Advice on bathroom heat strategy

[SomethingSensible]

I’m doing a whole house renovation/extension including new roof, new slab and external wrap insulation. So everything should be well insulated. Downstairs will be UFH (‘slab UFH’). Upstairs the bedrooms will be rads with TRVs and the two bathrooms will have water-based heated towel rails and UFH (‘bathroom UFH’). (We had electric UFH in a bathroom before and it broke. Not doing that again!)

 

The plan is to install an ASHP (and accompanying DHW tank). 

I’d like the ASHP to provide ‘comfort cooling’ during periods of extreme heat.

I plan to install PV and battery at some point in the future, hopefully within 2 years, but do not have the budget for that right now.

 

The ASHP company I've been talking to are suggesting a Samsung Gen 7 EHS Mono R290 unit. (12 kW, subject to heat loss calculations, to be done once I commit to them). From what I can tell, this supports 2 heating zones. They can have different temperatures if a mixing valve is used.

 

Hopefully, most of the setup is as simple as it can be. However, I’d appreciate your collective wisdom on the best way to configure the bathrooms…

 

“Best” in this case means most efficient (energy and cost) way of delivering the following behaviour…

 

Winter

• Slab UFH and bedroom rads operate ‘normally’ (I’m not really sure what normal is yet)
• Bathroom UFH is on schedule to ensure bathrooms are comfortable
• bathroom UFH and towel rails - can be ‘boosted’ if necessary to dry towels and floor

 

Rest of year

• No space heating required.
• bathroom UFH and towel rails are boosted manually when someone has a shower - to dry towels and floor.

 

Heat wave

• Comfort cooling through slab UFH and bedroom rads
• Bathrooms can be left alone - everything will dry naturally!

 

By boosting manually, I’m thinking something like this https://www.heatingcontrolsonline.co.uk/horstmann-secure-15-30-60-boost-timer.html but would also consider a smarthome solution using smart buttons

 

My desire to have heat in the bathrooms when everything else is off seems to be a bit awkward.

 

As far as I've been able to work out from my research, options for bathroom heat source seem to be...

 

1. ASHP - heating circuit

Due to small load, I believe a buffer tank would be required for times when rest of house heating is off. This avoids short cycling but is bad for efficiency. If there was no buffer tank there would likely be short cycling which would also hurt efficiency. But which hit to efficiency is worse?

 

2. ASHP - DHW circuit

Run bathrooms in parallel with tank on DHW circuit.  Use return temperature limiters to avoid overheating the bathrooms.  (https://www.heatingcontrolsonline.co.uk/return-temperature-limiters-c-44_49_61.html). This requires DHW to be on at times when it wouldn't otherwise need to be and could be more expensive. It also feels like it could be complicated to control to also ensure the bathrooms aren't being heated overnight (which, until we get PV, will presumably be the cheapest time to heat the DHW tank).

 

3. Electric boiler

The heat source for bathrooms could be completely separate from ASHP and rest of house heating. Maximum/simple control but poor efficiency compared to ASHP running normally. But is it worse that ASHP running sub-optimally, as in option 1?

 

I hope this all makes sense. It seems all the options involve taking an efficiency hit in some way, but I'm not sure how to make a fair (numerical) comparison.

 

Thanks in advance for any pointers. 

[-rick-]

1 hour ago, SomethingSensible said:

3. Electric boiler

The heat source for bathrooms could be completely separate from ASHP and rest of house heating. Maximum/simple control but poor efficiency compared to ASHP running normally. But is it worse that ASHP running sub-optimally, as in option 1?

 

I'll leave the main system design for others to comment on (lots of experience here). But:

 

Why not put in towel radiators with water loops + electric heater. In winter, use the main system, in summer use electric if needed (doubt you'd need it much so putting in something that doesn't cost much to install but costs a little more to run if needed makes sense)?

[JohnMo]

2 hours ago, SomethingSensible said:

Bathroom UFH is on schedule

You are vastly overthinking this, each step add inefficiency.

 

Have all your heating as a single zone, leave bathroom heat on as with the rest of the house.

 

2 hours ago, SomethingSensible said:

supports 2 heating zones

No keep to one. Then no need for a buffer or volumiser.

 

No need for mixers or additional pumps either.

2 hours ago, SomethingSensible said:

Electric boiler

The heat source for bathrooms could be completely separate from ASHP

Sorry that is daft. You automatically excluded yourself from grant money. And why would you both when you have a heat pump?

 

Heat pump work most efficiently left to look after themselves. So one zones, no time schedule (except DHW), no boost settings, no buffer. Just a fully open system with a 3 port valve. Just set WC and let it do its own thing. It will only run as needed. Once your house temperature demands it flick over to cooling. Run above dew point, so about 12 degs and above. With a little tuning the system will run heat and cool as neededa

 

Towel rail I just use an electric one with timer/thermostat. Even in summer the bathroom floors are cooled.

 

DHW have the thermostat or temperature probe and run a time schedule to 1 or 2 times per day only.

 

Your scheme over will yield at a best a CoP of below 3 and even less taking into account the additional electric boiler. On the other hand a simple single zone scheme a CoP of 4 and above including DHW.

 

This is ours running, the big peaks are DHW heating via a fixed schedule. The shaded areas are heat pumps actually running in cooling mode. Note after the first DHW how the heat pump runs longer, this is just due to heat pump having to run harder to manage heat loading. Ove time it has nothing to do so it switches it's self off. The numbers in different colours below graph are the average CoP in the window (24 hrs)

 

Similar is true in heating mode.

 

[SBMS]

7 hours ago, JohnMo said:

You automatically excluded yourself from grant money

How come? Are you not allowed this?

[SteamyTea]

Right, I am very tired, but we see post like this quite often.

 

Without more details of the insulation levels, it is hard to really work out what is best.

Basic heat loss calculations really need to be done before you delve into detail.  12 kW ASHP seems quite large, but your house may be huge, without some more details we cannot properly comment.

 

Regarding the bathroom floor,

I went to a minor Public School, rough textured, cold concrete floors and tepid water, with the company of many other naked bodies, made me the person I am.

[JamesPa]

1 hour ago, SBMS said:

How come? Are you not allowed this?

Heat pump must be capable of heating the whole property to qualify for the BUS grant.

 

@JohnMo is giving sensible advice imho, keep it simple, one zone, no system separation, use an electric element for the towel rail separate to the heating/cooling (towel rails are anyway pretty useless for heating in a low temperature heating system, not enough output)

Edited July 19 by JamesPa

[SBMS]

5 minutes ago, JamesPa said:

Heat pump must be capable of heating the whole property to qualify for the BUS grant.

 

@JohnMo is giving sensible advice, keep it simple,use an electric element for the towel rail separate to the heating/cooling (towel rails are anyway pretty useless for heating in a low temperature heating system, not enough output)

I guess as long as it is sized to heat it doesn’t actually have to deliver heat to that room to be eligible for the grant though?

[JamesPa]

2 minutes ago, SBMS said:

I guess as long as it is sized to heat it doesn’t actually have to deliver heat to that room to be eligible for the grant though?

Arguably yes (or no) but why bother having this argument? (you will likely have other arguments with your installer which it's more important to win)

[SBMS]

10 minutes ago, JamesPa said:

Arguably yes (or no) but why bother having this argument? (you will likely have other arguments with your installer which it's more important to win)

It’s actually relevant for us as some rooms we may put AC in so wouldn’t be heated by heat pump. 

[JamesPa]

22 minutes ago, SBMS said:

It’s actually relevant for us as some rooms we may put AC in so wouldn’t be heated by heat pump. 

Ok potentially complicated.  You will have to negotiate both the BUS rules and the MCS ones.  I would suggest you need to read both and work out how you are going to convince an installer that what you propose complies (I'm not at all sure it does, but maybe you can find a way through!).

 

I would read the original regulations not guidance for this one as it's unusual so may not be adequately covered by simplified guidance.

Edited July 19 by JamesPa

[SBMS]

10 minutes ago, JamesPa said:

Ok potentially complicated.  You will have to negotiate both the BUS rules and the MCS ones.  I would suggest you need to read both and work out how you are going to convince an installer that what you propose complies (I'm not sure it does, but maybe you can find a way through!).

Yes agreed. I think MCS lets you discount rooms heated by other sources in heat pump design (MCs 3005). But I think you’re right that BUS requires 100% heat from heat pump. There’s a weird cross over I think that is unclear as to whether MCS when designed for BUS requires it and that’s probably an installer interpretation.

 

Still on the fence about cooling with ASHP and sticking fancoils upstairs and ditching the AC. 

[JohnMo]

2 hours ago, SBMS said:

How come? Are you not allowed this?

Read the grant BUS details, basically all DHW and Ch had to be supplied by the heat pump.

[JohnnyB]

41 minutes ago, JohnMo said:

Read the grant BUS details, basically all DHW and Ch had to be supplied by the heat pump.

My installer is suppling a heat pump that is large enough to heat the whole house but is putting the upstairs radiators down as phase 2 (It's a new build and I don't think I need rads upstairs but have installed the pipework just in case)

I think if you have an installer who can think outside the box a little there are options.

 

13 hours ago, SomethingSensible said:

They can have different temperatures if a mixing valve is used.

 

In my current house I have a Samsung Gen6 and if I understand the manual correctly it can supply 2 zones at different temperatures without mixing valves, but not at the same time, i.e. it would heat one zone at one temperature and then the other at a different temperature, but I haven't tried that as I have a single zone.

 

I'm also building a new build which will have a the R290 Samsung HP and I'm hoping to do a similar thing to you, but without the towel rails. This is still theory so I'm not sure how well it will work in practice, but my plan is to have 2 zones controlled by 2 room stats connected to the UFH controls. Downstairs UFH and FF rads (if fitted) will be on zone 1 then upstairs bathrooms UFH will be on the second zone. I'm expecting it all to run at the same flow temperature.

During the cold months I will run it with both room stats set to max and let the Samsung weather compensation control the flow temperature, it will be running as one zone.  When it warms up and I only want the bathrooms UFH on I will turn the downstairs UFH off by turning that room stat down but leave the bathroom UFH running.  All the time the HP can be controlled using the weather compensation so should be running efficiently.  I'm having a 50 ltr two port buffer so there is always enough water flowing in the system.

Currently I don't find a cold floor that useful for cooling the house, all I get is cold feet, but most days opening the doors and windows is enough to cool the house in the evening for our comfort levels, but I am putting power points beside the bedroom rad postions so I can add fan coils in the future if we want them.

Edited July 19 by JohnnyB

[SomethingSensible]

Thanks for all the replies.

 

@JohnMo "Vastly overthinking" things is a fair assessment. Still, I'd much rather consider the possibilities and reject the overcomplex/unworkable/daft than blindly install a system which can't do something which is important to me.

 

So, the electric boiler idea is 100% dead. Good riddance!

 

Running pure electric towel rails through the winter feels like I'd be missing out on all that ASHP COP goodness. But if they are only on for a short amount of time compared to the more-or-less continuous heating zone, then maybe it's better?

 

@-rick- I've looked a bit more at dual-fuel towel rail options and realised that the add-on heating elements aren't as expensive as I (mis)remembered.

 

Is there a reason why this should be avoided?

 

So whether I go for pure electric or dual-fuel, my towel drying concerns look like they are behind me.

 

My remaining worry is the floors during the season when space heating is not required. The comfort of the bathroom floors isn't so much a concern for me - I can handle a wee bit of chill! - rather, if the floor does not dry properly I'm worried we'll soon be dealing with mould growth.

 

 

[IGP]

8 hours ago, SomethingSensible said:

Running pure electric towel rails through the winter feels like I'd be missing out on all that ASHP COP goodness. But if they are only on for a short amount of time compared to the more-or-less continuous heating zone, then maybe it's better?

 

Why wouldn’t you just put UFH with the spacing for the required heat output for the room (or rads) on the ASHP open loop and have pure electric towel heaters just for warmed towels, timed for a few hours a day?

[JohnMo]

8 hours ago, JohnnyB said:

having a 50 ltr two port buffer

So will you have to an additional circulation pump? Is 50L enough to stop short cycling if zone 2 is only running?

 

A topic never really appreciated with low temperature UFH (assuming you are doing low temperature) is the nature of its self regulation. Especially when running weather compensation the floor surface temp in mild weather is 1 to 2 degs warmer that the room it heats. This small temperature difference means small fluctuations in room temperature, affect heat output from the floor. So as room gets close to floor temp heat output drops, as room gets colder floor output increases. This difference is sense by changes in return temp, to ASHP and it modulates output accordingly. So in effect you just don't need to zone. Your floor is your buffer, without buying stuff you don't need - buffer, additional circulation pumps, valves, thermostats, actuators, more antifreeze (if you use it)

 

8 hours ago, SomethingSensible said:

Running pure electric towel rails through the winter

They are just towel dryers not room heater, your UFH is the room heater. So you have on for an hour or so after baths or showers, think mine have a 300W heating element in. If room is already warm the thermostat doesn't kick in. So running is not excessive.

 

8 hours ago, SomethingSensible said:

if the floor does not dry properly I'm worried we'll soon be dealing with mould growth

That's down to your ventilation strategy, more than your heat strategy. So maybe something to wrap your head around in another topic?

[-rick-]

9 hours ago, SomethingSensible said:

@-rick- I've looked a bit more at dual-fuel towel rail options and realised that the add-on heating elements aren't as expensive as I (mis)remembered.

 

Is there a reason why this should be avoided?

As @JamesPa pointed out (and I forgot), if the heating system is designed as a low temperature system (as a system should be in a good quality well insulated house) then its not like the water will be very warm in the first place. So if you want warm towels, skip hooking it up to the central system and just use electric with timers for when you have showers.

[JamesPa]

10 hours ago, JohnnyB said:

 

23 hours ago, SomethingSensible said:

They can have different temperatures if a mixing valve is used.

 

In my current house I have a Samsung Gen6 and if I understand the manual correctly it can supply 2 zones at different temperatures without mixing valves, but not at the same time, i.e. it would heat one zone at one temperature and then the other at a different temperature

If you use a mixing valve you can have two temperatures simultaneously but your heat pump is running at the higher of the two so you are losing the efficiency benefits of running at a lower temperature.  Without a mixing valve you can only have one temperature at any given time.  All heat pumps and sofaik boilers work like thus hence the need for a 3 way diverter valve for dhw if you operate either a boiler or heat pump on wc.

 

The best and simple  way is to design all emitters to run at the same (low) flow temperature unless that is totally impossible,  in which case a compromise, for which there is a cost, must be made.

Edited July 20 by JamesPa

[JohnnyB]

4 hours ago, JohnMo said:

So will you have to an additional circulation pump? Is 50L enough to stop short cycling if zone 2 is only running?

My HP has integral pump but it will be several metres away from the house with about 15 metres of F&R pipework and I have been told I will need a second pump indoors to pump the water around the UFH. The pump on the HP isn't large enough to do everything.  The HP has a minimum water content of 30 ltrs so a 50 ltr buffer/volumiser is large enough to satisfy that requirement.  Whether it will be enough to stop short cycling I don't know, I haven't asked that question.  I don't intend to run it all the time when it is just the bathrooms where we want a warm floor, I intend to run it on a program so the floor is warm morning and evening when we are showering.  Running for a few hours each day I'm not expecting it to be an issue.

[JohnMo]

1 minute ago, JohnnyB said:

The HP has a minimum water content of 30 ltrs

That number is defrost support only - not for cycling.

 

2 minutes ago, JohnnyB said:

HP has integral pump but it will be several metres away from the house with about 15 metres of F&R pipework and I have been told I will need a second pump indoors to pump the water around the UFH

Mine is a similar distance from house, it would be worth doing the actual calculations yourself to double check. I get around 1.2m³/h flow through the UFH, without additional pumps, longest loop is 110m.  But having limited flow rate doesn't require a buffer in the system, you just add a fixed speed pump to the return piping and run at the lowest speed to achieve the flow you require (it's there to make up for a fixed amount head loss only).

[JohnMo]

This what Samsung say about water volume 

[SteamyTea]

5 hours ago, JohnnyB said:

I don't intend to run it all the time when it is just the bathrooms where we want a warm floor, I intend to run it on a program so the floor is warm morning and evening when we are showering

Not the best way to run a low temperature heating system, regardless of the heat source.

Day and night temperature difference can still be different, but that often comes naturally because overall heat losses are greater after sundown.

 

If you are used to high temperature gas boiler systems, it takes a while to appreciated the difference.

[saveasteading]

We have an old, primitive house with no floor insulation. Despite that we installed an electric blanket type under the tiles, with time control. Also an electrically heated heated towel rail, for which we simply push an 'on for 2 hours' button. as required.

Theoretically that is inefficient with 1. using straight electricity, 2. heat loss through the floor.

But all works well. With cosy feet all feels well with the world, and neither heat source is on for long.