's like this is to just ensure heat and hot water are never scheduled at the same time.

 

If that's already the case for you, it'll be something else - pics of the cylinder and plumbing might help someone who knows more about plumbing than me (not hard!) to work out what.

[Conor]

Could be 3-way valve not wired up properly and the heatpump flow is going through the cylinder coil instead of directly to the heating circuit. In practice means the hot wat tank will be cooled to the flow temp set for heating, e.g. 35c, rather than the usual 50c or whatever for hot water.

[kron77]

31 minutes ago, Nick Thomas said:

If it's plumbed in an "S plan" and you have both heating and hot water on at the same time, heat will be robbed from the cylinder to power the central heating. Mine is plumbed like this, and it's quite annoying - but apparently it's not "wrong", just not ideal. The solution when it's like this is to just ensure heat and hot water are never scheduled at the same time.

 

If that's already the case for you, it'll be something else - pics of the cylinder and plumbing might help someone who knows more about plumbing than me (not hard!) to work out what.

Thanks Nick! Yes, they are scheduled at different times. I'll try to take some photos but to be honest, it's a tight squeeze in there!

[kron77]

4 minutes ago, Conor said:

Could be 3-way valve not wired up properly and the heatpump flow is going through the cylinder coil instead of directly to the heating circuit. In practice means the hot wat tank will be cooled to the flow temp set for heating, e.g. 35c, rather than the usual 50c or whatever for hot water.

Hi conor thanks for the reply. Yes, this sounds about what's happening. Definitely drops down to around 37 (flow temp) if the central eating has been running for long enough. OK i'll get onto my new company and ask them to check the 3-way valve wiring. 

[JamesPa]

50 minutes ago, Nick Thomas said:

it's plumbed in an "S plan" and you have both heating and hot water on at the same time, heat will be robbed from the cylinder to power the central heating. Mine is plumbed like this, and it's quite annoying - but apparently it's not "wrong", just not ideal

Sorry but it's wrong and an installer who plumbed it this way doesn't know what they are doing!  As various have said scheduling them at different times is a solution, but it really shouldn't be possible in any system with weather compensation or low flow temperatures for space heating and water heating to operate simultaneously as it defeats the whole design principle.  Btw the comment applies also to fossil fuel boilers with weather comp (or that you want to run at low temp)

 

There are of course various work arounds, of which not scheduling them simultaneously is one, but the householder shouldn't have to work this out for themselves.

 

The common solution is a diverter valve (either-or) instead of the more common three way valve (ch or both), but you can also do it with two two way valves correctly wired 

Edited November 13, 2023 by JamesPa

[Beelbeebub]

If the installer used a 3 way valve that has a mid position, that might be the cause.

 

Traditional hearing systems would have a mid position where heat went to cylinder and radiators at the same time, not a problem when your boiler is set to a single high temp (eg 75c).

 

A HP should either run the heating OR the cylinder. It's the same physical valve, just a different actuator head and wiring.

[JamesPa]

For clarity @Beelbeebuband I are saying essentially the same, just slightly different terminology.  Basically wrong type of valve.  I don't know if it's possible just to change the head, @Beelbeebub may know.

[Beelbeebub]

13 minutes ago, JamesPa said:

For clarity @Beelbeebuband I are saying essentially the same, just slightly different terminology.  Basically wrong type of valve.  I don't know if it's possible just to change the head, @Beelbeebub may know.

I belive it is as the mechanism is physically the same. It's just the valve actuator - the motor bit) is different.

 

Even then it's not that different. A 3 or mispositioning valve has a resting position (when no power is applied) where all the water goes one way, an end position where power is supplied, the motor drives to the end position and stalls sending all the water the another way, and a mid position where some.swi4ches and electrical jiggery pokery make the motor stall half way so it "hovers" and water goes both ways.  You could wire up a 3 position valve to effectively be a diverter by simply not wiring up the mid position.

 

That said, I've never actually "converted" one as it's never been necessary.

Edited November 13, 2023 by Beelbeebub

[JohnMo]

 

1 hour ago, kron77 said:

What has my installer done wrong? They have gone bust so i'd like to know the answer before I approach a new company. 

 

Besides all the above about 3 and mid port valves, there is a safety issue here also.

 

If the cylinder is unvented, a mid point valve requires a fail closed 2 port valve between it and the cylinder heating coil - to give a positive close to stop the heat source, from supplying heat when power is removed by overheat thermostat.  A 3 port diverter does not need this additional 2 port valve, port A on a 3 port diverter is a fail closed port, this port is always directed to the cylinder heating coil, the other port to the heating circuit.

 

A cylinder being cooled by the heating circuit flow, is therefore a safety issue; as the port from a control valve should have been depowered when no call for heat and in the failed closed position.  A mid point valve has zero place within a heat pump system or as @JamesPa says in a system with WC.

[JamesPa]

3 minutes ago, JohnMo said:

If the cylinder is unvented, a mid point valve requires a fail closed 2 port valve between it and the cylinder heating coil - to give a positive close to stop the heat source, from supplying heat when power is removed by overheat thermostat.  A 3 port diverter does not need this additional 2 port valve, port A on a 3 port diverter is a fail closed port, this port is always directed to the cylinder heating coil, the other port to the heating circuit

Surely this is a bit academic when the heat source being switched by the valve is a heat pump, which is incapable of boiling the water ... unless of course there is a 'backup' heater fitted as well inline with the heat pump

[ProDave]

To clarify if everyone is making the right diagnosis, can you post some pictures of the plumbing, in particular what motorised valves you have, both general views and close up views so we can read the model numbers.

 

My preference is not to allow a 3 port valve into my house.  I much prefer 2 or more 2 port valves.

 

But basically an ASHP should NEVER fo heating and DWH at the same time. It is always one or the other.  That does not mean you have to set the programmer so they are on at different times.  Usually when both are on, DHW will be priority, so when the tank needs some heat, it will stop heating the house.  My own LG ASHP will set time limits on how long it will spend at a time doing DHW if you think there is any chance of the house cooling down while it is heating the DHW.

[ProDave]

3 minutes ago, JamesPa said:

Surely this is a bit academic when the heat source being switched by the valve is a heat pump, which is incapable of boiling the water ... unless of course there is a 'backup' heater fitted as well inline with the heat pump

The safety feature is one of the things the plumber checked before giving me G3 signoff.

 

You might argue an ASHP cannot possibly heat the water anywhere near boiling, but my ASHP as a 3kW backup heater inside it and a possible fault condition is that gets stuck on.

 

In normal use, my tank does not use the traditional tank thermostat.  It has it's own temperature probe in a thermostat pocket.  The traditional tank thermostat is only there to shut off the 2 port valve if the tank goes over temperature, and the temperature it is set to is much higher than the normal tank operating temperature.

[JamesPa]

Just now, ProDave said:

You might argue an ASHP cannot possibly heat the water anywhere near boiling, but my ASHP as a 3kW backup heater inside it and a possible fault condition is that gets stuck on.

As I said, surely academic unless there is a backup heater.

[JohnMo]

3 minutes ago, JamesPa said:

As I said, surely academic unless there is a backup heater.

Why is it academic, it's not a lawful install. Many heat pumps have an immersion directly inside it's casing or are capable of controlling one in a buffer cylinder. A secondary thermostat has to be able to close a positive shut off valve that fails to that position when power is removed. It cannot (maybe in this case shouldn't have been) be given an installation certificate without it.

[ProDave]

6 minutes ago, JamesPa said:

As I said, surely academic unless there is a backup heater.

My plumber did not think so.  He did not even ask if my ASHP had a backup heater.  He just expected the cylinder thermostat so shut the motorised valve feeding the tank.  Perhaps he is just a typical old school plumber and cannot understand the difference between a 5kW ASHP and a 20kW gas boiler.  Perhaps more modern plumbers would not insist on that?

[JohnMo]

2 minutes ago, ProDave said:

old school plumber and cannot understand the difference between a 5kW ASHP and a 20kW gas boiler

The rules are the same either way, they may change in 20 years time, but the rules as they are now, don't care what the heat source is.

[JamesPa]

11 hours ago, JohnMo said:

The rules are the same either way, they may change in 20 years time, but the rules as they are now, don't care what the heat source is.

Can't see why the rules need to change, they are written in a way that allows sensible interpretation in almost any circumstance:

 

G3. (1) There must be a suitable installation for the provision of  heated wholesome water or heated softened wholesome water to: (a) any washbasin or bidet provided in or adjacent to a room containing a sanitary convenience; (b) any washbasin, bidet, fixed bath and shower in a bathroom; and (c) any sink provided in any area where food is prepare (2) A hot water system, including any cistern or other vessel that supplies water to or receives expansion water from a hot water system, shall be designed, constructed and installed so as to resist the effects of temperature and pressure that may occur either in normal use or in the event of such malfunctions as may reasonably be anticipated, and must be adequately supported.

(3) A hot water system that has a hot water storage vessel shall incorporate precautions to:

(a) prevent the temperature of the water stored in the vessel at any time exceeding 100˚C; and

(b) ensure that any discharge from safety devices is safely conveyed to where it is visible but will not cause a danger to persons in or about the building. 

[JohnMo]

 

16 minutes ago, JamesPa said:

Can't see why the rules need to change, they are written in a way that allows sensible interpretation in almost any circumstance:

You may as well cut and paste the whole doc, because everything that comes after the cut and paste is mandatory anyway. There is very little room for interpretation.

 

They may say guidance, but are all covered by the statement

 

In the Secretary of State’s view Requirement G3(x) will be met if:

[JamesPa]

 

9 minutes ago, JohnMo said:

You may as well cut and paste the whole doc, because everything that comes after the cut and paste is mandatory anyway. There is very little room for interpretation.

 

They may say guidance, but are all covered by the statement

 

In the Secretary of State’s view Requirement G3(x) will be met if:

 

I could, but the preamble to the document says

 

"If guidance in an Approved Document is followed there will be a presumption of compliance with the requirement(s) covered by the guidance. However, this presumption is not conclusive, so simply following guidance does not guarantee compliance in an individual case. It is also important to note that there may well be other ways of achieving compliance with the requirements. There is therefore no obligation to adopt any particular solution contained in this Approved Document if you would prefer to meet the relevant requirement in some other way. However, persons intending to carry out building work should always check with their Building Control Body (BCB), either the local authority or an Approved Inspector, that their proposals comply with Building Regulations."

 

This makes it clear a) that the guidance is just that, it is a way (probably) to comply (note that it doesn't guarantee compliance) but not necessarily the only way.

 

Given that it doesn't guarantee compliance there is still an implied requirement actually to think about the system.  Also the guidance includes the following at 3.17: 'The selection of safety devices should take account of the physical location of the devices, and the design, configuration, location of components and performance characteristics of the system to which they are attached'.

 

Of course I concede that following the guidance is the sensible route in the vast majority of cases, but slavishly following it may not be, given that it does not guarantee compliance.   I also concede that whatever PL plumbers have may include terms which insist that the guidance be slavishly followed! 

 

My point nevertheless is that the actual rules do not need to change.  The industry may need to wake up to changes in technology (and perhaps, persuade the Sec of State to extend the guidance), but (this part of) the building regulations themselves are (extraordinarily) well written in a way which does not inhibit innovation nor lead to a situation where people can say 'I was only following the rules' as an excuse for poor practice.

 

 

[JohnMo]

But your real arguement is about the need for venting to drain, as you have mentioned several times on other threads.

 

The requirements of G3 are also supplemented with manufacturer instructions, any UVC sold in the UK comes with a set of instructions and the required safety devices. To not install as defined would go against G3 and OEM requirements.

 

For example my cylinder came with the following

 

• Cold water inlet PRV combination valve/expansion relief

• Lift up pressure and temperature relief valve - pre installed 

• Control thermostat

• Energy cut-out thermostat

• Energy cut-out motorised valve (indirects only)

• Tundish

 

Complete with instructions how to install

 

But back to this thread.

Having communication between the heating system and cylinder coil, should be a real concern, for the original poster. Which needs to be fixed asap

 

[JamesPa]

40 minutes ago, JohnMo said:

Having communication between the heating system and cylinder coil, should be a real concern, for the original poster. Which needs to be fixed asap

 

I certainly wouldn't disagree with that!  Nor would I disagree with following the manufacturer instructions of any equipment used (at least in most cases, I am sure there are exceptions although probably relatively few, if any, related to UVCs supplied with internal immersion heaters). 

 

Having said that I still suspect (as others do including, I would imagine, you) that the problem OP has highlighted is basically due to a wrong valve (3 way instead of diverter).  

 

PS: my thoughts on 'But your real argument is about the need for venting to drain, as you have mentioned several times on other threads.' are now mostly related to circumstances where there is no immersion heater in the tank.  For example the Vaillant R290 Heat pumps don't rely at all on a tank immersion, and SOFAIK aren't even capable of controlling one.  As R290 becomes more common I imagine that this may be replicated by other manufacturers. 

 

In this case there is a possibility that one could install a tank, supplied by a heat pump, without an immersion, a situation which is rarely if ever considered.  It seems to me that there are some interesting possible variants associated with this.

Edited November 14, 2023 by JamesPa

[JoeBano]

Mine did this, I got a relay fitted so if the hot water was on it automatically shut the central heating zone valve. On the grant heat pump it does say don’t schedule for the same time. 

[sharpener]

Not unique to heat pumps. I had to fit a relay to my Vokera boiler to inhibit the weather compensation when there is a call for the HW. A side benefit is that this gives fast warmup for the CH first thing in the morning. I might change the timings though as this takes it out of condensing mode which is not ideal.

 

Really good explanation of the various 3 port valve wiring etc with animated diagrams here.

[JamesPa]

34 minutes ago, sharpener said:

Not unique to heat pumps.

Indeed.  My gas boiler features two separate flow temperatures for Ch and dhw, but only if you fit their overpriced diverter valve (it occurs to me that it may be possible to fool it into thinking their valve is fitted).  And a friend of mine has rigged up a circuit to fool his weather compensating boiler that it's -2 outside when it's heating dhw, to force it to max flow temp (the friend in question wasn't prepared to pay the extortionate amount the boiler manufacturer wanted for a diverter valve).